trends in CELL BIOLOGY (Vol. 10) May 2000
hypothesis
Why cytoplasmic
signalling proteins
should be recruited
to cell membranes
FORUM
Upon stimulation, certain protein kinases, phosphatases and other players in signal transduction relocate to membranes, cytoskeletal structures, scaffolding
proteins or organelles1–3. Here we take receptor tyrosine kinase (RTK) signalling as our main example
(Fig. 1). Stimulation of RTKs is linked to the activation
of mitogen-activated protein kinase (MAPK) cascades
through a cytoplasmic protein Sos (a homologue of
the Drosophila melanogaster ‘Son of sevenless’ protein)
and the small GTP-hydrolysing protein Ras, anchored
to the cell membrane4. Sos is a GDP/GTP exchange factor that catalyses the conversion of inactive Ras (i.e.
its GDP-bound form) to active Ras (its GTP-bound
form). The adaptor protein Grb2 (growth-factor-receptor-binding protein 2) mediates the binding of
Sos to activated RTKs, such as the epidermal growth
factor receptor (EGFR). Grb2 binds to the activated
EGFR directly or through another adaptor protein,
tyrosine-phosphorylated Shc (src homology and
collagen domain protein). The EGFR does not phosphorylate Sos, nor does the catalytic activity of Sos
towards Ras change upon Sos binding to the receptor5.
When Sos is recruited to the membrane by activated
EGFR, Sos can interact with the membrane polyphosphoinositides through an N-terminal pleckstrin
homology (PH) domain.
This interaction pattern raises a number of questions about the role of the plasma membrane relocation in signal transduction. Why should the
Grb2–Sos complex bind to the membrane receptor
if Sos catalytic activity is not activated by the receptor? What prevents direct interaction of cytosolic
Sos with the membrane-bound Ras from activating
the latter? Why is Ras anchored to the membrane?
Should anchoring itself be a regulatory event? What
is essentially different in the activated versus the
nonactivated RTK? It has been proposed that the recruitment of Sos to the proximity of the membranebound Ras is a key feature in the activation of Ras
by phosphorylated EGFR6–8. But what does recruitment mean? If it means that Sos is first bound to the
EGFR and then moves to Ras by two-dimensional
diffusion, then why should this accelerate signal
transduction? Cytosolic Sos still requires the same
amount of time to reach the EGFR. Binding to EGFR
would slow down its diffusion unless Sos dissociated
again, but then Sos would escape back to the cytosol
rather than bind to Ras.
To clarify the effect of membrane localization, we
consider two extremes. When two protein molecules
form a productive complex (i.e. transduce the signal)
after each diffusive encounter, the signal-transduction
process is ‘diffusion-limited’. If only a small fraction
of the collisions leads to binding that lasts long
enough to transfer the information, the signal transduction is ‘reaction-limited’. In this case, the reaction
rate is controlled by the alignment of reactive patches
in the correct orientation or by the intrinsic chemical
transformation rather than by the Brownian collisions of the molecules. The two protein molecules then
associate and dissociate several times before signal
transduction takes place. We will now analyse the
consequences of the membrane translocation for
diffusion- and reaction-limited signal transduction.
Boris N. Kholodenko, Jan B. Hoek
and Hans V. Westerhoff
It has been suggested that localization of signal-transduction
proteins close to the cell membrane causes an increase in their rate
of encounter after activation. We maintain that such an increase
in the first-encounter rate is too small to be responsible for truly
enhanced signal transduction. Instead, the function of membrane
localization is to increase the number (or average lifetime) of
complexes between cognate signal transduction proteins and hence
increase the extent of activation of downstream processes. This is
achieved by concentrating the proteins in the small volume of the
area just below the plasma membrane. The signal-transduction
chain is viewed simply as operating at low default intensity because
one of its components is present at a low concentration. The steady
signalling level of the chain is enhanced 1000-fold by increasing
the concentration of that component. This occurs upon ‘piggyback’
binding to a membrane protein, such as the activated receptor,
initiating the signal-transduction chain. For the effect to occur, the
protein translocated to the membrane cannot be free but has to
remain organized by being piggyback bound to a receptor, membrane
lipid(s) or scaffold. We discuss an important structural constraint
imposed by this mechanism on signal transduction proteins that
might also account for the presence of adaptor proteins.
Does membrane localization enhance diffusionlimited signal transduction?
