Aggregation-related and post-aggregation-related cohesive systems in Dictyostelium discoideum involve stage-specific, Ca 2+-dependent, D1- or polymeric associations of membrane-bound moieties

Abstract

The cells of D. discoideum acquire developmentally regulated cohesive properties during aggregation and fruiting body construction. On the bases of genetic, serological, and physiological evidence, it has been suggested that two distinct cohesive systems operate: an aggregation-related (AR) system that facilitates the formation of multicellular aggregates and post-aggregation-related (PAR) system that maintains the integrity of the aggregate thereafter. We had previously demonstrated that ghosts and membrane fragments retain the cohesive properties of the cell from which they were derived. Here, we describe a two-phase assay involving the Ca 2+-dependent binding of 125I-labeled cell ghosts in suspension to their unlabeled counterparts immobilized on plastic surfaces. Using this assay we show that the ghosts of newly aggregation-competent (8 h) cells and of cells from the ‘Mexican hat (18 h) stage’ of fruit construction can bind, each to its immobilized counterpart, but not heterologously. Furthermore, neither binds to the immobilized ghosts of vegetative cells. This provides direct, functional evidence demonstrating the existence of the two stage-specific cohesive systems. It also suggests that both cohesive acts involve at least dimeric associations of molecules or molecular complexes located within or on juxtaposed membranes. Using immobilized 8 and 18 h ghosts, the specific binding activities of ghosts prepared from cells harvested at stages throughout the morphogenetic sequence were assayed in order to describe the developmental kinetics of the two cohesive systems. The binding data suggest that the AR system appears soon after the start of the morphogenetic sequence, peaks during early aggregation and is progressively diminished thereafter. The PAR system makes its appearance after aggregation and accumulates thereafter. Both systems are present in migrating slugs.