Function analysis of VIP36 related to protein sorting in intracellular transport

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The vesicular integral protein of 36 kDa (VIP36) is an intracellular lectin that recognizes high-mannose type glycan containing α1→2 mannose residue. We isolated the overexpressed clones of VIP36 (+VIP) and the mutant VIP36 (Asp→Asn) (mVIP), which did not show binding activities to high-mannose type glycans by in vitro assay, from MDCK cells. In either clone, VIP36 or mVIP36 was overexpressed in the apical membranes about five times more than in the basolateral ones. In the +VIP clone, apical glycoproteins contained high-mannose sugar chains about four to five times more than basolateral glycoproteins corresponding to the relative same ratios as the VIP36 localization. On the other hand, the glycoproteins containing high-mannose type sugar chains in the mVIP clone were transported to the apical domain two times more than to the basolateral domain as wild type, unrelated to the ratios of overexpressed mVIP36. These results indicated that VIP36 was involved in the intracellular transport of glycoproteins containing high-mannose type sugar chains to the final destinations.