On the involvement of Pi-binding proteins in Pi-uptake in the yeastCandida tropicalis

Immunological evidence for the involvement of cell wall proteins in phosphate uptake in the yeast Saccharomyces cerevisiae

Immunological cross-reactivity between cell wall proteins obtained from two yeast genera (Candida tropicalis and Saccharomyces cerevisiae) is reported. Specific retention of two cell wall proteins from Saccharomyces cerevisiae by an immunoabsorbent column coupled with antibodies against phosphate binding protein 2(PiBP2) from Candida tropicalis allowed to generate antibodies against the proteins from S. cerevisiae. These antibodies were effective in inhibiting phosphate uptake by S. cerevisiae cells. The proteins from S. cerevisiae displayed a phosphate binding activity which was inhibited in the presence of the forementioned antibodies. These results and the observation that the amount of these proteins in the shock fluid was dependent of the growth conditions (i.e., in the presence or in the absence of phosphate) support the idea that these proteins are involved in the high affinity phosphate transport system.

Phosphate uptake in the yeast Candida tropicalis: purification of phosphate-binding protein and investigations about its role in phosphate uptake

The purification of a phosphate-binding protein (PiBP2) by immunoadsorption is described. The entire anti phosphate-binding protein 2 antibodies as well as the Fab fragments obtained from these antibodies inhibit Pi uptake by whole cells. The inhibition is a mixed type of inhibition (Vm and Km are affected). These results should be regarded as a possible involvement of phosphate-binding protein 2 in Pi uptake. The binding of 125I-labelled fragments prepared from anti phosphate-binding protein 2 antibodies to whole cells, to shocked cells and to protoplasts has been investigated. The results confirm the release of phosphate-binding protein by osmotic shock and during protoplast formation. From these findings, a cell-wall localisation, near the cell surface of the phosphate-binding protein should be proposed.

Identification of a plasma membrane protein involved in Pi transport in the yeast Candida tropicalis

A plasma membrane protein was found to contain antigenic determinants in common with a Pi-binding protein released by osmotic shock from Candida tropicalis. This plasma membrane protein (M.P. 30) has a molecular mass of 30 kDa as measured by SDS-PAGE and anti-M.P. 30 antibodies inhibit Pi uptake in protoplasts while only the corresponding Fab fragments inhibit Pi transport in whole cells. This plasma membrane protein may be the Pi plasma membrane carrier.

Inorganic phosphate uptake by protoplasts and whole cells of yeast Candida tropicalis: absence of high affinity transport system in protoplasts

Pi-uptake by protoplasts obtained from the yeast Candida tropicalis as well as in whole cells is an energy dependent process. However, the apparent affinity constant of Pi transport by protoplasts about 0.4-0.5 mM whereas Km of 5-6 microM, occurs in whole cells as previously shown. Therefore it can be concluded that protoplasts take up actively Pi but the high affinity capacities are lost. In addition, the Pi-binding proteins are released during protoplasts formation. The presence of Pi-binding proteins in the "cellulase fluid" is demonstrated by immunological tests and by their binding activities after partial purification. These results are in good agreement with the involvement of Pi-binding proteins in Pi uptake in the yeast Candida tropicalis.