Adam and Delbrück suggested that the reduction
in dimensionality might enhance reaction rates between solutes that bind to membranes and membranebound species9; the solutes should not get lost by
wandering off into the bulk phase. The relevance and
magnitude of this enhancement has been studied
extensively in various biological systems10–12.
Conservative estimates can be made of the time
taken by signal transduction proteins in the cell
0962-8924/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0962-8924(00)01741-4
173
FORUM
hypothesis
Ligand
Ligand
unlikely to be an enhancement of the encounter
rates of signal transduction proteins.
Shc Grb2 SO Ras
Ras SO Grb2
Raf
Raf
RTK
RTK
Grb2 SO
MEK
MEK
Shc
SO
Erk
Grb2
Erk
trends in Cell Biology
FIGURE 1
Receptor tyrosine kinase (RTK)-mediated activation of mitogen-activated protein kinase
(MAPK) cascade through Ras. The upper part of the figure emphasizes the situation
where signal transduction occurs at the membrane surface. The bottom part of the
figure indicates that some of the signal transduction proteins might also reside in the
cytosol, raising the question of whether a signal could be transmitted through the cytosol
or why binding of Grb2/Sos to the activated receptor might be required. SO, Sos.
Boris N.
Kholodenko and
Jan B. Hoek are in
the Dept of
Pathology,
Anatomy and Cell
Biology, Thomas
Jefferson University,
1020 Locust St,
Philadelphia,
PA 19107, USA;
and Hans V.
Westerhoff is in
the Dept of
Molecular Cell
Physiology and
Mathematical
Biochemistry,
BioCentrum
Amsterdam,
De Boelelaan 1087,
NL-1081 HV
Amsterdam,
The Netherlands.
E-mails:
boris.kholodenko@
mail.tju.edu;
[email protected]
174
membrane or in the cytosol to encounter their first
partner molecule by free diffusion13. A spherical cell
with a radius of 10 mm has a surface area of 1260 mm2.
If it contains 10 000 copies of each signal transduction
protein in its membrane, then at 0.35 mm spacing
the protein molecules occupy the entire cell surface.
Partners in signal transduction should then be approximately L 5 0.25 mm apart. The average time for
them to meet a neighbour should be approximately
L2/2D, where D is the lateral diffusion coefficient. As
the membrane diffusion coefficient of the protein is
approximately 10–9–10–10 cm2/s (Refs 14 and 15), it
should take about 0.3–3 s before the partner proteins
hit each other when diffusing in two dimensions. For
three-dimensional signal transduction with 10 000
proteins per cytosol, the partner proteins will be on
average 0.6 mm apart. Using a diffusion coefficient of
10–8 cm2/s for cytosol diffusion (see Ref. 16 and references therein), this leads to a time of L2/(3D) 5 0.1 s,
which is faster, not slower, than the 0.3–3s for the
2-D scenario.
These estimates show that, for homogeneous protein distribution, hooking proteins up to the plasma
membrane causes very little, if any, increase in their
encounter rates. Indeed, it has been argued that the
fastest route to diffusion is through the cytosol, not
the membrane, because of two orders of magnitude
difference in the diffusion coefficients17. In Box 1, a
more rigorous comparison of the rates of diffusionlimited protein associations in the cell membrane
and in the cytosol leads to the same conclusion: the
function of attachment to the plasma membrane is
Membrane association might enhance
reaction-limited signal transduction
In the reaction-limited extreme, the catalytic rate is
much slower than the association and dissociation
rates. The influence of first-encounter rates controlled
by diffusion on the overall reaction rate can be
neglected, and the signal transduction rate is determined by the fraction of molecules in the associated
state multiplied by a reaction rate constant13. The
effect of association to the membrane is that of an
increased local concentration. This causes an increase
in the apparent affinity and can enhance the association of two membrane-associated proteins compared
with two cytosolic proteins. Using order-of-magnitude
reasoning, Haugh and Lauffenburger estimated that
the increase in reaction-limited protein association
could be as high as 102–103 (Ref. 20). The enhancement depends on many molecular details including
the reversibility of the binding12, which implies that
the dissociation rate constant might also change
upon membrane relocation. For instance, affinity
enhancement due to ‘macromolecular crowding’
appeared to be caused partly by a decrease in the
dissociation rate constant22.
A gain in the number of signalling complexes
is more important than an increase in the
encounter rate
The calculations above showed that encounter
times are of the order of 0.1–1 s. Thus, an increase in
first-encounter rate can be important for very fast
signalling processes, which are diffusion-limited.
