The p115 protein forms part of a tether that leads from GRASP65 on Golgi membranes, through GM130 and then p115, and finally to the integral membrane protein giantin on vesicles. Loss of this link in the presence of continued vesicle formation could lead to vesiculation of the Golgi, and indeed during mitosis GM130 is phosphorylated, such that it no longer binds p115. Consistent with this theory, Puthenveedu and Linstedt find that the injection of p115 antibodies results in Golgi fragmentation.
But, antibodies to giantin and GM130 (which sever the link by blocking p115 binding to either giantin or GM130) do not cause Golgi fragmentation, and do not prevent reformation of the Golgi after mitosis. This leads the authors to suggest that the tether function of p115 is nonessential; its absence may allow vesicles to wander further from the Golgi but leave them as fusion-competent as they were before.
In contrast, a p115 function that is independent of giantin and GM130 is essential. This essential function may involve loading of p115 at the ER by interaction with COPII coats (used for ER to Golgi transport). This p115 could then serve as a target for the docking of Golgi-derived vesicles, which might effect the maturation of an intermediate compartment into a cis-Golgi compartment. As for mitosis, inhibition of this essential p115 function, and perhaps other vesicle events, may cause mitotic vesiculation. ▪