Multisite phosphorylation by MAPK

Reversible protein phosphorylation plays a fundamental role in signal transduction networks. Phosphorylation alters protein function by regulating enzymatic activity, stability, cellular localization, or binding partners. Over three-quarters of human proteins may be phosphorylated, with many targeted at multiple sites. Such multisite phosphorylation substantially increases the scope for modulating protein function—a protein with n phosphorylation sites has the potential to exist in 2n distinct phosphorylation states, each of which could, in theory, display modified functionality. Proteins can be substrates for several protein kinases, thereby integrating distinct signals to provide a coherent biological response. However, they can also be phosphorylated at multiple sites by a single protein kinase to promote a specific functional output that can be reversed by dephosphorylation by protein phosphatases. On page 233 of this issue, Mylona et al. (1) reveal an unexpected role for multisite phosphorylation, whereby a protein kinase progressively phosphorylates sites on a transcription factor to promote and then subsequently limit its activity independently of dephosphorylation.