The objective of this program is to explore the molecular basis and physiological functions of ion transport in eukaryotic microorganisms, fungi in particular. The water mold Blastocladiella emersonii, well known to developmental biologists for its remarkable life cycle, is also a favorable organism for electrophysiological research. Earlier work from this laboratory has suggested that the organism drives a combined circulation of potassium ions and protons with the aid of a potassium-translocating ATPase. We shall continue ongoing efforts to isolate the plasma membrane and to characterize its ion-transport ATPases. The findings will serve as the basis for examining the movements of K+, M+, Na+ and Ca2+ ions in the context of the cell's growth and development. Circumstantial evidence from several laboratories implicated protons and calcium ions in apical extension and the control of branching. Experiments with Neurospora and Achlya will explore the effects of ionophores on growth and cytoplasmic calcium levels. We shall seek to discover whether changes in the calcium level serve as regulatory signals; whether colonial growth in certain mutants reflects altered ion fluxes; whether there is a gradient of Ca2+ ions within hyphae and what this has to do with polarized growth; and how ionic signals interact with other signals to create the integrated process that we call growth.