The isolation from a unicellular organism, Dictyostelium discoideum, of a highly-related cdc2 gene with characteristics of the PCTAIRE subfamily.

It has been suggested that cell-type determination in Dictyostelium discoideum is dependent on the position of a cell in the cell cycle at the time of starvation. In order to understand the molecular basis of this phenomenon, we initiated studies on the cell cycle and have recently described the isolation of a Dictyostelium gene encoding a homolog of the Cdc2 kinase. We have been unable to isolate additional cdc2 genes from Dictyostelium using polymerase chain reaction technology, but have isolated a gene that is highly related to cdc2. The encoded product is a protein of 33 kDa that shares over 60% identity to the cell-cycle-dependent Cdc2 kinases. However, despite this high level of identity, the gene is not capable of complementing the temperature-sensitive cdc28 mutant of Saccharomyces cerevisiae. Furthermore, the gene product shares some characteristics with the recently described PCTAIRE proteins; it contains a PCTAIRE motif instead of the Cdc2 kinase conserved PSTAIRE sequence, it does not possess the conserved GDSEID sequence that is involved in the activation of the enzyme and it has a Ser in the position equivalent to Thr-161. However, the Dictyostelium protein exhibits a slightly higher level of identity to the Cdc2 kinases than to the PCTAIRE proteins and is smaller than any of the PCTAIRE proteins thus far identified. Since the gene product has characteristics of both Cdc2 kinases and PCTAIRE proteins we have designated the gene product Crp (Cdc2-Related PCTAIRE) kinase. The gene is expressed as two transcripts of 1.5 and 1.8 kb and the expression is developmentally regulated with low levels of mRNA in vegetative cells and significantly higher levels throughout the remainder of the differentiation process. These results suggest the possibility that the gene product is involved in Dictyostelium differentiation rather than growth. This report is the first evidence for a highly-related cdc2 gene in unicellular eukaryote. It also demonstrates for the first time that a unicellular eukaryote expresses a protein containing the PCTAIRE sequence.