The yeast orthologue of GRASP65 forms a complex with a coiled-coil protein that contributes to ER

Formation of the fi rst compartment of the Golgi apparatus involves recognition events between several different membranes. These include tethering of both anterograde COPII vesicles and retrograde COPI vesicles to the cis-Golgi. In addition, intermediate compartment structures that formed in the cell periphery fuse with the cis-Golgi membranes after movement to the cell center, and Golgi stacks undergo homotypic fusion to form the elongated ribbon characteristic of vertebrate cells. Finally, the cis-Golgi is attached to an adjacent medial compartment in the stack. Several proteins have been proposed to guide these membrane recognition events, including the coiled-coil proteins GM130 and p115, the multisubunit complexes transport protein particle (TRAPP) and conserved oligomeric Golgi (COG), and the PDZ-like protein GRASP65 (Whyte and Munro, 2002; Barr and Short, 2003). This latter protein was identifi ed using an in vitro assay for the postmitotic reassembly of mammalian Golgi stacks (Barr et al., 1997). It consists of two PDZ-like domains fl anked by a C-terminal Ser/ Pro-rich domain and an N-terminal myristoylation site, which is required for its association with the cis-Golgi (Barr et al., 1998; Kondylis et al., 2005). GRASP65 recruits the coiled-coil protein GM130 to the cis-Golgi by binding to the latter’s C terminus (Barr et al., 1997). Golgi association of the two proteins appears mutually interdependent, as mutation of the interfacial residues in either protein causes a loss of localization (Barr et al., 1998). GRASP55, a GRASP65 paralogue unique to vertebrates, is found on the medial Golgi but may also contribute to GM130 recruitment (Shorter et al., 1999; Vasile et al., 2003). GM130 also interacts with the coiled-coil protein p115 and the small GTPase Rab1, and it has been proposed that the GRASP65– GM130 complex acts in a variety of tethering interactions at the cis-Golgi (Nakamura et al., 1997; Weide et al., 2001; Kondylis et al., 2005; Puthenveedu et al., 2006). Despite these compelling physical interactions, analysis of the importance of GRASP65 in vivo has not yet reached a clear consensus. Removal of the protein by RNAi has been reported to affect the formation of Golgi ribbons, the number of cisternae, the structure of the cisternae themselves, and even the formation of the mitotic spindle (Kondylis et al., 2005; Sutterlin et al., 2005; Puthenveedu et al., 2006). However, in all cases, transport through the Golgi appeared relatively normal, and a similar result has been reported for the loss of GM130 (Vasile et al., 2003). These rather variable and perhaps surprisingly mild phenotypes may refl ect a degree of redundancy in membrane traffi c steps at the cis-Golgi. Not only are there other large The yeast orthologue of GRASP65 forms a complex with a coiled-coil protein that contributes to ER to Golgi traffi c