Recently we have shown that the peptidyl-prolyl
cis/
trans isomerase parvulin 17 (Par17) interacts with tubulin in a GTP-dependent manner, thereby promoting the formation of microtubules. Microtubule assembly is regulated by Ca
2+-loaded calmodulin (Ca
2+/CaM) both in the intact cell and under
in vitro conditions via direct interaction with microtubule-associated proteins. Here we provide the first evidence that Ca
2+/CaM interacts also with Par17 in a physiologically relevant way, thus preventing Par17-promoted microtubule assembly. In contrast, parvulin 14 (Par14), which lacks only the first 25 N-terminal residues of the Par17 sequence, does not interact with Ca
2+/CaM, indicating that this interaction is exclusive for Par17. Pulldown experiments and chemical shift perturbation analysis with
15N-labeled Par17 furthermore confirmed that calmodulin (CaM) interacts in a Ca
2+-dependent manner with the Par17 N terminus. The reverse experiment with
15N-labeled Ca
2+/CaM demonstrated that the N-terminal Par17 segment binds to both CaM lobes simultaneously, indicating that Ca
2+/CaM undergoes a conformational change to form a binding channel between its two lobes, apparently similar to the structure of the CaM-smMLCK
796–815 complex.
In vitro tubulin polymerization assays furthermore showed that Ca
2+/CaM completely suppresses Par17-promoted microtubule assembly. The results imply that Ca
2+/CaM binding to the N-terminal segment of Par17 causes steric hindrance of the Par17 active site, thus interfering with the Par17/tubulin interaction. This Ca
2+/CaM-mediated control of Par17-assisted microtubule assembly may provide a mechanism that couples Ca
2+ signaling with microtubule function.
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