The
oculocerebrorenal syndrome of Lowe (OCRL) is a rare X-linked disorder characterized by severe
mental retardation, congenital
cataracts and
renal Fanconi syndrome. OCRL1
protein is a
phosphatidylinositol 4,5-bisphosphate 5-phosphatase with a C-terminal
RhoGAP domain. Considering the pleiotropic cellular functions of
Rho GTPases (Rho, Rac and Cdc42) and their dysregulation in several forms of
mental retardation, we have investigated the so far unexplored function of the
RhoGAP domain of OCRL1. Activated Rac
GTPase was found to stably associate with the OCRL1
RhoGAP domain in vitro and to co-immunoprecipitate with endogenous OCRL1. Contrasting with other GAPs, OCRL1
RhoGAP exhibited a significant interaction with
GDP bound Rac in vitro. As compared to Rac, other
Rho GTPases tested showed reduced (Cdc42) or no binding (RhoA, RhoG) to OCRL1
RhoGAP. Immunofluorescence studies in HEK and COS7 cells and Golgi perturbation assays with
Brefeldin A demonstrated that a fraction of endogenous Rac co-localizes with OCRL1 and
gamma-adaptin in the trans-Golgi network. The OCRL1
RhoGAP domain showed low Rac GAP activity in vitro, and when expressed in Swiss 3T3 cells induced specific inhibition of RacGTP dependent ruffles, consistent with OCRL1 being an active RacGAP. OCRL1 appears to be a bifunctional
protein which, in addition to its PIP2 5-phosphatase activity, binds to Rac
GTPase. This novel property may play a role in localizing OCRL1 to the trans-Golgi network. Moreover, loss of OCRL1
RhoGAP and the resulting alteration in Rho pathways may contribute to
mental retardation in
Lowe syndrome, as illustrated in other forms of
X-linked mental retardation.