For example, a rapid onset of EGFR phosphorylation
in response to growth factors implies that receptor
dimerization proceeds as an almost diffusion-limited
step23. For reaction-limited processes, which are
slower, a change in first-encounter rate is irrelevant.
Instead, the mechanism underlying an increase in
signal transfer rate involves an increase in the number
of signalling complexes that act as catalysts activating
downstream processes. We submit that membrane
localization serves to enhance the extent of complex
formation of signal-transduction proteins and hence
increases the intensity of the signal being transduced. If the extent of complex formation is indeed
the important issue, then the following, comparatively simple, analysis can be made of the effect of
membrane localization.
Membrane anchoring of only one of two
interacting proteins does not lead to a gain in
the number of signalling complexes
Does the number of complexes formed by signalling
proteins depend on whether one of the interacting
proteins is bound to the membrane (rather than both
proteins diffusing in the cytosol)? We assume that
the standard free-energy difference of the binding
reaction does not depend on the spatial localization
of the complex, so that the equilibrium constant Kd
is the same regardless of whether one of the proteins
is associated with the membrane.
trends in CELL BIOLOGY (Vol. 10) May 2000
FORUM
hypothesis
BOX 1 – COMPARISON OF THE RATES OF DIFFUSION-LIMITED PROTEIN ASSOCIATIONS IN THE CELL MEMBRANE
AND IN THE CYTOSOL
Modern theories of diffusion-limited reactions show that a two-dimensional association rate constant is not a constant parameter but depends slightly on time18. Based on these theories, Lamb and Pugh estimated the encounter rate between
membrane proteins using the sum of their diffusion coefficients (Dm) and the sum of their radii (rprot)19. If we express the
concentrations of the proteins based on the whole cell volume, the association rate constant is predicted to be:
k m (t ) =
4pD m N A(V / M)
2
ln(4D mt / r prot
) − 2g
(13)
where t is the time from the onset of reaction, NA is Avogadro’s number, V is the volume of the cell, M is the surface area
of the membrane and g 5 0.58 is Euler’s constant. When the time increases from 0.05 to 100 s, the association rate
constant decreases by a factor of 2 (for typical values of Dm and rprot)19. Therefore, in the time frame of seconds, the time
dependency of the two-dimensional rate constant can be neglected.
The encounter rate in three dimensions is described by the following expression for the second-order rate constant, kc 5
4pNADcrprot, where Dc is the sum of the diffusion coefficients of the encountering cytosolic proteins. The rate enhancement due to the confinement of proteins to the plasma membrane can be estimated as the ratio (h) of the association
rate constant in two dimensions to that in three dimensions. For diffusion-limited association of two proteins located in
the plasma membrane of a spherical cell or delocalized over the cytosol volume, the ratio h can be approximated as
h 5 (0.02 – 0.05)(Dm/Dc)(rcell/rprot). The cell radius, rcell, appears in the equation as the result of dividing the cell volume
by the cell surface area. For rcell of 10 mm and rprot of about 10–2 mm, h ≈ (20 – 50)Dm/Dc. Because the diffusion coefficients
of proteins in the membrane are two orders of magnitude lower than in the cytosol, the rate enhancement due to
association with the membrane (if any) is moderate for protein–protein associations limited by diffusion. The diffusionlimited association rate might actually decrease in the membrane20. We conclude that the function of localization at the
plasma membrane is unlikely to be an enhancement of diffusion rates. However, if both interacting proteins are targeted
to specific membrane domains (e.g. membrane rafts21) constituting a small fraction of the whole plasma membrane, then
an increase in diffusion-limited association rates can be significant.
BOX 2 – INTERACTIONS OF A AND T RESULTING IN THE FORMATION OF AN AT COMPLEX
Let A be a cytoplasmic protein and T a membrane-associated target protein that contains a specific A-binding domain
(Fig. 2). We denote by Vm the volume of a water layer adjacent to the membrane to which protein T is confined and by
Vc the cytosol volume. At equilibrium, the law of mass action relates the concentrations of A, T and the complex AT to
the dissociation constant, Kd (the number of molecules in the cell will be designated by the same symbols as the
corresponding species, in italics):
 A  ⋅T 
 Vc   Vm   1  A ⋅ T
⋅
=
Kd =
AT
 Vc 
 AT 
 Vm 
(
)
(1)
The numbers A, T and AT of the corresponding molecules in the cell are restricted by the total numbers, Atot, Ttot,
as follows:
A + AT = A tot
T + AT = T tot
(2)
(3)
As only Vc remains in Eqns 1–3, it follows that the number of complexes AT does not depend on whether one of the interacting proteins is anchored to the plasma membrane or both proteins are in the cytosolic fraction of the cell.
