Degradation of a cAMP-binding protein ofDictyostelium discoideumby an endogenous protease

Abstract

When cells of Dictyostelium discoideum are starved they aggregate to form multicellular organisms that eventually can differentiate into fruiting bodies of spores and stalk cells, cAMP plays a dual role in this process: it is the chemotactic agent that allows cells on a moist surface to aggregate to a common point to form the multicellular organism [1], and it is also involved in the process of differentiation. Pulsatile additions of cAMP stimulate cell differentiation [2,3]. High concentrations of cAMP can induce stalk cell formation [4,5] and, under certain conditions, spore cell production can be induced by cAMP [6,7]. Cyclic AMP has also been shown to induce differentiation in the multicellular aggregate [8]. Further evidence of the involvement of cAMP in differentiation is given by the finding that addition of cAMP to cells affects the expression of developmentally regulated proteins [9] and the expression of specific genes [10]. Receptors for cAMP have been identified by many workers. Membrane-associated cAMP-binding proteins have been reported [11-14], and soluble cAMP-binding proteins have also been identified [14-19]. There has been a report that the soluble cAMP-binding proteins are associated with protein kinase activity [16] but this report has not been confirmed [17,18]. Wallace and Frazier [14] reported that the soluble cAMP-binding proteins are of M r 26 000, 33 000, and 36 000, but Cooper et al. [19] have reported that the M r are higher, in the 36 000-40 000 range. Here we describe our studies indicating that the endogenous cAMP-binding protein of M r 40 000 is extremely sensitive to endogenous protease activity, and the lower M r binding proteins are produced during the process of freezing the cells. When fresh extracts are used, only the 40 000 M r binding protein is observed. Tos-LysCH2C1, an inhibitor of proteinase I of Dictyostelium [20,21] inhibits this degradative reaction.