19S cytosolic malate synthase. A small pool characterized by rapid turnover.

A pool of 19S malate synthase was detected in the cytosol. This pool was separated from the microsomal and glyoxysomal malate synthase when cotyledons of germinating seed of Cucumis sativus were fractionated. An early stage of seed germination was selected for our investigations, when 100S "microsomal" malate synthase was also present. 1) When L-[35S]methionine was applied in vivo to label cellular proteins the small pool of 19S malate synthase was found to contain the highest specific activity compared to microsomal or glyoxysomal malate synthase. 2) The kinetics of specific radioactivity in malate synthase during a pulse chase-labelling experiment established that 19S malate synthase was a precursor of microsomal malate synthase. 3) By means of anti-malate synthase antibodies, 100S malate synthase could be recovered on protein A-Sepharose and thus separated from the endoplasmic reticulum. The data suggest that both microsomal and glyoxysomal malate synthase are synthesized in the cytosol rather than at the endoplasmic reticulum according to the signal hypothesis.