RESUMO
CD45 is an abundant cell-surface protein tyrosine phosphatase (PTPase) of hematopoietic cells that mediates specific tyrosine dephosphorylation in lymphocyte Ag activation. The hypothesis that CD45 PTPase activity varies during progression through the cell cycle was examined in this study. Expression of CD45 PTPase activity was determined in pre-B lymphoma (70Z/3.12) and cytolytic T lymphocyte (CTLL-2) cell lines after fractionation by counterflow centrifugal elutriation into cell cycle-stage-enriched subpopulations. For both cell lines, CD45 PTPase activity was elevated 2- to 10-fold in late G2 + M fractions compared with the PTPase activity of asynchronous cells. FACS and SDS-PAGE analyses indicated that there was only a minor increase in CD45 protein expression during progression through the cell cycle, suggesting that increased CD45 PTPase activity was a result of increased enzymatic activity. Cyclin B expression in 70Z/3.12 cells was evaluated in elutriated subpopulations and intact cyclin B was present in all cell cycle phases from G1 to late G2 + M, disappearing in mitosis and just before those fractions containing maximum CD45 activity. This observation indicated that the peak PTPase activity of CD45 occurred in late mitosis or in cytokinesis. The elevation of CD45 PTPase activity in mitosis supports the hypothesis that CD45 has a role in phosphorylation events occurring during cell division.
