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The unique insert in myosin VI is a structural calcium–calmodulin binding site

6 avr. 2004Proceedings of the National Academy of Sciences

DOI : 10.1073/pnas.0306892101

Auteurs

Amel Bahloul, Guillaume Chevreux, Amber L. Wells, Davy Martin, Jocelyn Nolt, Zhaohui Yang, Li-Qiong Chen, Noëlle Potier, Alain Van Dorsselaer, Steve Rosenfeld, Anne Houdusse, H. Lee Sweeney

Résumé

Myosin VI contains an inserted sequence that is unique among myosin superfamily members and has been suggested to be a determinant of the reverse directionality and unusual motility of the motor. It is thought that each head of a two-headed myosin VI molecule binds one calmodulin (CaM) by means of a single “IQ motif”. Using truncations of the myosin VI protein and electrospray ionization(ESI)-MS, we demonstrate that in fact each myosin VI head binds two CaMs. One CaM binds to a conventional IQ motif either with or without calcium and likely plays a regulatory role when calcium binds to its N-terminal lobe. The second CaM binds to a unique insertion between the converter region and IQ motif. This unusual CaM-binding site normally binds CaM with four Ca 2+ and can bind only if the C-terminal lobe of CaM is occupied by calcium. Regions of the MD outside of the insert peptide contribute to the Ca 2+ –CaM binding, as truncations that eliminate elements of the MD alter CaM binding and allow calcium dissociation. We suggest that the Ca 2+ -CaM bound to the unique insert represents a structural CaM, and not a calcium sensor or regulatory component of the motor. This structure is likely an integral part of the myosin VI “converter” region and repositions the myosin VI “lever arm” to allow reverse direction (minus-end) motility on actin.

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