For the first time, scientists have described a way for cells to add phosphate groups to proteins that doesn't involve an ATP donor. A group at Johns Hopkins University in Baltimore synthesized diphosphoinositol pentakisphosphate (IP7) and radiolabeled the putative donor pyrophosphate. In mouse brain and yeast extracts, gel electrophoresis and autoradiography revealed that the radiolabeled IP7 phosphorylated as many proteins as radiolabeled ATP.1 "IP7 looks to act quite universally," says Solomon Snyder, lead author who had previously hinted that inositol pyrophosphates might play such roles.2 "And while the numbers are similar, it looks as if IP7 and ATP are phosphorylating different proteins."
"What's also remarkable is we have evidence not just of phosphorylation of proteins, but of pyrophosphorylation, adding a phosphate on top of a phosphate. That's completely unheard of," Snyder says. IP7 phosphorylation appears selective for eukaryotic organisms, with none seen in