THE RHO-GEF ROM2P LOCALIZES TO SITES OF POLARIZED CELL-GROWTH AND PARTICIPATES IN CYTOSKELETAL FUNCTIONS IN SACCHAROMYCES-CEREVISIAE

Rom2p is a GDP/GTP exchange factor for Rho1p and Rho2p GTPases; Rho pr oteins have been implicated in control of actin cytoskeletal rearrange ments. ROM2 and RHO2 were identified in a screen for high-copy number suppressors of cik1 Delta, a mutant defective in microtubule-based pro cesses in Saccharomyces cerevisiae. A Rom2p::3XHA fusion protein local izes to sites of polarized cell growth, including incipient bud sites, tips of small buds, and tips of mating projections. Disruption of ROM 2 results in temperature-sensitive growth defects at 11 degrees C and 37 degrees C. rom2 Delta cells exhibit morphological defects. At permi ssive temperatures, rom2 Delta cells often form elongated buds and fai l to form normal mating projections after exposure to pheromone; at th e restrictive temperature, small budded cells accumulate. High-copy nu mber plasmids containing either ROM2 or RHO2 suppress the temperature- sensitive growth defects of cik1 Delta and kar3 Delta strains. KAR3 en codes a kinesin-related protein that interacts with Cik1p. Furthermore , rom2 Delta strains exhibit increased sensitivity to the microtubule depolymerizing drug benomyl. These results suggest a role for Rom2p in both polarized morphogenesis and functions of the microtubule cytoske leton.