Effects of proline analogues on the formation of alkaline phosphatase in Escherichia coli.

Two of the four proline analogues tested for their effect on the formation and activity of Escherichia coli alkaline phosphatase were able to substitute for proline in protein synthesis in a proline auxotroph. One of these, 3,4-dehydroproline, effectively replaced proline and led to formation of an active enzyme under conditions where no proline was present in the polypeptides. Substitution of azetidine-2-carboxylate for proline prevented active enzyme formation, producing instead altered monomeric forms of the alkaline phosphatase. These were detected with antibodies specific to denatured forms of the enzyme, and they were also characterized, together with cellular proteins, by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Alkaline phosphatase, as well as several other proteins, is localized exterior to the bacterial cell cytoplasm in the periplasmic space. In the presence of azetidine-2-carboxylate, a substantial number of these periplasmic proteins retain their specific site of localization, and the denatured subunits of alkaline phosphatase were only detected in the periplasmic fraction of the cell. Thus, secretion of these proteins does not appear to require a high degree of specificity in the native structure of the polypeptide chain. The analogues 4-allohydroxyproline and 4-thiazolidine carboxylate were unable to substitute for proline in protein synthesis but they inhibited growth of E. coli.