280s Biochemical Society Transactions ( 1 996) 24
Low probability of dystrophin and utrophin coiled coil
regions forming dimers
Steven J. WINDER, Toby J. GIBSON. and John KENDRICKJONES. MRC Laboratory of Molecular Biology, Hills Road
Cambridge, CB2 2QH, UK. *EMBL. Postfach 10.2209,
Meyerhofstrasse 1, D69012, Heidelberg, Germany.
Dystrophin and utrophin form flexible links between the
actin cytoskeletonand the cell membrane. Utrophin is found
in all cell types whilst dystrophin expression is restricted to
muscle and neuronal tissues only. Mutations in the
dystrophin gene cause a break in this protein link leading to
membrane damage and cell death as typified by the x-linked
myopathies, Ouchenne and Becker muscular dystrophies.
The N-terminal regions of dystrophin and utrophin are
anchored to the actin cytoskeleton while their C termini are
linked to a group of integral transmembrane proteins. The
central repeating coiled coil region is believed to form a
flexible shock-absorber function separating the N and C
termini and effectively protecting the cell membrane from
being damaged by the underlying contractile machinery.
The high degree of sequence and predicted structural and
functional similarity between dystrophin and utrophin and
other members of the spectrin family of proteins has lead to
the assumption that dystrophin and utrophin associate with
themselves in a manner similar to that seen with a- and pspectrin and a-actinin, namely, as antiparallel dimers.
Genetic Data Environment (GDE) analysis of the coiled coil
repeat regions of dystrophin and utrophin in comparison with
the spectrins [ l ] has allowed us to compare the structural
arrangement of the coiled coil structures in these proteins.
Fourier analysis of dystrophin repeats [2] revealed a 'one
long, one short' helix arrangement rather than the triple
helical bundle originally proposed by Speicher and Marchesi
[3], with the three helix repeat being formed by the nested
I-----------helix
A--------l
spco
...LQ.F..DA.DL..WI.EK..LL.S.D.G.
spcb
...L.FFF.DA.DL..WI.E...LL.S.D.G.
dys
utr
...L . . L . . . L . . L . . W L . . L E . . L . . . L . . .
...L..L...L..L..WL..LE..L...L...
I--------------helix
&-----------I
spcb
..V . . L L K K A . . L . . D L . . R . . R L . . L . . . A . . L L . . G H
DL ..V..LLKKH..F..EL.....RL..L...A..LL..GH
dys
D...L...L..LK.L..DL..K...L..L...A..LL....
utr
D...L...L..LK.L..EL..H...L..L......LL....
spco
DL
I-----------helix
spca
spcb
dys
utr
C-----------
I
..... I . .RL.. L . .RW.. LKELA. .RR.KL..
.....I..RL..L..LW..L.ELA..R...L..
.....L L ...L..LN.RW..L..RL.ER...L.
.....LL ...L . . L N . R W . . L . . . L . D R . . . L .
Figure 1. GDE derived consensuses from 111 for; a-spectrin,
(spca); p-spectrin, (spcb); dystrophin, (dys) and utrophin,
(utr) coiled coil repeats. Only residues conserved by
property in >55% of repeats are shown. The C helix in one
repeat is continuouswith the A helix in the following repeat.
Abbreviations used; GDE, Genetic Data Environment.
AAAAAAAAAAAAAAAAAAAA
..
dy*mphh
I
I
I I
I
I
I
I
1
I
1
I
1 . .
I
I
I
A
I
I
I
I .
1
I
I
I
1
I
1
I .
1
I
I
I
I
1 1
I
I
I
I
. I 1
A
Figure 2. Hypothetical arrangement of a-spectrin (top) and
dystrophin (bottom) as antiparallel dimers, with potential
dimerisation interfaces shaded, each box and accompanying
shaded area represents one coiled coil repeat. Arrowheads
represent interfaces completely in register
overlap of the end of one long helix with the start of the next
in the following repeat. GOE analysis confirmed these earlier
findings but also highlighted other differences between the
repeats of the spectrins and dystrophin and utrophin. It can
be seen from the derived consensus (shown in Fig. 1.) that
there are considerably more conserved residues in the
spectrins than in dystrophin and utrophin. For the most part
the residues that are conserved between the spectrins and
dystrophin and utrophin are the hydrophobic residues in the
a and dpositions of the heptad, involved in the packing of the
coiled coil. The extra conserved residues, predominantly
charged and hydrophobic, are mainly in helix A and the first
part of helix B, and are involved in the association of one
helical bundle with another, i.8. the dimerisation interface.
This interface is in 7 out of 24 and 5 out of 22 repeats in
dystrophin and utrophin respectivelycompared with 20 out of
20 in a-spectrin. Furthermore, not evident in the
consensuses in Fig. 1 is the variability in repeat length and a
number of other insertions within the repeats themselves in
dystrophin and utrophin, which further add to the irregularity
of the repeating structures. This irregularity is highlighted in
Fig. 2 where dystrophin, with a limited dimerisation interface
(shown shaded) is presented aligned with an apposing
dystrophin molecule as a dimer. It is clear from this
representation, by comparison with a-spectrin, that the
limited dimerisation interface, the irregularity in repeat length
and insertions in dystrophin (and utrophin) repeats, means
that dystrophin and utrophin are so out of alignment that
there is insufficient overlap of these regions to form an
interface for dimerisation to occur. Therefore, the lack of a
conserved dimerisation interface and the variability in repeat
length strongly suggest that dystrophin and utrophin are
unable to form dimers over their whole lengths.
The degree of conservation between repeats and the
phasing in Fig. 2 could possibly allow for a small amount of
dimerisation, possibly between repeats 2-5 in each of two
monomers, but any more is unlikely. The widely held belief
that dystrophin (and utrophin) form dimers is therefore
unlikely to be correct.
Acknowledgement. We are grateful to the Muscular
Dystrophy Group of Great Britain and Northern Ireland for
financial support.
References.
1. Winder, S.J., Gibson, T. J. & Kendrick-Jones, J. (1995)
FEBS Lett. 369,27-33
2. Cross, R.A., Stewart, M. & Kendrick-Jones, J. (1990)
FEBS Lett. 262.87-92.
3. Speicher, D.'W. & Marchesi, V.T. (1984) Nature 311, 177180.