Structure and Switch Cycle of SRβ as Ancestral Eukaryotic GTPase Associated with Secretory Membranes

G proteins of the Ras-family of small GTPases trace the evolution of eukaryotes. The earliest branching involves the closely related Arf, Sar1, and SR beta GTPases associated with secretory membranes. SRb is an integral membrane component of the signal recognition particle (SRP) receptor that targets ribosome-nascent chain complexes to the ER. How SRb integrates into the regulation of SRP-dependent membrane protein biogenesis is not known. Here we show that SR beta-GTP interacts with ribosomes only in presence of SR alpha and present crystal structures of SRb in complex with the SRX domain of SRa in the GTP-bound state at 3.2 angstrom, and of GDP- and GDP.Mg2+-bound SR beta at 1.9 angstrom and 2.4 angstrom, respectively. We define the GTPase switch cycle of SR beta and identify specific differences to the Arf and Sar1 families with implications for GTPase regulation. Our data allow a better integration of SR beta into the scheme of protein targeting.