Rho is a member of the Ras-related family of small molecular weight GTP-binding 
proteins, and works as a molecular switch by shuttling between the GDP-bound 
inactive form and the GTP-bound active form. Cellular functions of Rho have been 
studied by two ways; one is to express or microinject constitutively active Rho 
mutants in cells to identify the active phenotype of Rho, and the other is to 
use botulinum C3 exoenzyme that specifically ADP-ribosylates and inactivates Rho 
in cells to find out phenotypes of Rho inactivation. These analyses have 
revealed that Rho is involved in cell to substratum adhesion and motility, cell 
contraction and cytokinesis through the reorganization of the actincytoskeleton 
and modulation of its activity. These actions of Rho are mediated by downstream 
Rho effectors. Several putative Rho effectors have been isolated on the basis of 
their selective binding to the GTP-bound form of Rho. Among them, the ROCK 
family of Rho-associated serine/threonine protein kinases inactivates cofilin 
and myosin phosphatase to induce stabilization of filamentous actin and increase 
in the actomyosin-based contractility. mDia binds profilin likely to promote 
actin polymerization. Thus, these effectors in combination are supposed to work 
in organization of various forms of the actin cytoskeleton. Furthermore, 
analyses using a ROCK specific inhibitor Y-27632 have suggested that the 
Rho-ROCK pathway works in contractions of vascular and bronchial smooth muscles 
under various pathological conditions and is involved in malignant cell 
transformation and tumor invasion and metastasis.