VIP36 localisation to the early secretory pathway

VIP36, an integral membrane protein previously isolated from epithelial MDC K cells, is an intracellular lectin of the secretory pathway. Overexpressed VIP36 had been localised to the Golgi complex, plasma membrane and endocyt ic structures suggesting post-Golgi trafficking of this molecule (Fiedler e t al,, 1994), Here we provide evidence that endogenous VIP36 is localised t o the Golgi apparatus and the early secretory pathway of MDCK and Vero cell s and propose that retention is easily saturated. High resolution confocal microscopy shows partial overlap of VIP36 with Golgi marker proteins. Punct ate cytoplasmic structures colocalise with coatomer and ERGIC-53, labeling ER-Golgi intermediate membrane structures. Cycling of VIP36 is suggested by colocalisation with anterograde cargo trapped in pre-Golgi structures and modification of its N-linked carbohydrate by glycosylation enzymes of media l Golgi cisternae, Furthermore, after brefeldin A treatment VIP36 is segreg ated from resident Golgi proteins and codistributes with ER-Golgi recycling proteins.