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Vol. 12, Issue 12, 3773-3782, December 2001
and
Departments of *Biomolecular Chemistry and
Ssbs of Saccharomyces cerevisiae are
ribosome-associated molecular chaperones, which can be cross-linked to
nascent polypeptide chains. Because Ssbs are members of a divergent
subclass of Hsp70s found thus far only in fungi, we asked if the
structural requirements for in vivo function were similar to those of
"classic" Hsp70s. An intact peptide-binding domain is essential and
an alteration of a conserved residue in the peptide-binding cleft
(V442) affects function. However, Ssb tolerates a number of alterations
in the peptide-binding cleft, revealing a high degree of flexibility in
its functional requirements. Because binding of Ssb to peptide substrates in vitro was undetectable, we assessed the importance of
substrate binding using the chimera BAB, in which the peptide binding
domain of Ssb is exchanged for the analogous domain of the more
"classical" Hsp70, Ssa. BAB, which binds peptide substrates in
vitro, can substitute for Ssb in vivo. Alteration of a residue in the
peptide-binding cleft of BAB creates a protein with a reduced affinity
for peptide and altered ribosome binding that is unable to substitute
for Ssb in vivo. These results indicate that Ssb's ability to bind
unfolded polypeptides is likely critical for its function. This binding
accounts, in part, for its stable interaction with translating
ribosomes, even although it has a low affinity for peptides that
detectably bind to other Hsp70s in vitro. These unusual properties may
allow Ssb to function efficiently as a chaperone for ribosome-bound
nascent chains.
Bacteriology, University of Wisconsin, Madison,
Wisconsin 53706
Both authors contributed equally to this work.
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