To compare direct interactions with receptor-mediated interactions of cytoplasmic A with T, we estimate the Kd value,
which allows more than 50% of the total A protein to be associated with T. It follows from Eqns 1–3 that more than
half of the molecules of Atot will be in the complex AT if Ttot ≥ Atot/2 and if the dissociation constant of specific binding
is less than:
K d (direct binding, AT ≥ 0.5A tot ) ≤ (1− A tot / 2T tot ) ⋅ T

trends in CELL BIOLOGY (Vol. 10) May 2000
tot

Vc 
(4)
175
FORUM
hypothesis
T
amount to 1000 (Ref. 2). Therefore, one possibility for
a signal-transduction chain to enhance the extent of
association of its proteins (or reduce the number of
protein molecules per cell required to achieve the
same extent of association) is to have these proteins
anchored to the membrane.
AT
A
trends in Cell Biology
FIGURE 2
Direct interaction between a cytosolic protein A and a
membrane-anchored target protein T, leading to the
formation of the complex AT.
The analysis in Box 2 shows that the fraction of a
cytosolic protein A that is complexed to a membrane
target T (Fig. 2) is independent of whether T is
confined to the membrane or allowed to delocalize
all over the cytosol. Direct interaction of A and T
will result in significant binding only if Kd does not
exceed the concentration of the target protein dissolved in the cytoplasmic volume (Eqn 4). We conclude that membrane localization of only one of
the interacting proteins does not enhance complex
formation and signal transduction.
Membrane anchoring of both interacting
proteins might increase the number of
signalling complexes
If both signal transduction partners are anchored
to the membrane, then they exist in a reduced volume and association should be favoured. The analysis is given by the equations in Box 2, provided that
the cytosolic volume Vc is replaced by volume of the
shell near the membrane that is accessible to the
proteins, Vm. A consequence is that the dissociation
constant Kd required for a certain degree of association of two membrane-linked proteins exceeds that
required for the same degree of association of two
cytosolic proteins, by the ratio Vc/Vm. For a spherical
cell of radius 10 mm and a submembrane layer of
thickness 3 nm (corresponding to the dimensions
of an anchored protein), the ratio Vc/Vm might well
T
R
RAT
A
T
(1)
(2)
RA
trends in Cell Biology
FIGURE 3
Piggyback mechanism of the complex formation between a
cytosolic protein A and a membrane-anchored target
protein T. Interaction of A and T is promoted by first
binding A to the receptor R after activation of R, which
recruits A to the membrane surface.
176
Piggyback riding: receptor-mediated membrane
localization increases the number of complexes
formed by a cytoplasmic protein and a
membrane-anchored protein
A gain in the number of signalling complexes involving a cytosolic protein A and its target T can also
be brought about by a reversible, ‘piggyback’ mechanism. Here, A binds to a ‘piggy’ protein receptor (R)
that is itself bound irreversibly to the membrane
(Fig. 3). Protein A then rides piggyback until it meets
membrane-bound target T. It then forms a complex
with T, while continuing to ride piggyback on R.
A simple analysis of quasi-equilibrium binding is
given in Box 3. Equations 10 and 12 in Box 3
demonstrate that piggyback riding leads to a strong
reduction of the apparent (dissociation) equilibrium
constant. The factor decrease can be as high as the
factor Vc/Vm. Consequently, under conditions where
nonmembrane-associated proteins A and T would fail
to bind to each other, a significant fraction of piggyback riding A should bind to membrane-anchored T.
Active compartmentation: a tool to activate
signal transduction
In signal transduction, it is the extent of activation
– defined as the signal ratio between the active and
the inactive state of the pathway – that matters. A
100–1000-fold activation can constitute an effective
switch between the ‘on’ and ‘off’ states of a phosphorylation cascade26,27. The piggyback effect should
be suitable for the switch between the active and
the inactive states of the signal transduction chain.
For this to occur the receptor should have affinity
for A only after activation by its own signal. Such
regulated translocation of the kinases and phosphatases can generate switch-like responses due to
enzyme-saturation effects28.
