Results from a number of laboratories suggest that intracellular Ca is involved in the regulation of Dictyostelium discoideum growth and development. To learn more about the regulation and function of intracellular Ca in this organism, we have cloned and sequenced cDNAs that encode a putative P-type Ca ATPase designated patA. The deduced protein product of this gene (PAT1) has a calculated molecular mass of 120,718 daltons. It exhibits about 46% amino acid identity with Ca ATPases of the plasma membrane Ca ATPase family and lower identity with sarco(endo)plasmic reticulum Ca ATPase family members and monovalent cation pumps. However, PAT1 lacks the highly conserved calmodulin-binding domain present in the C-terminal region of most plasma membrane Ca ATPase-type enzymes. When Dictyostelium amoebae are adapted to grow in the presence of 80 mM CaCl, both the patA message and protein product are up-regulated substantially. These cells also exhibit an increase in the rate and magnitude of intracellular P-type Ca uptake activity. Immunofluorescence analysis indicates that PAT1 colocalizes with bound calmodulin to intracellular membranes, probably components of the contractile vacuole complex. The presence of PAT1 on the contractile vacuole suggests that in Dictyostelium this organelle might function in Ca homeostasis as well as in water regulation.

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