A Functional GTPase Domain , but not its Transmembrane Domain , is Required for Function of the SRP Receptor b - subunit

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 a and SR b , 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 b . This notion is supported ( a ) by Srp102p’s sequence similarity to SR b ; ( b) by its disposition as an ER membrane protein; ( c ) by its interaction with Srp101p, the yeast SR a 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 GTPbinding 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