PDBsum entry 2bki

Motor protein/metal-binding protein

PDB id: 2bki

Contents

Protein chains

824 a.a.

145 a.a.

78 a.a.

Ligands

SO4

Metals

_CA ×4

Waters ×49

References listed in PDB file

Key reference

Title

The structure of the myosin VI motor reveals the mechanism of directionality reversal.

Authors

J.Ménétrey, A.Bahloul, A.L.Wells, C.M.Yengo, C.A.Morris, H.L.Sweeney, A.Houdusse.

Ref.

Nature, 2005, 435, 779-785. [DOI no: 10.1038/nature03592]

PubMed id

15944696

Abstract

Here we solve a 2.4-A structure of a truncated version of the reverse-direction myosin motor, myosin VI, that contains the motor domain and binding sites for two calmodulin molecules. The structure reveals only minor differences in the motor domain from that in plus-end directed myosins, with the exception of two unique inserts. The first is near the nucleotide-binding pocket and alters the rates of nucleotide association and dissociation. The second unique insert forms an integral part of the myosin VI converter domain along with a calmodulin bound to a novel target motif within the insert. This serves to redirect the effective 'lever arm' of myosin VI, which includes a second calmodulin bound to an 'IQ motif', towards the pointed (minus) end of the actin filament. This repositioning largely accounts for the reverse directionality of this class of myosin motors. We propose a model incorporating a kinesin-like uncoupling/docking mechanism to provide a full explanation of the movements of myosin VI.

Figure 4.
Figure 4: A new CaM-binding motif that interacts strongly with 4Ca^2+-CaM. a, The overall conformation and polarity of the insert-2 -CaM complex (new 1-6-14 motif) is compared with those observed when CaM interacts with myosin light chain kinase (MLCK) (classic 1-8-14 motif) (target peptides superimposed). Note that in both cases the C-lobe of CaM in an open conformation grips the N-terminal region of the target sequence, largely through the first anchoring hydrophobic residue (W793/W800). b, In contrast, comparison of the N-lobes (helices A and D superimposed) shows differences in their conformation (closure differs by 20°) and in the target peptide position (note the 14th anchoring residue position) within the lobe. Note that the sixth anchoring residue of the 1-6-14 motif (W798) interacts strongly with both lobes of CaM (helices A and H). c, Sequence comparison of the two CaM-binding motifs. The letters n, c and b indicate whether each residue of these motifs interacts with the N-lobe, the C-lobe or both lobes of CaM, respectively.

Figure 5.
Figure 5: Directionality of movement and power stroke in myosin motors. a, b, Schematic drawings of the myosin VI (a) and myosin V (b) structural models (see Methods) before and after force generation. Similar colours to those in Fig. 1 are used. Note in particular how conformational changes in the relay (yellow) and SH1 helix (red) lead to the rotation (black arrow) of the converter (green). The red arrow represents the predicted F-actin displacement (stroke) for these models; the green arrow indicates the converter contribution for this stroke. c, For reverse myosin I, the solid arrow indicates the stroke that would be produced with a lever arm of about 4 nm (that equivalent to one IQ motif) and the dotted arrow corresponds to the stroke generated by an approximately 14-nm lever arm as described for this engineered motor28. d, e, Two mechanisms could account for the 12-nm stroke of the myosin VI MD^ins2IQ. If the converter remains coupled to the motor domain (d), it must adopt an orientation that differs by about 90° from that found for plus-end motors in the pre-powerstroke state. Alternatively, unwinding of the SH1 helix in the weak actin-binding states would decouple the converter from the motor domain (e). In this case, the relay -converter interactions would be maintained but the relay helix would not be bent in the pre-powerstroke state because steric clashes with the SH1 helix are eliminated. Thus, the converter would be biased towards the plus end of the actin filament. Recoupling of the converter to the motor domain would occur on strong binding to actin.

The above figures are reprinted by permission from Macmillan Publishers Ltd: Nature (2005, 435, 779-785) copyright 2005.