Cell Biology and Metabolism
Acid-activatable Cysteine Proteinases in the Cellular Slime Mold Dictyostelium discoideum*

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Studies of the cysteine proteinases of the cellular slime mold Dictyostelium discoideum have been aided by a simple acid treatment step that was incorporated into the standard one-dimensional gelatin-sodium dodecyl sulfate-polyacrylamide gel electrophoresis assay procedure. The step involved immersing the separating gel in 10% (v/v) glacial acetic acid for 30–60 s immediately after electrophoresis. This modified approach revealed the presence of acid-activatable forms of some enzymes with noticeable increases in their ability to hydrolyze gelatin, a substrate present in the sodium dodecyl sulfate-polyacrylamide gels, and peptidyl amidomethylcoumarins. The activation has been analyzed using extracts of dormant spores from which cysteine proteinase activity had previously appeared low or virtually absent. The major acid-activatable proteinase had an apparent molecular mass of 48 kDa. Its activation was not due to autocatalysis as it was not prevented by mercuric chloride, an inhibitor of the enzyme, and was not accompanied by a significant change in electrophoretic mobility. It was most likely due to a conformational change and/or the removal of a low molecular weight inhibitor. The acid treatment has also revealed the presence of acid-activatable cysteine proteinases in vegetative cells, in which cysteine proteinase activity is present at high levels, as well as among enzymes from the developmental cells which have much lower cysteine proteinase activity. Indeed novel developmental forms were detected at some stages. These results provide additional insight concerning cysteine proteinase expression at various stages during development in the slime molds. A developmental model is presented which suggests that the crypticity of the cysteine proteinases in dormant spores may be governed by proton pumps and endogenous lysosomotropic agents.

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This work was supported in part by a NATO Collaborative Research Grant and by the Smith Kline (1982) Foundation (to M. J. N.) and the Natural Sciences and Engineering Research Council of Canada (to D. A. C.). The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.