Gene families encode the major encystment-specific proteins of Physarum polycephalum plasmodia

The encystment of Physarum polycephalum plasmodia, also called spherulation, involves the synthesis of many specific mRNAs and proteins. Most of these molecules accumulate at the onset of the major morphological and physiological changes typical of this differentiation pathway and are not present during the other two transitions leading to dormancy in Physarum, namely sporulation and encystment of amoebae. The nucleotide sequences of apparently full-length cDNA copies of the four major encystment-specific mRNAs were determined. The four sequences included the entire coding regions and at least 26 nucleotides of the 5'-nontranscribed leaders. The encoded proteins were named spherulins. We found that spherulins 1a and 1b are 81% homologous and are thus members of a gene family. They both possess putative signal peptides and N-glycosylation sites, suggesting that they are cell-wall glycoproteins. Spherulin 2a and spherulin 3a are non-homologous proteins. The absence of signal peptides suggests that they are intracellular structural proteins. Low-stringency Southern hybridizations showed that each also belongs to a two-member gene family.