New Mediator of Phosphorylation

CELLULAR SIGNALING

Move over, ATP: scientists have turned up a second cellular molecule that donates phosphate groups to proteins.

Cells tack phosphate groups onto proteins as their primary means of communicating information. The sole source of phosphate groups has long been thought to be adenosine triphosphate, ATP. Now, a group led by Solomon H. Snyder of Johns Hopkins University School of Medicine has shown that another cellular molecule--diphosphoinositol pentakisphosphate, or IP₇--can act as the phosphate donor.

IP₇ was first identified in the early 1990s by researchers in the U.S. and Europe, who drew attention to the molecule's high-energy pyrophosphate bond and the fact that it's rapidly utilized by cells. Noting that ATP shared these same characteristics, Snyder began to wonder whether IP₇ could also phosphorylate proteins.

Hoping to prove his hunch, Snyder's lab tracked down the enzyme that adds the seventh phosphate to IP₇'s precursor. Postdoc Adolfo Saiardi has used this enzyme to make ³²P-radiolabeled IP₇. When he and postdoc Rashna Bhandari mixed the labeled IP₇ with mouse cell extracts, they found that the molecule transfers its ³²P-labeled phosphate group to a whole slew of proteins [Science, 306, 2101 (2004)].

"We think IP₇ phosphorylation of proteins is as universal as ATP phosphorylation," Snyder says. The two donors differ in a major way, however: ATP's donated phosphate is normally attached to the protein target by an enzyme mediator known as a kinase, whereas IP₇ phosphorylates proteins without any enzymatic help.

Stephen B. Shears of NIH's National Institute of Environmental & Health Sciences calls the team's in vitro experiments "elegant," noting that if the findings turn out to be correct, Snyder and colleagues "will have discovered a completely new signaling paradigm." Shears cautions that the authors have yet to prove that IP₇-mediated protein phosphorylation occurs in living cells.

Snyder and his team are working to do just that. They're also trying to determine where and how IP₇ phosphorylates proteins. Each of the IP₇-phosphorylated proteins that they've found so far contains a string of serines. But they couldn't yet pinpoint which serine is phosphorylated because they don't have sufficient quantities of IP₇ for mass spectrometric studies. Snyder is working with University of Utah chemist Glenn D. Prestwich to develop a versatile synthetic route to IP₇, based in part on one devised by J. R. Falck of the University of Texas Southwestern Medical Center.

Snyder speculates that IP₇ may be phosphorylating an existing phosphoserine residue, creating an unheard-of pyrophosphorylated serine. Such a novel modification could explain why cells might rely on two different phosphate donors, he says. "I think we may find that IP₇-mediated protein phosphorylation transmits a different kind of information."