Plasma membrane proteins of Dictyostelium: The spore coat proteins

Abstract

Spore coats were isolated following germination ofDictyostelium discoideum spores. They were found to contain five major proteins of apparent molecular weights of 60,000 (SP60), 68,000 (SP68), 70,000 (SP70), 96,000 (SP96), and 200,000 (SP200). Of these SP68 and SP96 were found to be glycoproteins. Analysis of the tryptic and chymotryptic fingerprints indicated that the major spore coat proteins are independent gene products and not derived by proteolytic cleavage of common percursors. These spore coat proteins are the major newly synthesized proteins seen in membranes late in development and account for the majority of the changes seen in membranes during culmination. They can be observed in membranes isolated from spores, most likely while in transit to the spore coat. SP60, SP68, and SP70 appear to be on the outer face of the spore coat since they are sensitive to proteolysis by Pronase added to intact spores while SP96 and SP200 are protected until the spore case is broken during germination. The spore coat proteins are not found in stalk cells or in several mutant strains blocked in spore formation including one strain, HL-6, which undergoes many of the biochemical differentiations of culmination but forms only stalk cells. The spore coat proteins do accumulate in a spore shape mutant, HU26, which forms small round spores rather than ellipsoid ones. Characterization of the spore coat proteins further defines the changes in membrane proteins during development ofDictyostelium and the processes of terminal differentiation in spores.