Model for stathmin/OP18 binding to tubulin

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Abstract
Stathmin/OP18 is a regulatory phosphoprotein that controls microtubule (MT) dynamics. The protein does not have a defined three‐dimensional structure, although it contains three distinct regions (an unstructured N‐terminus, N: 1–44; a region with high helix propensity, H 1: 44–89; and a region with low helix propensity, H 2: 90–142). The full protein and a combination of H 1 and H 2 inhibits tubulin polymerization, while the combination of H 1 and the N‐terminus is less efficient. None of the individual three regions alone are functional in this respect. However, all of them cross‐link to α‐tubulin, but only full‐length stathmin produces high‐molecular‐weight products. Mass spectrometry analysis of α‐tubulin–stathmin/OP18 and its truncation products shows that full‐length stathmin/OP18 binds to the region around helix 10 of α‐tubulin, a region that is involved in longitudinal interactions in the MT, sequestering the dimer and possibly linking two tubulin heterodimers. In the absence of the N‐terminus, stathmin/OP18 binds to only one molecule of α‐tubulin, at the top of the free tubulin heterodimer, preventing polymerization.