Fig. 2. The P1 polyproline sequence is required for Srv2 binding to profilin. (A) Sequence alignment of the two mouse Srv2/CAP isoforms (CAP1 and CAP2) and yeast Srv2. Positions of the proline-to-alanine substitutions in Srv2-201, Srv2-202 and Srv2-203 mutants are indicated below the sequences. (B) Quantification of four independent supernatant depletion pull-down assays. Wild-type Srv2 and Srv2-203 bind profilin efficiently, whereas Srv2-201 and Srv2-202 fail to bind profilin in this assay. Standard deviations are indicated by error bars. (C) Tryptophan fluorescence assay of wild-type Srv2_253-373 and mutant Srv2-201_253-373. Wild-type Srv2_253-373 but not Srv2-201_253-373 induces a saturable increase in tryptophan fluorescence of 1 µM profilin.