Proton- and sodium-coupled phosphate transport systems and energy status of Yarrowia lipolytica cells grown in acidic and alkaline conditions

In this study we have used a newly isolated Yarrowia lipolytica yeast strain with a unique capacity to grow over a wide pH range (3.5-10.5), which makes it an excellent model system for studying H⁺- and Na⁺coupled phosphate transport systems. Even at extreme growth conditions (low concentrations of extracellular phosphate, alkaline pH values) Y. lipolytica preserved tightly-coupled mitochondria with the fully competent respiratory chain containing three points of energy conservation. This was, demonstrated for the first time for cells grown at pH 9.5-10.0. In cells grown at pH 4.5, inorganic phosphate (P_i) was accumulated by two kinetically discrete H⁺/P_i-cotransport systems. The low-affinity system is most likely constitutively expressed and operates at high P_i concentrations. The high-affinity system, subjected to regulation by both extracellular P_i availability and intracellular polyphosphate stores, is mobilized during P_i-starvation. In cells grown at pH 9.5-10, Pi uptake is mediated by several kinetically discrete Na⁺dependent systems that are specifically activated by Na⁺ ions and insensitive to the protonophore CCCP. One of these, a low-affinity transporter operative at high P_i concentrations is kinetically characterized here for the first time. The other two, high-affinity, high-capacity systems, are derepressible and functional during P_istarvation and appear to be controlled by extracellular P_i. They represent the first examples of high-capacity, Na⁺driven Pi transport systems in an organism belonging to neither the animal nor bacterial kingdoms. The contribution of the H⁺- and Na⁺-coupled P_i transport systems in Y. lipolytica cells grown at different pH values was quantified. In cells grown at pH values of 4.5 and 6.0, the H⁺-coupled P_i transport systems are predominant. The contribution of the Na⁺/P_i cotransport systems to the total cellular Pi uptake activity is progressively increased with increasing pH, reaching its maximum at pH 9 and higher.