The simple principle for signal transduction is as
follows. Upon binding of its extracellular ligand, a
receptor protein develops an affinity for a cytosolic
membrane protein A. This leads to an increased steadystate concentration of A (as the receptor–protein
complex RA) near the membrane for as long as the
receptor remains activated. Because of this enhanced
concentration, more of the target signal-transduction
protein complexes with A, enhancing its signal
more than 100-fold. In this picture, diffusion rates
are irrelevant; it is all mass-action balance at steady
state, the balance being tipped by concentrating A
close to the membrane.
Structural constraints and scaffolds
In the analysis above, it was RA (the receptor–protein
complex) and not just A (the free protein) that bound
the target T. If only free A was able to bind to T, the
presence of the receptor should decrease rather than
increase association of A and T and hence decrease the
trends in CELL BIOLOGY (Vol. 10) May 2000
FORUM
hypothesis
BOX 3 – PIGGY-BACK MECHANISM OF THE FORMATION OF A COMPLEX INVOLVING A CYTOSOLIC A AND A
MEMBRANE-ANCHORED T
We designate by R, RA and RAT the numbers of molecules of the receptor R, of its complex with A and of the ternary complex RAT per cell (Fig. 3). Rtot is the total number of activated receptor molecules. The binding of A and R is characterized
by the dissociation constant K dR. To compare a direct binding of A and T with a two-step interaction involving the association of A and R, we assume that the association of both cytoplasmic A and the complex RA with membrane-bound T is
characterized by the same dissociation constant, Kd. At equilibrium, the concentrations of interacting molecules and their
complexes are related through the law of mass action:
(
K dR =  1  ⋅ A ⋅ R
RA
 Vc 
R + RA + RAT = R tot
)
(
K d =  1  ⋅ RA ⋅ T
RAT
 Vm 
(5)
A + RA + RAT = A tot
(7)
(8)
)
(6)
T + RAT = Y tot
(9)
If all A molecules were bound to the receptor R, Eqn 1 (Box 2) and Eqn 6 (Box 3) would be identical after multiplying Eqn 6
by Vm/Vc and substituting Kd by K dapp 5 Kd(Vm/Vc). However, only part of A is bound to R. When Atot is significantly less than
Rtot, the changes in R due to the variation in RA and RAT complexes can be neglected. In this case Eqn 7 can be omitted
and R is approximated as Rtot. Using Eqn 5 to express A in terms of RA and substituting into Eqn 8, Eqns 5–9 are reduced to
three relations that are identical to Eqns 1–3 after substitution of Kd by the apparent dissociation constant K dapp, defined as:
R
K
V
K dapp = K d ⋅  m  ⋅ (1+ a), a = totd
 Vc 
R Vc
(10)
The value of the dimensionless factor a is determined by the ratio of the dissociation constant for the interactions of A
with R and the concentration of the activated receptor based on the whole cytoplasmic volume. Data obtained for receptor tyrosine kinase (RTK) show that Kd is in the range 1–100 nM (Refs 24,25). The total concentration of RTKs (normalized to the cell volume) is in the order of 100 nM and ~20–50% of the total amount is activated by the stimuli (see
Ref. 23 and references therein). We conclude that for membrane recruitment by RTKs, the value of a does not exceed 1.
In the general case, the number of activated receptor molecules (Rtot) might be comparable to the number of A molecules
(Atot). For a two-step mechanism of A and T interaction involving the binding of A and R, we can estimate the value of Kd
that allows ≥ 50% of A molecules to be bound to T (in the complex RAT). Obviously, this requires the total numbers of
the interacting molecules (A, T and R) to satisfy the restrictions: Rtot ≥ Atot/2; Ttot ≥ Atot/2. Substituting the inequality
RAT/Atot ≥ 0.5 into Eqns 5–9 and expanding the solution of the resulting quadratic equation using the above restrictions,
we find that the corresponding Kd should satisfy the inequality:
K d (RAT ≥ 0.5A tot ) ≤ (1− A tot / 2T tot ) ⋅
(1− A tot / 2R tot ) T tot 
⋅
Vm 

(1+ a)
(11)
Equations 11 and 4 (Box 2) can be used to compare the values of Kd that allow ≥ 50% of A molecules to be bound to T
for a piggyback or direct mechanism of binding:
K d ( piggy − back mechanism , RAT ≥ 0.5A tot )
K d (direct binding , AT ≥ 0.5A
tot
)
(
tot
tot
 V  1− A / 2R
≈ c ⋅
(1+ a)
 Vm 
activation of T: a membrane protein with affinity to a
cytosolic protein cannot increase the free concentration of the latter near the membrane. A must be a twodomain protein, where the binding of one domain to
R should not occlude the other domain from T.
