THE VANADATE-TOLERANT YEAST HANSENULA-POLYMORPHA UNDERGOES CELLULAR REORGANIZATION DURING GROWTH IN, AND RECOVERY FROM, THE PRESENCE OF VANADATE

When present at intracellular concentrations above micromolar, vanadat e becomes toxic to most organisms. However, the yeast Hansenula polymo rpha is able to grow on vanadate concentrations in the millimolar rang e, showing at the same time modifications in cellular ultrastructure a nd polyphosphate metabolism. Here, the development of the ultrastructu ral changes, and of vacuolar and secretory activities, during exponent ial growth on vanadate and upon a return to vanadate-free conditions w as investigated. External invertase secretion was inhibited by vanadat e, as shown by a decrease in external invertase activity, an intracell ular accumulation of small vesicles and a cytoplasmic accumulation of internal invertase. An aberrant appearance of the cell wall and defect s in cellular surface growth, possibly linked to defects in secretion, were also observed. However, inhibition of the secretory pathway was not complete since the activity of another secreted enzyme, exoglucana se, increased in the presence of vanadate. Growth on vanadate was also accompanied by an enhancement of vacuolar proteolysis, as indicated b y an increase in carboxypeptidase Y activity. However, these modificat ions were all reversible upon return to vanadate-free conditions, with the normalization process being complex and involving new and dramati c ultrastructural changes and activation of an autophagic mechanism. T his mechanism is involved in the elimination/resorption of the observe d vanadate-induced aberrant cell structures and/or sites involved in v anadate accumulation, a necessary prerequisite for restoration of conv entional ultrastructure and metabolic functions.