Characterization of clathrin‐independent endocytosis in epithelial cells

Abstract

Major histocompatibility complex class I (MHCI) and more recently, membrane transport glycoprotein, CD98, were shown to be internalized via clathrin‐independent endocytosis (CIE) in HeLa cells. Following internalization, a fraction of vesicles containing MHCI fuse with early endosomes, defined by the presence of early endosomal autoantigen 1 (EEA1) and containing the transferrin receptor, a clathrin cargo. MHCI is then routed to late endosomes for degradation or to distinct tubular carriers that return CIE cargo back to the plasma membrane. In contrast to MHCI, CD98 bypasses the merge with early endosomes and directly enters into the tubular carriers. To study CIE in different epithelial cell types, surface antibody internalization assays were used to follow the trafficking of MHCI and CD98 in A431 and ARPE‐19 cells. Like HeLa cells, MHCI enters via CIE in vesicles that fuse with early endosomes and traffic to late endosomes for degradation. However, after endocytosis, CD98 reaches late endosomes although it bypasses the EEA1‐early endosome. Recycling of MHCI and CD98 is observed in both cell lines although tubular carriers seen in HeLa cells are absent. These observations confirm that CIE exists in different cell types but that itineraries for distinct cargo may vary.