Figure 2.

Keratinocytes phosphorylate only serine residues of basonuclin. A quarter-confluent culture was starved for phosphate for 18 hr, labeled with ³²P_i (9,000 Ci/mmol) at concentration of 0.25 mCi/ml of culture medium for 3 hr, washed, and then lysed. The basonuclin was purified by immunoprecipitation and SDS/PAGE, and then subjected to phosphoamino acid analysis. Added unlabeled phosphoamino acids were detected with ninhydrin, and ³²P was located with the PhosphorImager. (A) The positions of electrophoretically resolved phosphoserine, phosphothreonine, phosphotyrosine, and inorganic phosphate are indicated by PS, PT, PY, and P_i. Ori indicates origin. Only serine and inorganic phosphate are labeled with ³²P. (B) Chromatography gives the same result.