The gic2-sec3 chimera is able to rescue synthetic lethality between sec3ΔN and exo70-38. (A) sec3ΔN is synthetic lethal with exo70-38. sec3ΔN and exo70-38 were expressed under SEC3 and EXO70 promoters in CEN plasmids. The sec3ΔN exo70-38 double mutant supplemented with a CEN, URA3, SEC3 balancer was streaked out on the plates with (right) or without (left) 5-FOA and incubated for 5 d at 25°C. The sec3ΔN and exo70-38 single mutants were used as controls. exo70-38 sec3ΔN could not survive when losing the SEC3 balancer on the 5-FOA plate (right). (B) Diagram of Sec3 and gic2-sec3 chimera in which the N terminus of Sec3 (aa 1–307) was replaced with the N terminus of Gic2 (aa 1–155; gray). The interaction with Cdc42 is indicated by the arrows. (C) The chimera gic2-sec3 is able to rescue the synthetic lethality between sec3ΔN and exo70-38. gic2-sec3 was expressed under the endogenous SEC3 promoter in a CEN plasmid supplemented with a CEN, URA, SEC3 balancer. Although sec3ΔN and exo70-38 were synthetic lethal, the exo70-38 gic2-sec3 grew well at 25°C in the presence of 5-FOA. (D) The CRIB domain and polybasic region at the N terminus of Gic2 are essential for the functional replacement of wild-type SEC3 with gic2-sec3 chimera in yeast. Chimeras, including gic2-sec3 and gic2 (CRIB mutation)-sec3, in which the key residues of the CRIB domain are replaced by alanine (I134A, S135A, and P137A), and gic2 (CRIBΔ)-sec3, gic2 (CRIBΔ & polybasicΔ)-sec3, and gic2-sec3 (K109A, K110A, K119A, K120A, and K121A), in which the polybasic region of Gic2 was mutated, were expressed under the SEC3 promoter in CEN plasmids and tested for their synthetic lethality with exo70-38. All chimeras except gic2-sec3 are synthetic lethal with exo70-38.