Influence of Cytokinins and Sulfhydryl Group-Reacting Agents on Calcium Transport in Fungi

Achlya is a filamentous coenocytic fungus that grows only in the presence of Ca2+. The Ca2+ transport (uptake and binding) system of this organism was studied and shown to consist of two components: a low molecular weight Ca2+ binding glycopeptide present within the cell wall matrix and a Ca2+ uptake component associated with the cell membrane. Certain thiol group-reacting agents and cytokinin-active compounds enhanced Ca2+ binding and uptake, respectively. Ca2+ transport by intact cells showed pH dependence with optima at pH 6.5 and 8.5 and temperature dependence with maximal transport occurring at 40° (without activation) and 15° (with activation). Ca2+ transport proceeded by facilitated diffusion. The system is insensitive to metabolic inhibitors. Compounds with known cytokinin activity stimulated the release of Ca2+ bound to the cell wall glycopeptide, and these same compounds enhanced Ca2+ intake into osmotically shocked cells (lacking the glycopeptide). The half-saturation values for Ca2+ intake into cells and release from the glycopeptide are essentially identical.