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© The Rockefeller University Press, 0021-9525/1998//341 $5.00
The Journal of Cell Biology, Volume 142, Number 2, , 1998 341-354

Articles

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



Stephen C. Ogg, Wolfgang P. Barz, and Peter Walter

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

Key Words: protein targeting • signal recognition particle • Saccharomyces cerevisiae • endoplasmic reticulum • GTPase

Abbreviations used in this paper: CPY, carboxypeptidase Y; DPAP-B, dipeptidyl aminopeptidase B; FB, flotation buffer; SR, SRP receptor; SR{alpha}, SRP receptor {alpha}-subunit; SRβ, SRP receptor β-subunit; SRP, signal recognition particle; TMD, transmembrane domain.

The current addresses of Stephen C. Ogg is Department of Biochemistry, University of Dundee, Dundee DD1 4HN, Scotland. The current address of Wolfgang P. Barz is Department of Membrane Biochemistry, Max-Planck-Institute for Biochemistry, D-82152 Martinsried, Germany.

Address all correspondence to Peter Walter, Howard Hughes Medical Institute and Department of Biochemistry and Biophysics, University of California School of Medicine, San Francisco, CA 94143-0448. Tel.: 415-476-5017; Fax: 415-476-5233; E-mail: walter{at}cgl.ucsf.edu


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