CHO-K1 cells were transiently transfected with 6 μg of each of the control empty vector plasmid (vector) or plasmids that encoded wild-type (wt) or Y111A, D166A, R184A, R323A and E364A mutant PSS 1. (A) Cell lysates of the transfectants were assayed for serine base-exchange activity. Three independent experiments gave similar results; the values shown are for one of the three independent experiments, and are the averages of duplicate determinations with a variation of <12% between duplicates. (B) Cell lysates (10 μg of protein) of the transfectants were separated by SDS/PAGE, and then analysed by Western blotting with an anti-PSS 1 N-terminal peptide antibody.