Molecular mechanism of "transportsomes" involved in Ca^<2+-> activated Cl^- transport

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It has been recognized that various physiological function, in addition to ion permeation, can be regulated by anion channels (CFTR, CLC etc.) expressed on cell membrane. Among them, CLCA proteins have been shown to be multifunctional, with roles in, for example, secretion, cell adhesion, differentiation, and tumor suppressing action. We hypothesized that rCLCA make distinct molecular complexes (transportsome) with adaptor or scaffold proteins to exert such multifunctions. We found that full-length of a rat CLCA homologue (rCLCAl-f) is responsible for modulating Ca^<2+>-dependent Cl^- transport. More recently, we presented in vivo evidence of a physiological role for rCLCA1-f in transepithelial Cl^- transport in the ductal system of rat submandibular gland (SMG). This isoform was expressed in early and recycling endosomes labeled by their specific markers. We then described a distinct expression and physiological function of a splicing isoform of rat rCLCA (rCLCAl-t). rCLCAl-t weakened cell attachment to a greater extent than rCLCAl-f did, and was found to associate with RACK1 (receptor for activated C kinase) which is reported to interact with β_1-integrin and to promote cell adhesion. Immunohistochemistry revealed rCLCA1-t to be located in the perinuclear region of the basal, undifferentiated cells of the rat SMG excretory duct. These suggest that cell-specific splicing of rCLCA mRNA may contribute to the differences in physiological function probably due to formation of distinct types of transportsome among epithelial cells.