Disruption of the NCS-1/frequenin-related ncsA gene in Dictyostelium discoideum accelerates development

To learn more about the function of intracellular Ca2+ in Dictyostelium discoideum, we searched databases for sequences encoding potential members of the neuronal calcium sensor (NCS) family of Ca2+-binding proteins. As a result, genes for five new putative Ca2+-binding proteins were identified. Based on amino acid sequence alignments and phylogenetic analyses, one of these genes (ncsA) was determined to be closely related to NCS-1/frequenin genes in other organisms. The protein product of ncsA (NcsA) binds 45Ca2+ and exhibits a dramatic gel mobility shift in the presence of Ca2+, suggesting that it is a Ca2+ sensor. ncsA-null cells grow normally in axenic culture. However, on bacterial lawns, the ncsA-null clones expand slowly and development begins prematurely within the plaques. In larger clones, ncsA-null cells form narrow growth zones with evenly spaced aggregates along the inner edge, and closely packed fruiting bodies. An analysis of intracellular cyclic adenosine monophosphate (cAMP) levels, developmental timing on phosphate-buffered saline (PBS) agar, and stage-specific gene expression indicate that development of ncsA-null cells is accelerated by 3-4 h. Together, these results suggest that NcsA might function in Dictyostelium to prevent cells from entering development prematurely in the presence of environmental nutrients.