CLA4 functions are required for viability in ncs1Δ cells. (A) Cla4p lacking the PAK domain cannot support viability in the absence of Ncs1p. Strains SY3364, SY3365, SY3380 (cla4Δ::TRP1 ura3::cla4ΔPAK-MYC::URA3 <YCpHIS3cla4-75>), and SY3379 (ncs1Δ::LEU2 cla4Δ::TRP1 ura3::cla4ΔPAK-MYC::URA3 <YCpHIS3cla4-75>) (sectors 1 to 4, respectively) were grown at 25°C on YEPD medium; single colonies were assayed for growth by streaking onto prewarmed YEPD medium and incubating for 3 days at 37°C. (B) Cells lacking Ncs1p require an active Cla4p kinase. Yeast strain SY3378 (ncs1Δ::HIS3 cla4Δ::TRP1 <pRS316ADE8CLA4>) was transformed with pRS315 (sector a), pRS315ADE8cla4K549R (which encodes a kinase-dead Cla4p), (sector b), or pRS315ADE8CLA4 (sector c), and the ability of pRS315ADE8cla4K549R to substitute for pRS316ADE8CLA4 was assayed by growth on synthetic complete medium lacking leucine (left) or supplemented with 5′-FOA (right) for 3 days at 30°C.