The Yeast Ca²⁺-ATPases and Ca²⁺/H⁺ Exchangers of the Secretory Pathway and Their Regulation

Abstract

We propose a new model for yeast Ca²⁺ homeostasis that considers the roles of different Ca²⁺/H⁺ exchangers and Ca²⁺-ATPases which function in all secretory organelles. Majority of these Ca²⁺-ATPases are blocked by cyclopiazonic acid and thapsigargin in a similar fashion as SERCA. The exchangers and Ca²⁺-ATPases are activated ~7- and ~2-fold by extracellular glucose and contribute ~80 % and 20 %, respectively, to Ca²⁺ efflux from the cytosol. Vacuoles do not represent major storage organelles, contributing 20–35 % in Ca²⁺ uptake (an efflux from the cytosol). VCX1 and PMC1 and their respective vacuolar transporters positively regulate both types of transporters from all secretory organelles, whereas PMR1 and Pmr1p negatively regulate Ca²⁺ pumps from the ER and NE. Calcineurin is a positive regulator of Ca²⁺-ATPases and the exchangers from secretory organelles, whose capacity is modulated depending on the energy supply. Calcineurin activates Ca²⁺-ATPases and the exchangers under normal growth conditions and under high Ca²⁺ stress in the absence of glucose. Glucose and high Ca²⁺ together additively stimulate Ca²⁺-ATPases, whereas Ca²⁺/H⁺ exchangers demonstrate higher activity than that observed under Ca²⁺ stress in the absence of glucose but lower activity than that observed with glucose alone. Modulation of the exchanger activities under Ca²⁺ stress correlates with that of V H⁺-ATPase, suggesting indirect regulation of the exchangers by calcineurin via regulation of this H⁺ pump. The presence of Ca²⁺-ATPases and exchangers in all secretory organelles is discussed from the point of view of local and specific Ca²⁺ signaling.