Defective cell cohesivity expressed late in the development of a Dictyostelium discoideum mutant

Abstract

Mutant JC-5, derived from strain FR-17 which is itself a mutant of Dictyostelium discoideum, exhibits late in development a temperature-sensitive morphogenetic defect associated with the loss of EDTA-resistant cell cohesivity. At the permissive temperature (22°C) the morphogenetic sequence followed by JC-5 is in quality and timing like that of the parent strain. At the restrictive temperature (27°C) the mutant initially forms normal aggregates but starting at 11–12 hr these rapidly disperse to a smooth lawn of dissociated cells which nevertheless complete their differentiation into spores and stalk cells on schedule. Aggregates shifted from 22 to 27°C at 11 hr disperse promptly and completely, and those shifted even as late as 13–14 hr regress markedly. Aggregates shifted from 27 to 22°C at 11 hr continue normal morphogenesis uninterrupted and those shifted 1–2 hr later after significant dispersion has occurred can be rescued. These events observed on solid substratum are precisely correlated with the maintenance, loss, or recovery of EDTA-resistant cohesivity assayed in shaken suspension. Past the critical 11-hr stage (but not before) mutant cells can be made to lose cohesivity even at the permissive temperature by exposure to cycloheximide and, having lost it at the restrictive temperature, can regain cohesivity at the permissive temperature only in the absence of the inhibitor. In contrast, cycloheximide leaves the cohesivity of the parent strain unaffected at any stage. Ghosts derived from cohesive cells exhibit normal cohesivity when assayed at the permissive or restrictive temperature. Ghosts derived from noncohesive cells are noncohesive at either temperature. These results indicate that the defect lies in the inability of mutant cells late in development at the restrictive temperature to maintain or mobilize a critical level of a plasma membrane-associated moiety required for cohesion. If present, however, it can function at either temperature. Expression or repair of the cohesive defect was not accompanied by significant changes in the levels of contact site-A glycoprotein, assayed by electrophoretic mobility and labeling with [ 14C]acetate, or of carbohydrate binding protein (“discoidin”), assayed by hemagglutinating activity. Correlative changes were observed in several proteins capable of binding with 125I-labeled wheat germ agglutinin or concanavalin A but there is no indication as yet that any of them bears a causative relation to the defect.