Intra-Golgi Golgin PpSgm1 and GRIP domain Golgin PpImh1 synergistically mediate Golgi cisternal stacking

Regulation of the distinctive stacked Golgi morphology remains an unresolved subject. Using the budding yeast Pichia pastoris, we have previously documented the role of GRIP domain Golgin P. pastoris (Pp)Imh1 in cisternal stacking, regulated by the Arl3-Arl1 GTPase cascade switch. Expanding our work with the present study, we demonstrate the participation of PpSgm1, another conserved Golgin, in this stacking process alongside PpImh1. Null mutation of P. pastoris SGM1 caused partial unstacking of the late cisternae from the Golgi stack, implicating its role in cisternal stacking. When we overexpressed PpImh1 or PpSgm1 independently, each failed to restore stacking in the absence of the other, suggesting neither of them is sufficient for cisternal stacking alone. On the other hand, a double knockout of PpIMH1 and PpSGM1 led to a dramatic phenotype, causing complete separation of the late cisternae from the Golgi stack and significantly increasing TGN peeling, as seen in electron microscopy and live-cell imaging. Our results suggest a synergistic collaboration of PpImh1 and PpSgm1 in cisternal stacking, with implications for a conserved mechanism across species.