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.