In polarized epithelial MDCK cells, all known endogenous endocytic receptors are found on the basolateral domain. The influenza virus hemagglutinin (HA) which is normally sorted to the apical plasma membrane, can be converted to a basolateral protein by specific mutations in its short cytoplasmic domain that also create internalization signals. For some of these mutations, sorting to the basolateral surface is incomplete, allowing internalization of two proteins that differ by a single amino acid of the internalization signal to be compared at both the apical and basolateral surfaces of MDCK cells. The rates of internalization of HA-Y543 and HA-Y543,R546 from the basolateral surface of polarized MDCK cells resembled those in nonpolarized cells, whereas their rates of internalization from the apical cell surface were fivefold slower. However, HA-Y543,R546 was internalized approximately threefold faster than HA-Y543 at both membrane domains, indicating that apical endocytic pits in polarized MDCK cells retained the ability to discriminate between different internalization signals. Slower internalization from the apical surface could not be explained by a limiting number of coated pits: apical membrane contained 0.7 as many coated pits per cell cross-section as did basolateral membranes. 10-14% of HA-Y543 at the apical surface of polarized MDCK cells was found in coated pits, a percentage not significantly different from that observed in apical coated pits of nonpolarized MDCK cells, where internalization was fivefold faster. Thus, there was no lack of binding sites for HA-Y543 in apical coated pits of polarized cells. However, at the apical surface many more shallow pits, and fewer deep, mature pits, were observed than were seen at the basolateral. These results suggest that the slower internalization at the apical surface is due to slower maturation of coated pits, and not to a difference in recognition of internalization signals.

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