Perhaps this structural constraint is sometimes met by
so-called adaptor proteins. Recruitment of a protein to
the membrane by binding to another functional protein has the potential disadvantage that one face of
either protein is occluded from its normal activity.
Adaptor proteins might serve to bind to proteins while
keeping them far enough apart for their surface to
retain their normal activity. In addition, they might
orient A so as to enhance its affinity to T.
trends in CELL BIOLOGY (Vol. 10) May 2000
)
(12)
Our analysis also applies to translocation to scaffolds. Scaffolds bring together signalling proteins,
organizing and coordinating the function of entire
signalling cascades17. Again, binding to a scaffold
should not compromise the ability of the signaltransducing proteins to bind to each other. Adaptor
proteins might also be required to achieve this end.
Are membrane rafts required for promoting
fast, diffusion-limited signalling processes?
For most cell geometries and with homogeneous
protein distribution in the membrane, the translocation to the plasma membrane cannot increase
first-encounter rates significantly (if at all). However,
177
FORUM
hypothesis
as noted above (Box 1), when signalling molecules
are concentrated within special membrane domains,
such as membrane ‘rafts’21, diffusion rates increase
as the ratio of the membrane surface to the total domain area. If this ratio is much greater than one, the
diffusion-limited rates will exceed such rates in the
cytosol. Therefore, targeting all participants of a signalling chain to a limited area within a raft will enhance very fast diffusion-limited signalling processes. However, the number of signalling complexes
formed within these domains would also increase by
the same factor. Therefore, the rate of reaction-limited
signal transduction that does not depend on firstencounter rates will also increase by a factor equal to
the membrane surface/domain area, i.e. over a f
actor of 1000 compared with such a rate in the
cytosol.
Acknowledgements
We dedicate this
paper to the
memory of
Paul Srere, teacher
of the importance
of macromolecular
organization for
cell function. We
thank Frank
Bruggeman,
Oleg Demin,
Jorrit Hornberg,
Jan Lankelma and
Oscar Somsen for
discussions, and
NIH grants
GM59570-01A1,
AA01786,
AA07215,
AA08714 and the
Netherlands
Organization for
Scientific Research
(NWO) for
support.
178
Specifics of Sos–Ras interactions
Our example of the EGFR-signalling pathway is
used to explain why activation of membrane-bound
Ras by direct action of Sos from the cytosol is two or
three orders of magnitude less effective than EGFRmediated Sos association with Ras. It also explains
why activation of Sos by EGFR does not require any
modification of Sos; it arises from concentrating Sos
near the membrane. As the partners Ras and Sos are
to be localized for effective catalysis, the membrane
anchoring of Ras might be a regulatory process2.
Indeed, the inhibition of farnesyltransferase blocks
Ras function by preventing its posttranslational farnesylation, and has been suggested as a potential
anticancer treatment29.
An interesting variation on the topic of Ras activation by RTKs has been described for fibroblastgrowth-factor (FGF) receptors (FGFRs)30. Stimulation
of FGFRs by various FGFs induces cell proliferation,
differentiation and migration by activation of the
Grb2–Sos–Ras–MAPK signalling pathway. However,
unlike other RTKs, activated FGFR cannot bind to
Grb2 directly. Recently, a membrane-anchored protein known as FRS2 (FGFR substrate 2) has been
discovered30. Activated FGFR phosphorylates FRS2
protein. Tyrosine-phosphorylated FRS2 is able to
bind to Grb2 and, therefore, the Grb2–Sos complex.
In a familiar twist, the juxtaposition of the FRS2–
Grb2–Sos complex on the membrane might facilitate the Ras activation by Sos, providing a feasible
mechanism for linking FGFRs to the Grb2–Sos–Ras–
MAPK pathway.
Outlook
The advantages of membrane localization of signalling complexes are not limited to the specific example of Ras activation by Sos examined here.
Recruitment of signalling proteins to the membrane
occurs widely and several mechanisms are used to
regulate these events, including not only protein–
protein domain interactions but also recognition
of specific lipid species, covalent modification by
acylation or prenylation. Our analysis provides a
logical rationale to interpret these localization processes as switching devices to activate a particular
signalling pathway.
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trends in CELL BIOLOGY (Vol. 10) May 2000