rho3

Rho3 is a small GTPase of the Rho family found in budding yeast (Saccharomyces cerevisiae) and related fungi. It is encoded by the RHO3 gene and cycles between an active GTP-bound form and an inactive GDP-bound form. As with other Rho family proteins, Rho3 undergoes post-translational lipid modification at its C-terminus to promote membrane association and proper cellular localization.

Functionally, Rho3 plays a key role in polarized exocytosis and growth. It localizes to sites of cell growth, such as the incipient bud site and the growing daughter cell tip, and regulates the delivery of secretory vesicles to the plasma membrane. In conjunction with components of the exocyst complex (including Exo70) and the actin-myosin transport system (notably Myo2), Rho3 promotes vesicle tethering and fusion at the cell surface, supporting the secretion of membrane and cell wall components necessary for polarized growth.

Regulation of Rho3 involves cycling between GTP- and GDP-bound states, controlled by guanine nucleotide exchange factors and GTPase-activating proteins, as well as by lipid modifications that anchor it to membranes. Its localization and function are coordinated with the broader cellular polarity machinery.

Genetically and evolutionarily, in Saccharomyces cerevisiae, RHO3 functions alongside related GTPases and exocytic regulators to ensure efficient secretion; mutations in RHO3 cause growth and trafficking defects but are not universally lethal. Homologs of Rho3 exist in other fungi, reflecting a conserved role in exocytosis and polarity across species.