Abstract
The long neck of unconventional myosin V is composed of six tandem "IQ motifs," which are fully occupied by calmodulin (CaM) in the absence of calcium. Calcium regulates the activity, the folded-to-extended conformational transition, and the processive run length of myosin V, and thus, it is important to understand how calcium affects CaM binding to the IQ motifs. Here we used electron cryomicroscopy together with computer-based docking of crystal structures into three-dimensional reconstructions of actin decorated with a motor domain-two IQ complex to provide an atomic model of myosin V in the presence of calcium. Calcium causes a major rearrangement of the bound CaMs, dissociation of CaM bound to IQ motif 2, and propagated changes in the motor domain. Tryptophan fluorescence spectroscopy showed that calcium-CaM binds to IQ motifs 1, 3, and 5 in a different conformation than apoCaM. Proteolytic cleavage was consistent with CaM preferentially dissociating from the second IQ motif. The enzymatic and mechanical functions of myosin V can, therefore, be modulated both by calcium-dependent conformational changes of bound CaM as well as by CaM dissociation.
Highlights
Myosin V is a double-headed, processive motor involved in transport of organelles, mRNA, and membrane trafficking
Atomic Model of Myosin V Bound to Actin in the Presence of Calcium—Electron cryomicroscopy and helical reconstruction techniques were used to generate three-dimensional maps for a myosin V motor domain-2IQ (MD-2IQ) construct bound to actin in the presence of calcium and in the absence of nucleotide (Fig. 1)
This study provides the first structural information on how calcium binding to CaM affects the actin-bound structure of myosin V
Summary
Myosin V is a double-headed, processive motor involved in transport of organelles, mRNA, and membrane trafficking (for review, see Ref. 1). We have previously developed atomic models for a myosin V motor domain-two IQ complex bound to actin in the absence of calcium and in several nucleotide states that mimic the progression through the ATPase cycle [12]. A high resolution crystal structure of apoCaM bound to the first two IQ motifs of murine myosin V was solved recently, which serves as a model for the entire neck of mammalian myosin V [13]. It was unclear how apoCaM could bind an IQ motif, because unbound apoCaM exists with both the N-lobe and the C-lobe in a closed, non-gripping conformation. IQ Motifs of Myosin V interactions with apoCaM, the overall lobe conformation (semi-open C-lobe, closed N-lobe) remains the same
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