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The unique insert in myosin VI is a structural calcium-calmodulin binding site.
Amel Bahloul1, Guillaume Chevreux, Amber L Wells
1Structural Motility, Institut Curie Centre National de la Recherche Scientifique, Unité Mixte de Recherche 144, 26 Rue d'Ulm, 75248 Paris 05, France.
Summary
Myosin VI binds two calmodulin molecules per head, not one. A unique insert site binds calcium-bound calmodulin, structurally enabling reverse motility on actin.
Area of Science:
- Molecular and Cellular Biology
- Biochemistry
- Structural Biology
Background:
- Myosin VI is unique among myosins, possessing an inserted sequence linked to its reverse directionality and unusual motility.
- It was previously hypothesized that each myosin VI head binds one calmodulin (CaM) via an IQ motif.
Purpose of the Study:
- To investigate the stoichiometry and binding characteristics of calmodulin interaction with Myosin VI.
- To elucidate the role of the unique inserted sequence in Myosin VI's interaction with calmodulin and its functional implications.
Main Methods:
- Utilized protein truncations of Myosin VI.
- Employed electrospray ionization mass spectrometry (ESI-MS) to analyze calmodulin binding.
Main Results:
- Demonstrated that each Myosin VI head binds two calmodulin molecules.
- Identified two distinct calmodulin binding sites: one to a conventional IQ motif (potentially regulatory) and a second to a unique insertion site.
- The second calmodulin binds calcium-dependent (Ca2+-CaM) and is essential for Myosin VI's structural integrity and function.
- Regions outside the insert peptide contribute to Ca2+-CaM binding stability.
Conclusions:
- Myosin VI binds two calmodulins per head, challenging previous assumptions.
- The Ca2+-CaM bound to the unique insert acts as a structural element, not a calcium sensor.
- This structural CaM is integral to the Myosin VI converter region, repositioning the lever arm for minus-end directed motility on actin.