Single point mutations in either gene encoding the subunits of the heterooctameric yeast phosphofructokinase abolish allosteric inhibition by ATP

Yeast phosphofructokinase is a heterooctameric enzyme subject to a complex allosteric regulation. A mutation in the PFK1 gene, encoding the larger cy-subunits, rendering the enzyme insensitive to allosteric inhibition by ATP was found to be caused by an exchange of proline 728 for a leucine residue. By in vitro mutagenesis, we introduced this mutation in either PFK1 or PFK2 and found that the exchange in either subunit drastically reduced the sensitivity of the holoenzyme to ATP inhibition. This was accompanied by a lack of allosteric activation by AMP, fructose a,6-bisphosphate, or ammonium and an increased resistance to heat inactivation. Yeast cells carrying either one mutation or both in conjunction did not display a strong phenotype when grown on fermentable carbon sources and did not show any significant changes in intermediary metabolites. Growth on non-fermentable carbon sources was clearly impaired. The strain carrying both mutant alleles was more sensitive to Congo Red than the wildtype strain or the single mutants indicating differences in cell wall composition. In addition, we found single pfk null mutants to be less viable than wild type at different storage temperatures and a pfk2 null mutant to be temperature-sensitive for growth at 37 degreesC. The latter mutant was shown to be respiration-dependent for growth on glucose.