| Summary: | There is no biochemical evidence for the activation of Rac and Cdc42 in hemopoietic cells, nor is the mechanism of activation of these small GTPases well characterized in hemopoietic cells. We demonstrate here that lnterleukin-3 (IL-3) induced activation of endogenous Rac-1, Rac-2 and Cdc42. Rac-1 was also activated by colony-stimulating factor-1 (CSF-1), Steel locus factor (SLF), granulocyte-macrophage colony-stimulating factor (GM-CSF), lnterleukin-5 (IL- 5), or by cross-linking the B-lymphocyte receptor for antigen (BCR). The molecules found to be upstream activators of Rac-1 in hemopoietic cells were Pl-3K, p21 Ras, and M-Ras. The activation of Rac-1, Rac-2, and Cdc42 by IL-3 and other hemopoietic growth factors is likely to be an important component of their actions in promoting growth, survival and function. While studying the Rac GEF, smgGDS, we serendipitously observed a protein which was up-regulated in activated lymphocytes. We purified, from actively dividing T-lymphocytes this novel, highly conserved cytoplasmic phospho-protein, which we term Caprin-1. We found that expression of endogenous Caprin-1 correlates with the proliferative status of cells, being up-regulated in actively dividing cells and down-regulated in quiescent cells. We identified Caprin-1 and a homologous protein, Caprin-2, as members of a novel protein family characterized by two novel protein domains, termed homology regions—1 and —2 (HR-1, HR-2). We also observed that over-expression of a fusion protein of GFP and Caprin-1 induced a specific, dose-dependent suppression of the proliferation of NIH 3T3 cells, further supporting a role for Caprin-1 in cellular proliferation.
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