ENDOCYTOSIS OCCURS INDEPENDENTLY OF ANNEXIN-VI IN HUMAN A431 CELLS

Annexin VI is one of a family of calcium-dependent phospholipid-bindin g proteins. Although the function of this protein is not known, variou s physiological roles have been proposed, including a role in the budd ing of clathrin-coated pits (Lin et al., 1992. Cell. 70:283-291.). In this study we have investigated a possible endocytotic role for annexi n VI in intact cells, using the human squamous carcinoma cell line A43 1, and report that these cells do not express endogenous annexin VI, a s judged by Western and Northern blotting and PCR/Southern blotting. T o examine whether endocytosis might in some way be either facilitated or inhibited by the presence of annexin VI, a series of A431 clones we re isolated in which annexin VI expression was achieved by stable tran sfection. These cells expressed annexin VI at similar levels to other human cell types. Using assays for endocytosis and recycling of the tr ansferrin receptor, we report that each of these cellular processes oc curs with identical kinetics in both transfected and wild-type A431 ce lls. In addition, purified annexin VI failed to support the scission o f coated pits in permeabilized A431 cells. We conclude that annexin VI is not an essential component of the endocytic pathway, and that in A 431 cells, annexin VI fails to exert any influence on internalization and recycling of the transferrin receptor.