Involvement of VIP36 in Intracellular Transport and Secretion of Glycoproteins in Polarized Madin-Darby Canine Kidney (MDCK) Cells

Abstract

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...

Role of N-glycosylation in trafficking of apical membrane proteins in epithelia

“…VIP36 was also found to promote apical sorting of several glycoproteins. The apical membrane content of this lectin in MDCK cells was found to be twice as high as the basolateral content, and the plasma membrane glycoproteins recognized by VIP36 were also twofold enriched in the apical membrane compared with the basolateral membrane (34). Overexpression of VIP36 in MDCK cells increased the relative apical content of both VIP36 and VIP36-recognized glycoproteins.…”

“…So far, only two lectins have been identified as potential apical sorting receptors: the mannose-binding lectin VIP36 and the galactose-binding lectin galectin-3. It is remarkable that VIP36 has been found to facilitate apical distribution of two raft-associated proteins (34,35), whereas galectin-3 has been shown to be critical for apical location of three non-raft proteins but not a raft-associated protein (15). Possibly, VIP36 and galectin-3 facilitate apical distribution of glycoproteins by two different mechanisms, raft dependent and raft independent.…”

“…One potential apical sorting receptor, the mannose-binding lectin VIP36 (43,44,79), is predominantly localized in ERGIC and the cis-Golgi (36,83), and to a lesser extent in the TGN and plasma membrane (34,82). VIP36 plays a role in the export of glycoproteins from the ER (36,83) (Table 2).…”

“…Furthermore, VIP36 has been shown to bind to clusterin and ␣-amylase and to increase the relative apical abundance of these proteins (34,35). Consistent with a specificity of VIP36 for high-mannose-type glycans (43,44,79), both clusterin and ␣-amylase carry not only complextype N-glycans but also the high-mannose-type N-glycans (34,35). However, since the majority of mature glycoproteins contain predominantly complex-type N-glycans, VIP36 is unlikely a universal apical sorting receptor.…”

“…Additionally, the mechanisms by which sorting information encoded by N-glycans is recognized by cellular sorting machinery have not been defined. Two lectins have been identified as potential sorting receptors that recognize N-glycans as apical sorting signals (15,17,34,35). However, it is not clear how glycan-lectin interactions that must occur in the luminal compartments of TGN/endosomes are converted into a cascade of cytoplasmic events resulting in apical delivery of N-glycosylated proteins.…”