IDENTIFICATION OF A NOVEL CA2-REGULATED PROTEIN THAT IS ASSOCIATED WITH THE MARGINAL BAND AND CENTROSOMES OF CHICKEN ERYTHROCYTES()

We have identified a novel Ca2+-regulated protein, p23, that is expres sed specifically in avian erythrocyte and thrombocyte lineages. Sequen ce analysis of this 23 kDa protein reveals that it bears no homology t o any known sequence, In mature definitive erythrocytes p23 exists in equilibrium between a soluble and a cytoskeletal bound pool. The cytos keletal fraction is associated with the marginal band of microtubules, centrosomes and nuclear membrane under conditions of low free [Ca2+]. An increase in free [Ca2+] to 10(-6) M is sufficient to induce dissoc iation of >95% of bound p23 from its target cytoskeletal binding sites , yet this [Ca2+] has little effect on calmodulin-mediated MB depolyme rization. Analysis of p23 expression and localization during erythropo iesis together with results from heterologous p23 expression in tissue cultured cells demonstrates that this protein does not behave as a bo ne fide microtubule-associated protein. In addition, the developmental analysis revealed that although p23 is expressed early in definitive erythropoeisis, its association with the MB, centrosome and nuclear me mbrane occurs only in the final stages of differentiation. This cytosk eletal association correlates with marked p23 stabilization and accumu lation at a time p23 expression is being markedly downregulated. We hy pothesize that the mechanism of p23 association to the MB and centroso mes may be induced in part by a decrease in intracellular [Ca2+] durin g the terminal stages of definitive erythropoiesis.