Identification of proline carrier in Escherichia coli K-12

Proline carrier, a product of the putP gene of Escherichia coli, was identified as a 35 kDa cytoplasmic membrane protein by SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Its identification was based on the following evidence: First, the density of the band corresponding to a 35 kDa protein correlated with the proline-binding activity of cytoplasmic membranes from putP-deficient and putP-amplified strains. Second, by the differential labeling method, the 35 kDa protein was specifically labeled with radioactive N-ethyl-maleimide. The 35 kDa protein was found to aggregate on heat treatment and to show abnormal mobility on SDS-PAGE.