A discrete signaling function for an inositol pyrophosphate.

Inositol pyrophosphates were, until recently, without clearly defined functions. Two recent papers in Science have now clearly defined a function for an IP(7) pyrophosphate (inositol hexaphosphate with one pyrophosphate) that is the product of the enzyme encoded by the Vip1 gene in Saccharomyces cerevisiae. This IP(7) with a pyrophosphate tentatively assigned to be on either the 4 or 6 position is a cofactor that is required for inactivating the cyclin-cyclin-dependent kinase complex of Pho80, Pho81, and Pho85. Inhibition of the kinase results in the nuclear translocation of Pho4, which is a transcription factor that promotes expression of genes required for phosphate assimilation under conditions of low phosphate. When grown in low-phosphate media, IP(7) accumulates, which leads to the expression of genes involved in the acquisition of phosphate.