Yeast flocculation: Calcium specificity

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Abstract
Expression of flocculation in yeast requires the presence of multi‐charged cations. Calcium ions fulfil this role over a broad pH range. At near neutral pH, magnesium and a variety of transition elements can induce flocculation. Calcium‐induced flocculation was competitively inhibited by excess sodium ions whereas magnesium‐induced flocculation was not competitively inhibited. Potassium and lithium ions caused similar inhibition but to a lesser extent. 45Ca effux from yeast cells was greatly increased by the external presence of buffer and cations. Inhibition of flocculation by chelating agents, EDTA or EGTA, was overcome by excess calcium ions. Flocculation could not be induced, under these conditions, by excess magnesium or transition‐element salts.It was concluded that yeast flocculation has a direct specific requirement for calcium ions. Other ions cause flocculation indirectly by effecting calcium ion leakage from cells, which is then able to initiate flocculation. Low calcium concentrations were susceptible to competitive inhibition by sodium ions present in most acidic buffers. In addition, citrate ions inhibited flocculation, probably by sequestration of leaked calcium. Flocculation by salts, other than calcium, was thus restricted to a neutral or high pH range.