Relationship between an animal lectin VIP36 and the cytoskeletal proteins in quality control mechanism

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Vesicular integral protein of 36kDa (VIP36) is an intracellular animal lectin binding protein and lipids that is glycosylated with high-mannose glycans. By using this grant we generated polyclonal and monoclonal specific antibodies against three of its protein domains using synthetic peptides, NGSLSYDHSKDGRWS (amino acids 185-199, identified as a part of CRD of rat VIP36), QKRQERNKRFY (348-358, cytosolic domain of VIP36) and NFLKSPKDNVDDPTGNFR (299-316, stalk domain). We have obtained the antibodies and clarified the following three points.(1)We have found that VIP36 is highly expressed in rat hepatocytes. Two types of VIP36 exist in the Golgi apparatus and localize in heterogeneous form in the ER and the Golgi apparatus. The heterogeneous distribution was confirmed by the different antibodies against different domains of VIP36. This strongly suggests that the heterogeneity of VIP36s may play some roles in the sorting of secretory proteins in cells.(2)We have found by immunoelectron microscopy that VIP36 may function as sorting protein in the post-Golgi secretory pathway, located in the VIP36-positive vesicular structures identified as endosomal component through the recycling trafficking between the Golgi apparatus and the plasma membrane.(3)VIP36 is highly expressed in hepatocytes and we have now developed the sandwich enzyme-linked immunosorbent assay (ELISA) system to quantify these levels. We could not fully clarify the pathways that regulate the increase in VIP36 in obese Zucker rats but our present data suggest that there is an association between the VIP36 levels and liver metabolism. We contend therefore that VIP36 may play some roles in the formation of a fatty liver, and that its measurement may be used for the future diagnosis and prognosis of metabolic disorders such as obesity.