Phosphorylation disrupts the central helix in Op18/stathmin and suppressesbinding to tubulin

Protein phosphorylation represents a ubiquitous control mechanism in living cells. The structural prerequisites and consequences of this important pos t-translational modification, however, are poorly understood. Oncoprotein 1 8/stathmin (Op18) is a globally disordered phosphoprotein that is involved in the regulation of the microtubule (MT) filament system. Here we document that phosphorylation of Ser63, which is located within a helix initiation site in Op18, disrupts the transiently formed amphipathic helix. The phosph oryl group reduces tubulin binding 10-fold and suppresses the MT polymeriza tion inhibition activity of Op18's C-terminal domain. Op18 represents an ex ample where phosphorylation occurs within a regular secondary structural el ement. Together, our findings have implications for the prediction of phosp horylation sites and give insights into the molecular behavior of a globall y disordered protein.