DISTINCT INTRACELLULAR-LOCALIZATION OF LCK AND FYN PROTEIN-TYROSINE KINASES IN HUMAN T-LYMPHOCYTES

Two src family kinases, lck and fyn, participate in the activation of T lymphocytes. Both of these protein tyrosine kinases are thought to f unction via their interaction with cell surface receptors. Thus, lck i s associated with CD4, CD8, and Thy-1, whereas fyn is associated with the T cell antigen receptor and Thy-1. In this study, the intracellula r localization of these two protein tyrosine kinases in T cells was an alyzed by immunofluorescence and confocal microscopy. Lck was present at the plasma membrane, consistent with its proposed role in transmemb rane signalling, and was also associated with pericentrosomal vesicles which co-localized with the cation-independent mannose 6-phosphate re ceptor. Surprisingly, fyn was not detected at the plasma membrane in e ither Jurkat T cells or T lymphoblasts but was closely associated with the centrosome and to microtubule bundles radiating from the centroso me. In mitotic cells, fyn co-localized with the mitotic spindle and po les. The essentially non-overlapping intracellular distributions of lc k and fyn suggest that these kinases may be accessible to distinct reg ulatory proteins and substrates and, therefore, may regulate different aspects of T cell activation. Anti-phosphotyrosine antibody staining at the plasma membrane increases dramatically after CD3 cross-linking of Jurkat T cells. The localization of lck to the plasma membrane sugg ests that it may participate in mediating this increase in tyrosine ph osphorylation, rather than fyn. Furthermore, the distribution of fyn i n mitotic cells raises the possibility that it functions at the M phas e of the cell cycle.