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Sandro F. Ataide
All cells face the problem of trafficking secretory and
membrane-bound proteins during or shortly after translation on
the ribosome. The signal recognition particle (SRP) neatly solves
this problem by targeting ribosome-nascent chain complexes (RNCs)
to membrane spanning pores called translocons, by docking to a
membrane-bound SRP receptor (SR). In bacteria, this targeting
process involves recognition and binding of a hydrophobic signal
sequence on the RNC by the SRP protein Ffh, a GTPase that also
binds tightly to the 4.5S RNA. Upon GTP binding and association
with the receptor protein FtsY, itself a GTPase, the ribosome
and associated signal peptide are transferred to the translocon
from the RNC-SRP-receptor complex via an uncharacterized
mechanism. GTP hydrolysis by the SRP-receptor complex allows
its dissociation and a new protein targeting cycle to resume. In
eukaryotes, six proteins comprise the SRP (heterodimer SRP 9/14,
SRP 19, SRP 54, and heterodimer SRP 68/72), along with a larger
RNA moiety, the 7S RNA. 7S RNA includes two functionally distinct
domains: the S domain binds to the proteins SRP 19, 54, 68 and
72 and recognizes the signal peptide, whereas the Alu domain
binds proteins SRP 9 and 14 and is involved in arresting the
translating ribosome. Despite its larger size, the eukaryotic
SRP includes core components homologous to the bacterial SRP,
and fundamental mechanistic features appear to be conserved.
Structural studies using cryo-electron microscopy have
demonstrated the existence of two conformations for SRP: open
and closed. Recent studies in the Doudna lab have indicated
that upon SRP-FtsY formation the closed conformation adopted
by the bacterial SRP places the GTPase domain, signal peptide
binding site and 4.5S RNA in close proximity. These data imply
the importance of conformational rearrangements in SRP during
the protein targeting cycle. The essential 4.5S RNA plays an
important yet incompletely understood role in the cycle, and
may undergo different conformational changes during the pathway
as seen in the free 4.5S RNA structure and in complex with the
Ffh M domain. Understanding how the proteins involved in the
SRP pathway interact and the function of the conserved 4.5S RNA
remain important goals for the understanding of the bacterial
system. We are using the Escherichia coli system to
determine the conformational changes that SRP undergoes during
the protein targeting cycle and solve the crystal structure of
the SRP-FtsY complex.
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