On the role of calcium during chemotactic signalling and differentiation of the cellular slime mould Dictyostelium discoideum.

Transient cytosolic calcium elevations are required for chemotaxis and differentiation of Dictyostelium discoideum since Ca2+ chelating buffers introduced into the cells by scrape loading inhibited motility as well as orientation in a Ca2+ specific manner. Ca2+ changes are provided either by intrinsic cytosolic Ca2+ oscillations that can be determined as periodic Ca2+ efflux, or by receptor-mediated Ca2+ liberation from the InsP3-sensitive store and Ca2+ influx. Cytosolic Ca2+ homeostasis as well as oscillations seem to be regulated by two different Ca2+ stores, the acidosomes and the InsP3-sensitive store, both of which are dependent on Ca2+ pumps and V-type H+ ATPases. Ca2+ transients are sensed by calmodulin-binding proteins. The latter have been detected in Dictyostelium by 125I-calmodulin labeling. A calmodulin-dependent protein phosphatase, calcineurin A, was cloned, sequenced, purified and characterized biochemically. Overproduction of calcineurin A as well as antisense constructs will help to the elucidation of its function in signal transduction. Surprisingly, protein synthesis is also controlled by Ca2+/calmodulin. An integral ribosomal protein of the 60S subunit, L19, proved to be a calmodulin-binding protein and calmodulin antagonists of different classes, inhibited in vitro translation of Dictyostelium and wheat germ extracts.