Hematopoietic-specific Rho GTPases Rac2 and RhoH and human blood disorders

highlights
• Rho GTPases are Ras-related proteins that regulate cell cytoskeleton.
• Rac2 and RhoH GTPases are expressed only in hematopoietic cells
• Gene targeted mice have identified specific functions of Rho GTPases in hematopoiesis
• Rac2 is implicated in phagocytic function and RhoH in T cell receptor signaling
• RAC2 and RHOH mutations have recently been implicated in human immunodisorders

The small guanosine triphosphotases (GTPases) Rho proteins are members of the Ras-like superfamily. Similar to Ras, most Rho GTPases cycle between active GTP-bound, and inactive GDP-bound conformations and act as molecular switches that control multiple cellular functions. While most Rho GTPases are expressed widely, the expression of Rac2 and RhoH are restricted to hematopoietic cells. RhoH is an atypical GTPase that lacks GTPase activity and remains in the active conformation. The generation of mouse knock-out lines has led to new understanding of the functions of both of these proteins in blood cells. The phenotype of these mice also led to the identification of mutations in human RAC2 and RHOH genes and the role of these proteins in immunodeficiency diseases. This review outlines the basic biology of Rho GTPases, focusing on Rac and RhoH and summarizes human diseases associated with mutations of these genes.