Adjacent cisternae of the Golgi apparatus are stacked and linked by tubules to from a Golgi ribbon (Nakamura et al. 2012). GORASP1 (GRASP65), a protein localizing to membranes of cis-Golgi cisternae, enables stacking by in trans dimerization/oligomerization through its PDZ domains (Tang et al. 2010). In mitosis, GORASP1 is phosphorylated by CDK1 and PLK1 (Preisinger et al. 2005). PLK1-mediated phosphorylation of GORASP1 prevents stacking of Golgi cisternae and contributes to unlinking and fragmentation of the Golgi apparatus, probably by interfering with GORASP1 oligomerization (Wang et al. 2003, Sengupta and Linstedt 2010). Similarly, GORASP2 (GRASP55), localized to median Golgi cisternae, promotes stacking by trans-oligomerization. Trans-oligomerization of GORASP2 is prevented by mitotic phosphorylation of GORASP2 downstream of MEK/ERK cascade, and contributes to the Golgi fragmentation in prophase (Xiang and Wang 2010).