Transport molecule forms a protective
structure to guide proteins to cell membrane
5 March 2015
groove that accommodates the transmembrane
region of the protein that's being targeted."
Membrane proteins are involved in a number of
essential cellular functions such as signaling,
catalyzing chemical reactions, and nutrient and ion
transport. These proteins contain hydrophobic
regions that allow them to be embedded in the
hydrophobic lipid bilayer of cell membranes. During
protein synthesis, which takes place inside the
aqueous environment of a cell, emerging
hydrophobic regions are predisposed toward
clumping together in order to avoid water. As
aggregation can be harmful, protective
mechanisms immediately shield these regions as
they emerge and then chaperone proteins to the
membrane.
A tail-anchored protein (magenta) is bound to a large
protective grove in a Get3 dimer. Green areas represent Unlike the majority of membrane proteins, tailanchored proteins contain only one hydrophobic
hydrophobic regions on Get3. Credit: Robert Keenan,
region that is usually the last to be synthesized.
University of Chicago
The molecular complex that guides an important
class of proteins to correct locations in cell
membranes does so by forming a dimeric structure
with a protective pocket, report scientists from the
University of Chicago in Science on Mar. 5. This
structure shields tail-anchored membrane proteins
- which have roles in a wide variety of cellular
functions from neurotransmitter release to insulin
production - from harmful aggregation or misfolding
as they move through the inner environment of a
cell. The findings clarify the mechanism behind a
fundamental biological process.
Because of this, the coordination of several
molecular factors - together comprising the GET
('guided entry of tail-anchored proteins') pathway are required to prevent tail-anchored proteins from
aggregating.
The key component of this pathway is a targeting
factor known as Get3, which captures and shields
the hydrophobic region of tail-anchored proteins.
The mechanism by which Get3 performs its
protective function has been debated, leaving the
study of the entire pathway in flux. Previous studies
have argued that four molecules of Get3 combine
to form a tetrameric complex that encase its cargo.
But other studies, including ones from Keenan and
co-author Ramanujan Hedge, PhD, of the Medical
Research Council in Cambridge, have pointed to a
"The cell is able to shield tail-anchored proteins
dimeric complex involving only two subunits of
and get them to the right membrane at the right
time through this two-subunit complex," said study Get3.
co-senior author Robert Keenan, PhD, associate
professor of biochemistry and molecular biophysics To resolve this question, the team, led by
at the University of Chicago. "The simple analogy Agnieszka Mateja, PhD, postdoctoral fellow at the
University of Chicago, first assembled
is that it's like a hot dog bun. It presents a large
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Get3-substrate complexes in the laboratory. Using factor in complex with its membrane protein cargo,
purified components to mimic physiological
Science ,
conditions, they found dimeric structures. Get3 has www.sciencemag.org/lookup/doi/10.1126/science.1
been shown to form dimers when linked to ATP, but 261671
Get3 also naturally degrades ATP over time, which
likely led to previous observations of a tetrameric
Get3-substrate complexes under non-physiological
conditions. When the researchers expressed a tail- Provided by University of Chicago Medical Center
anchored protein in bacteria with a modified version
of Get3 that was unable to degrade ATP, only
dimeric complexes were formed.
To visualize the physiologically-relevant structure,
the researchers used protein crystallization to study
Get3 while it was holding a hydrophobic tailanchored protein. In collaboration with Tony
Kossiakoff, PhD, professor of biochemistry and
molecular biophysics at the University of Chicago,
the team designed synthetic antibody fragments
that bound to specific parts of the Get3 dimer.
These antibodies facilitated packing of Get3 and
the substrate into the crystal. Analyzing this
complex atom by atom, the team found that Get3
functioned exactly as they had predicted - with two
Get3 subunits protecting the tail-anchored protein
in a large hydrophobic groove.
"There was a lot of painstaking engineering that
went into this, but ultimately we defined the
structure of the physiologic complex," Keenan said.
"It's a very important biological pathway, a hard
technical problem, and now only the second
instance where we have a glimpse of how a
hydrophobic transmembrane protein binds to one of
its targeting factors. This is a fundamental insight
into how a cell works."
With the structure solved and the Get3 protective
mechanism clarified, Keenan and his team are now
investigating the mechanism of other steps in the
GET pathway.
"The mechanistic details of the GET pathway are
different if you assume a dimer model or a tetramer
model," Keenan said. "By demonstrating that the
physiologically relevant targeting complex is
dimeric, we get a relatively simple, elegant working
model. It really clarifies thinking in this field."
More information: Structure of the Get3 targeting
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APA citation: Transport molecule forms a protective structure to guide proteins to cell membrane (2015,
March 5) retrieved 18 June 2017 from https://phys.org/news/2015-03-molecule-proteins-cellmembrane.html
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