Alkaline Phosphatase Subunits and Their Dimerization In Vivo

Abstract

A pool of alkaline phosphatase subunits has been found in cells of Escherichia coli which are actively synthesizing the enzyme. The radioactive subunits from pulse-labeled cells were specifically recognized by their capacity to produce, upon incubation with Zn ++ and nonradioactive monomers, radioactive dimers with the characteristics of alkaline phosphatase. The pool of subunits was larger (10 times or more) than the amount expected to be bound to ribosomes and was bound to a rapidly sedimentable fraction from which 60% was released by ribonuclease. In a culture pulse-labeled for one-third (8 sec) of the enzyme synthetic time, the pool of radioactive monomers was 81% of the radioactive enzyme and was totally (98%) in the endoplasm. The size of the pool was increased by decreasing the dimerization rate without affecting protein synthesis. This was achieved by decreasing Zn ++ in the growth medium. It was found that the cells contained a full complement of monomers, although the level of active enzyme was low. A process subsequent to the release of the monomers from the ribosomes was found to be limiting the formation of the finished enzyme. This process affects the level of the pool of monomers independently from their synthesis.