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J. Cell Biol.,
Volume 142, Number 2, July 27, 1998 341-354
-subunit
Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California School of Medicine,
San Francisco, California 94143-0448
The signal recognition particle and its receptor (SR) target nascent secretory proteins to the ER.
SR is a heterodimeric ER membrane protein whose
subunits, SR
and SR
, are both members of the GTPase superfamily. Here we characterize a 27-kD protein in Saccharomyces cerevisiae (encoded by SRP102) as a
homologue of mammalian SR. This notion is supported (a) by Srp102p's sequence similarity to SR; (b)
by its disposition as an ER membrane protein; (c) by its
interaction with Srp101p, the yeast SR homologue; and (d) by its role in SRP-dependent protein targeting
in vivo. The GTP-binding site in Srp102p is surprisingly
insensitive to single amino acid substitutions that inactivate other GTPases. Multiple mutations in the GTP-binding site, however, inactivate Srp102p. Loss of activity parallels a loss of affinity between Srp102p and
Srp101p, indicating that the interaction between SR subunits is important for function. Deleting the transmembrane domain of Srp102p, the only known membrane anchor in SR, renders SR soluble in the cytosol,
which unexpectedly does not significantly impair SR
function. This result suggests that SR functions as a regulatory switch that needs to associate with the ER
membrane only transiently through interactions with
other components.
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