Phosphoinositide 3-kinase: the protein kinase that time forgot.

Class I phosphoinositide 3-kinases were originally characterized as lipid kinases, although more than 10 years ago they were also found to phosphorylate protein serine residues. However, while there is a vast amount of data on the function of this lipid kinase activity, relatively little is known about the function of the protein kinase activity. We discuss the evidence that suggests that the protein kinase activity of phosphoinositide 3-kinases mediates important signalling functions in cells.