Mechanism of glycoprotein sorting and transport in the cells and its application for drug design

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FLAG-tagged ERGIC-53 was expressed with HA-tagged VIPL or VIP36 in 293T cells and pull down assay was performed from the cell lysates. HA-VIPL and HA-VIP36 were precipitated with ERGIC-53 indicating that these proteins interact with ERGIC-53. ERGIC-53 also interacted with MCFD2 in the presence of calcium, but neither VIP36 nor VIPL did. Surface plasmon resonance (SPR) experiment demonstrated that the interaction between ERGIC-53 and MCFD2 was susceptible to calcium ions, especially the concentration between 0.1 and 0.2 mM. MCFD2 mutants derived from F5F8D patients had three or four order of magnitude lower Ka values for ERGIC-53. Ultracentrifugation analysis and SPR experiment of soluble VIP36 indicating that sugar-binding activity of VIP36 was regulated by oligomer formation dependent on pH, which explains enhancement of avidity to sugar chains. Further FLET analysis using YGFP- and EGFP-fused VIP36 showed their interactions in the cells. In contrast, sugar-binding ability of ERGIC-53 … More was enhanced by MCFD2 interactions. The interaction of ERGIC-53 with MCFD2 depends on calcium concentration. By pull-down assay with VIP36, novel protein constitutively interacted with VIP36 was identified as a chaperone BiP. The interaction had different characteristics from those of chaperone substrates. Immunoelectron microscopic analysis showed that the binding of VIP36 to BiP was occurred especially in the endoplasmic reticulum (ER). Interestingly, VIP36 precipitated with BiP had sugar chains resistant to endo H suggesting that VIP36 transported retrogradely could bind to BiP in the ER. The binding of PE-labeled soluble ERGIC-53, VIP36, and VIPL tetramers to HeLaS3 cells showed their preferable binding to high mannose-type sugar chains. Inhibition analysis by a panel of high mannose-type sugar derivatives demonstrated the precise specificities of them. VIP36 and VIPL recognize Man α 2Man α 2Man arm and glucosylation of its non-reducing terminal abrogated their binding. ERGIC-53 preferencially bound to M8b isoform, which is a product of ER α-mannosidase I, indicating that newly synthesized glycoproteins may transferred from calnexin to ERGIC-53 via VIPL. VIP36 fused with CD8 transmembrane domain followed by intracellular domain of CD3ζ was expressed in HeLaS3 cells. The cell surface display system enabled us to pick up clones of mutated VIP36 having distinct sugar-binding specificities from random mutated VIP36 libraries. Less