Regulation of protein synthesis during the preaggregative period of Dictyostelium discoideum development: involvement of close cell associations and cAMP.

The preaggregative period of Dictyostelium discoideum is composed of two rate-limiting components which exhibit dramatic differences in either their dependency upon, or sensitivity to, close cell-cell associations, inhibitors of protein synthesis, temperature, and pH. The first component comprises the initial 4.5 hr and the second component the last 2.5 hr of the preaggregative period. By pulse-labeling cells with [35S]methionine, separating polypeptides by 2D-PAGE, and semiquantitatively comparing the rates of synthesis of 778 individual polypeptides by fluorography, the following results were obtained: a detailed program of protein synthesis accompanies the preaggregative (0-7 hr) and aggregative (7-10.5 hr) periods of development; this includes significant decreases in the rate of synthesis of 93 polypeptides synthesized during vegetative growth and significant increases in the rate of synthesis of 74 polypeptides either undetectable or synthesized at relatively low rates during vegetative growth; 35 polypeptides are transiently synthesized at different times during the preaggregative and aggregative periods; two peaks of activity are clearly defined for both increases and decreases; these peaks correlate temporally with the first and second rate-limiting components of the preaggregative period; the majority of changes (74%) which occur during the first rate-limiting component will occur in the absence of close cell-cell associations, but the majority (66%) which normally occur during the second rate-limiting component do not occur in the absence of close cell-cell associations; a high concentration of cAMP in the medium of continuous suspension cultures does not stimulate most of the changes which are dependent upon close cell-cell associations; even though cAMP stimulates progress through the second rate-limiting component in suspension cultures first allowed to associate for 4.5 hr ("competent" cells) prior to disaggregation it still does not stimulate most of the changes which are dependent upon close cell-cell associations; and synthesis of only 3 out of 778 polypeptides appears to be stimulated by addition of exogenous cAMP, and only in resuspended cultures of "competent" cells. The prominent role of close cell-cell association and the surprisingly minor effect of cAMP in the regulation of the program of protein synthesis accompanying the preaggregative and aggregative periods of Dictyostelium are discussed, especially as they relate to the effect of cAMP on protein synthesis in suspended cultures of postaggregative cells.