VIP36, an intracellular lectin that recognizes high mannose-type glycans (Hara-Kuge, S., Ohkura, T., Seko, A., and Yamashita, K. (1999) Glycobiology 9, 833-839), was shown to localize not only to the early secretory pathway but also to the plasma membrane of MadinDarby canine kidney (MDCK) cells. In the plasma membrane, VIP36 exhibited an apical-predominant distribution, the apical/basolateral ratio being ϳ2. Like VIP36, plasma membrane glycoproteins recognized by VIP36 were found in the apical and basolateral membranes in the ratio of ϳ2 to 1. In addition, secretory glycoproteins recognized by VIP36 were secreted ϳ2-fold more efficiently from the apical membrane than from the basolateral membrane. Thus, the apical/basolateral ratio of the transport of VIP36-recognized glycoproteins was correlated with that of VIP36 in MDCK cells. Upon overproduction of VIP36 in MDCK cells, the apical/basolateral ratios of both VIP36 and VIP36-recognized glycoproteins were changed from ϳ2 to ϳ4, and the secretion of VIP36-recognized glycoproteins was greatly stimulated. In contrast to the overproduction of VIP36, that of a mutant version of VIP36, which has no lectin activity, was of no effect on the distribution of glycoproteins to apical and basolateral membranes and inhibited the secretion of VIP36-recognized glycoproteins. Furthermore, the overproduction of VIP36 greatly stimulated the secretion of a major apical secretory glycoprotein of MDCK cells, clusterin, which was found to carry at least one high mannose-type glycan and to be recognized by VIP36. In contrast to the secretion of clusterin, that of a non-glycosylated apical-secretion protein, galectin-3, was not stimulated through the overproduction of VIP36. These results indicated that VIP36 was involved in the transport and sorting of glycoproteins carrying high mannose-type glycan(s).Newly synthesized secretory and membrane proteins exit from the ER 1 in transport vesicles targeted to the Golgi apparatus. Vesicular transport through the Golgi is often accompanied by post-translational modifications, such as glycosylation of cargo proteins until they have reached the trans-Golgi Network. In the trans-Golgi Network, proteins are sorted into vesicles bound for different destinations including the plasma membrane, the endosome/lysosome, and secretory granules. The protein sorting has been one of the most interesting issues in the study of vesicular protein traffic processes, but its molecular mechanisms remain largely unresolved.It has been recently demonstrated that intracellular lectins play important roles in vesicular transport: for example, mannose-6-phosphate receptor (1) as a receptor recognizing the marker for lysosomal enzymes, calnexin (2, 3) and calreticulin (4) as molecular chaperones, and ERGIC-53 (5) possibly as a transport cargo receptor. ERGIC-53 is an intermediate compartment marker (6), and it is identical to MR60, a mannosespecific membrane lectin (7) with a carbohydrate-binding domain homologous to that of lectins of leguminous plants (8). The N-term...