Structure of the Small Dictyostelium discoideum Myosin Light Chain MlcB Provides Insights into MyoB IQ motif Recognition [Cell Biology]

May 1st, 2014 by Liburd, J., Chitayat, S., Crawley, S. W., Munro, K., Miller, E., Denis, C. M., Spencer, H. L., Cote, G. P., Smith, S. P.

Dictyostelium discoideum MyoB is a class I myosin involved in the formation and retraction of membrane projections, cortical tension generation, membrane recycling, and phagosome maturation. The MyoB-specific, single-lobe EF-hand light chain MlcB binds the sole IQ motif of MyoB with sub-micromolar affinity in the absence and presence of Ca2+. However, the structural features of this novel myosin light chain and its interaction with its cognate IQ motif remain uncharacterized. Here, we describe the NMR-derived solution structure of apo-MlcB, which displays a globular four-helix bundle. Helix 1 adopts a unique orientation when compared to the apo-states of the EF-hand calcium-binding proteins calmodulin, S100B, and calbindin D9k. NMR-based chemical shift perturbation mapping identified a hydrophobic MyoB IQ binding surface that involves amino acid residues in helices I and IV, and the functional N-terminal Ca2+-binding loop; a site also maintained when MlcB adopts the holo-state. Complementary mutagenesis and binding studies indicated that residues Ile-701, Phe-705, and Trp-708 of the MyoB IQ motif are critical for recognition of MlcB, which together allowed the generation of a structural model of the apo-MlcB:MyoB IQ complex. We conclude that the mode of IQ motif recognition by the novel single-lobe MlcB differs considerably from that of stereotypical bilobal light chains such as calmodulin.