Abstract
Golgi fragmentation is a highly regulated process that allows division of the Golgi apparatus between the two daughter cells. The mitotic reorganization of the Golgi is accompanied by a temporary block in Golgi functioning, as protein transport in and out of the Golgi stops. Our group has previously demonstrated the involvement of the alternatively spliced variants, ERK1c and MEK1b, in mitotic Golgi fragmentation. We also found that ERK1c translocates to the Golgi at G2/M, but the molecular mechanism underlying this recruitment remains unknown. In this study, we narrowed the translocation timing to prophase/prometaphase and elucidated its molecular mechanism. We found that CDK1 phosphorylates Ser343 of ERK1c, thereby allowing the binding of phosphorylated ERK1c to a complex that consists of PI4KIII$\beta$ and 14-3-3$\gamma$ dimer. The stability of the complex is regulated by PKD phosphorylation of PI4KIII$\beta$. The complex assembly induces the Golgi shuttling of ERK1c, where it is activated by MEK1b, and induces Golgi fragmentation. Our work shows that protein shuttling to the Golgi is not completely abolished in G2/M, thus integrating several independent Golgi-regulating processes into one coherent pathway.
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