Clc1p is phosphorylated on serines, primarily at residues 52 and 112. Cells expressing wild-type Clc1p (GPY1080, B), Clc1p S52T (GPY1954, C) or Clc1p S112T (GPY1955, D) were labeled as described in Figure 1A. Immunoprecipitated Clc1p was subjected to acid hydrolysis and two-dimensional thin-layer cellulose chromatography electrophoresis to separate labeled amino acids. (A) Positions of phosphoamino acid standards.