Active Rap1, a small GTPase that induces malignant transformation of hematopoietic progenitors, localizes in the nucleus and regulates protein expression

Rap1, a member of the Ras superfamily, regulates cytoskeletal changes in lower eukaryots and integrin-mediated adhesion in hematopoietic cells. Sustained activation of Rap1 in mouse hematopoietic stem cells causes expansion of hematopoietic progenitors, followed by a myeloproliferative disorder mimicking chronic myeloid leukemia. Moreover, these mice develop a B-cell lymphoproliferative disorder resembling chronic lymphocytic leukemia. Here, we used HEK 293 cells as a tool to examine the molecular effects of Rap1. We observed that a constitutively active Rap1 mutant localized predominantly in the nucleus. Nuclear localization of endogenous Rap1-GTP was also detected upon physiologic activation. A potential consequence of nuclear localization of Rap1-GTP is the regulation of gene expression. We used a high throughput proteomic approach to identify gene products potentially modulated by Rap1-GTP. Out of 1000 proteins examined, 64 proteins were upregulated and 66 proteins were downregulated. The differentially expressed gene products belong to cytoskeletal regulator proteins, signaling molecules, transcription factors, viability regulators, and protein transporters. This analysis provides the first fingerprint of gene product expression regulated by Rap1 and may contribute to our understanding of malignant transformation mechanisms regulated by this small GTPase.