Plasma membrane proteins in Dictyostelium.

Abstract

Cell interactions are important in modifying cell behavior. Changes in cell cohesion, for example, are involved in such processes as gastrulation and neurulation. The cell surface is clearly involved in these interactions and the cellular slime molds present relatively simple systems with which to study such phenomena. This review commences with a discussion of the various methods used to isolate Dictyostelium plasma membranes, since this variability may partly account for the difficulties in comparing results from different laboratories. Ecto-enzymes such as cyclic AMP phosphodiesterase and folic acid deaminase are clearly involved in the chemotactic response of cells. Sugar transferases are found on the cytoplasmic face of the membrane and may participate in glycoprotein and glycolipid synthesis. The presence of so many ecto-enzymes metabolizing nucleotide phosphates (ATPase, protein kinase, ADPase, AMPase, nucleotide diphosphokinase) is puzzling and possible functions are discussed. The signalling system involved in cell aggregation includes cyclic AMP receptors along with obscure mechanisms linking receptor occupancy to adenylyl cyclase stimulation and the release of newly synthesized cyclic AMP from cells. Many descriptive studies of plasma membranes have been published, but very little is known about the function of the many glycoproteins and proteins described. Actin and myosin are usually prominent proteins in plasma membrane preparations and are presumably associated with the cytoplasmic face. Changes in protein composition have been detected using lectins, coomassie blue staining, cell surface labelling and antibodies as well as radioactive amino acids, acetate and sugars. The synthesis of at least a dozen proteins is developmentally regulated. Three such glycoproteins have been implicated in cell adhesion. Components of the adhesion mechanism appear to change during development and this may be related to the sorting out of the two cell types (pre-spore and pre-stalk cells) in the aggregates. Lectins synthesized during early aggregation and partly released by the cells have also been implicated in cell-cell contact. Rapid changes in plasma membrane composition are theoretically possible since the membrane can, for example, be internalized once every 45 min during the pinocytotic uptake of dextran. Starvation, cyclic nucleotide levels and cell contact have been implicated in the control of plasma membrane protein synthesis. Developmentally regulated changes in the plasma membrane proteins of another species, Polysphondylium pallidum, are described since they provide interesting comparisons with Dictyostelium. Finally, seven areas in which advances can be expected in the near future are briefly defined.