USO1 (p115) protein, localizing to membranes of ER to Golgi transport vesicles, binds GOLGA2 (GM130), localizing to membranes of cis-Golgi cisternae. Binding of USO1 to GOLGA2 enables tethering of ER to Golgi transport vesicles to cis-Golgi cisternae, and is facilitated by a Ras-related GTPase RAB1. Fusion of ER to Golgi transport vesicles with cis-Golgi succeeds tethering and depends on STX5 (syntaxin-5). In mitosis, phosphorylation of GOLGA2 by cyclin B-activated CDK1 prevents USO1 docking. This results in cessation of ER to Golgi transport. Halting ER to Golgi transport increases the number of transport vesicles at the expense of Golgi cisternae, since transport vesicles keep budding from the ER but are unable to fuse with Golgi cisternae and deliver their content (Lowe et al. 1998, Seeman et al. 2000, Diao et al. 2008).