Countin, a cell-counting factor in Dictyostelium discoideum, is considered to limit the maximum size of the multicellular structure, because a countin null strain forms a huge fruiting body compared to that of the wildtype. A novel gene, countin2, that is highly homologous to countin (40% identity in amino acid sequence) was identified in the D. discoideum genome. The countin2 null strain formed a 1.7-fold higher number of the aggregates, resulting in smaller fruiting bodies compared with those of wild-type cells. Thus, the Countin2 protein is thought to limit the minimum size of the multicellular structure. The size and number of aggregates formed by a mixture of countin null and countin2 null strains were the same as those of the wild-type. These findings demonstrate that a combination of Countin and Countin2 proteins determines the appropriate size of the multicellular structure of D. discoideum.

In the cellular slime mold Dictyostelium discoideum, Countin and Countin2 proteins are thought to control the size of the multicellular structure since the countinstrain forms a huge fruiting body and countin2strain forms a small fruiting body. Recently, the countin3 gene encoding a polypeptide homologous to Countin and Countin2 was identified in the D. discoideum genome. The countin3strain formed a 1.8-fold larger number of aggregates, resulting in smaller fruiting bodies compared to those formed by the wild-type. The extent of cell-cell adhesion was reduced in the mutant, indicating that Countin3 protein regulates size by controlling the amounts of proteins responsible for the adhesion.

Genes for the cell-counting factors in Dictyostelium discoideum, countin and countin2, are considered to control the size of the multicellular structure of this organism. A novel gene, countin3, that is homologous to countin and countin2 genes (49 and 39% identity in amino acid sequence, respectively) was identified in the D. discoideum genome. The expression of countin3 was observed in the vegetatively growing cells, decreased in the aggregating stage, increased in the mid-developmental stage and decreased again in subsequent stages. This expression pattern is different from that of countin and countin2. The distinct expression kinetics of three genes suggests that they would have unique roles in size control of D. discoideum.

In the cellular slime mold Dictyostelium discoideum, Countin2 and Countin3 proteins are thought to limit the minimum size of the multicellular structure since countin2 -and countin3 -strains form small fruiting bodies. Expression levels of the cell-cell adhesion proteins, gp24 and gp80, in wild type and null mutants were compared. During the process of aggregation, countin2 -cells expressed lower levels of gp24 and gp80 than those expressed by the wild type. The expression levels of gp24 and gp80 in countin3 -cells were almost the same as those in wild-type cells in the early stages of aggregation but were lower than wild-type levels in the late stages. These results indicate that Countin2 and Countin3 proteins enhance the expression of gp24 and gp80 in D. discoideum in different ways.