Characterisation of the spore coat proteins of Dictyostelium discoideum

Using a rapid purification protocol designed to minimise proteolysis in vivo and in vitro, combined with high-resolution one-dimensional and two-dimensional polyacrylamide gel electrophoresis, 11 spore coat proteins have been identified in Dictyostelium discoideum AX2 spores with apparent molecular weights of 170 000 (SP170), 103 000 (SP103), 97 000 (SP97), 90 000 (SP90), 82 000 (SP82), 76 000 (SP76), 72 000 (SP72), 55 000 (SP55), 39 000 (SP39), 33 000 (SP33) and 3400 (SP3). Control experiments rule out the possibility that any of these components are generated by crosslinking or proteolytic degradation artifacts. All of these spore coat proteins are disulphide-crosslinked in the spore coat except for SP103, the majority of which is extracted in the absence of thiol reagent. All of the spore coat proteins are single polypeptides as judged by two-dimensional polyacrylamide gel electrophoresis except for SP33 which is consistently composed of four isoelectric variants. SP170, SP103, SP97, SP90, SP82 and SP72 are extremely acidic with pI values in the range pH 4.5-4.7, SP76 and SP55 are slightly less acidic with pI values of 5.7 and 5.9 respectively, whilst the four SP33 polypeptides have pI values of pH 6.6-7.5. SP170, SP103, SP90, SP82 and SP76 are all glycoproteins as judged both by periodic acid/Schiff staining and radiolabelling in vivo with mannose, glucosamine and fucose.