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1 Institute of Biological Chemistry, Academia Sinica, Taipei, Taiwan
2 College of Life Science, Institute of Biochemical Science, National Taiwan University, Taipei, Taiwan
(Requests for offprints should be addressed to S-T Chu who is now at Institute of Biological Chemistry, Academia Sinica, PO Box 23-106, Taipei, Taiwan; Email: stc316{at}gate.sinica.edu.tw.)
It has been shown that mouse 24p3 protein is a dexamethasone-inducible secretory protein which suggests the involvement of an autocrine mechanism in the L929 cell line. The aim of this study was to investigate the possibilityof this mechanism in L929 cells and to also demonstrate further processing of this protein after association with L929 cells. We have characterized the internalization of 24p3 protein in L929 cells with fluorescein isothiocyanate- and Alexa555-labeled protein supplement instead of secreted 24p3 protein. As endocytotic inhibitors could reveal the inhibition of protein internalization, we confirmed the existence of receptor-mediated 24p3 protein internalization in L929 cells by carrying out an inhibition test. Plus-chase experiment of L929 cells with biotinylated 24p3 protein demonstrated a release of internalized 24p3 protein into the extracellular region. The protein recycling inhibitor, bafilomycin A1, arrests the transport of 24p3 protein by accumulating in early endosome, but no effect could be observed in protein release from early to late endosome by nocodazole. Theseresults providethe first evidence on recycling of apo-24p3 protein in L929 cells.
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