The mitotic phosphorylation cycle of the cis-Golgi matrix protein GM130

The cis-Golgi matrix protein GM130 is phosphorylated in mitosis on serine 2 5, Phosphorylation inhibits binding to p115, a vesicle-tethering protein, a nd has been implicated as an important step in the mitotic Golgi fragmentat ion process. We have generated an antibody that specifically recognizes GM1 30 phosphorylated on serine 25, and used this antibody to study the tempora l regulation of phosphorylation in vivo. GM130 is phosphorylated in prophas e as the Golgi complex starts to break down, and remains phosphorylated dur ing further breakdown and partitioning of the Golgi fragments in metaphase and anaphase. In telophase, GM130 is dephosphorylated as the Golgi fragment s start to reassemble. The timing of phosphorylation and dephosphorylation correlates with the dissociation and reassociation of p115 with Golgi membr anes. GM130 phosphorylation and p115 dissociation appear specific to mitosi s, since they are not induced by several drugs that trigger nonmitotic Golg i fragmentation. The phosphatase responsible for dephosphorylation of mitot ic GM130 was identified as PP2A. The active species was identified as heter otrimeric phosphatase containing the B alpha regulatory subunit, suggesting a role for this isoform in the reassembly of mitotic Golgi membranes at th e end of mitosis.