The glycogen-bound polyphosphate kinase from Sulfolobus acidocaldarius is actually a glycogen synthase

Inorganic polyphosphate (polyp) is obtained by the polymerization of the te rminal phosphate of ATP through the action of the enzyme polyphosphate kina se (PPK). Despite the presence of polyp in every living cell, a gene homolo gous to that of known PPKs is missing from the currently sequenced genomes of Eukarya, Archaea, and several bacteria. To further study the metabolism of polyp in Archaea, we followed the previously published purification proc edure for a glycogen-bound protein of 57 kDa with PPK as well as glycosyl t ransferase (GT) activities from Sulfolobus acidocaldarius (R. Skorko, J. Os ipiuk, and K. O. Stetter, J. Bacteriol. 171:5162-5164, 1989). In spite of u sing recently developed specific enzymatic methods to analyze polyp, we cou ld not reproduce the reported PPK activity for the 57-kDa protein and the p olyp presumed to be the product of the reaction most likely corresponded to glycogen-bound ATP under our experimental conditions. Furthermore, no PPK activity was found associated to any of the proteins bound to the glycogen- protein complex. We cloned the gene corresponding to the 57-kDa protein by using reverse genetics and functionally characterized it. The predicted pro duct of the gene did not show similarity to any described PPK but to archae al and bacterial glycogen synthases instead. In agreement with these result s, the recombinant protein showed only GT activity. Interestingly, the CT f rom S. acidocaldarius was phosphorylated in vivo. In conclusion, our result s convincingly demonstrate that the glycogen-protein complex of S. acidocal darius does not contain a PPK activity and that what was previously reporte d as being glycogen-bound PPK is a bacterial enzyme-like thermostable glyco gen synthase.