Aurora A phosphorylation-induced interaction of TACC3 and clathrin coordinates adjacent domains in each protein to create a microtubule-binding interface, whereas a distinct site in TACC3 recruits ch-TOG to mitotic spindles.

Myosin-binding protein C (MyBP-C) is a multidomain protein present in the thick filaments of striated muscles and is involved in both sarcomere formation and contraction regulation. The latter function is believed to be located at the N terminus, which is close to the motor domain of myosin. The cardiac isoform of MyBP-C is linked to hypertrophic cardiomyopathy. Here, we use NMR spectroscopy and biophysical and biochemical assays to study the three-dimensional structure and interactions of the cardiac-specific Ig-like domain C0, a part of cardiac MyBP-C of which little is known. The structure confirmed that C0 is a member of the IgI class of proteins, showing many of the characteristic features of this fold. Moreover, we identify a novel interaction between C0 and the regulatory light chain of myosin, thus placing the N terminus of the protein in proximity to the motor domain of myosin. This novel interaction is disrupted by several cardiomyopathy-linked mutations in the MYBPC3 gene. These results provide new insights into how cardiac MyBP-C incorporates in the sarcomere and how it can contribute to the regulation of muscle contraction.

Myosin binding protein C (MyBP-C) is a thick filament protein involved in the regulation of muscle contraction. Mutations in the gene for MyBP-C are the second most frequent cause of hypertrophic cardiomyopathy. MyBP-C binds to myosin with two binding sites, one at its C-terminus and another at its N-terminus. The N-terminal binding site, consisting of immunoglobulin domains C1 and C2 connected by a flexible linker, interacts with the S2 segment of myosin in a phosphorylation-regulated manner. It is assumed that the function of MyBP-C is to act as a tether that fixes the S1 heads in a resting position and that phosphorylation releases the S1 heads into an active state. Here, we report the structure and binding properties of domain C1. Using a combination of site-directed mutagenesis and NMR interaction experiments, we identified the binding site of domain C1 in the immediate vicinity of the S1–S2 hinge, very close to the light chains. In addition, we identified a zinc binding site on domain C1 in close proximity to the S2 binding site. Its zinc binding affinity (Kd of approximately 10–20 μM) might not be sufficient for a physiological effect. However, the familial hypertrophic cardiomyopathy-related mutation of one of the zinc ligands, glutamine 210 to histidine, will significantly increase the binding affinity, suggesting that this mutation may affect S2 binding. The close proximity of the C1 binding site to the hinge, the light chains and the S1 heads also provides an explanation for recent observations that (a) shorter fragments of MyBP-C unable to act as a tether still have an effect on the actomyosin ATPase and (b) as to why the myosin head positions in phosphorylated wild-type mice and MyBP-C knockout mice are so different: Domain C1 bound to the S1–S2 hinge is able to manipulate S1 head positions, thus influencing force generation without tether. The potentially extensive extra interactions of C1 are expected to keep it in place, while phosphorylation dislodges the C1–C2 linker and domain C2. As a result, the myosin heads would always be attached to a tether that has phosphorylation-dependent length regulation.

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Nebulin, a giant protein (molecular mass 800 kDa) specific for the skeletal muscle of vertebrates, has been suggested to be involved in the length regulation of the thin filament as a ‘molecular ruler’. Despite its size, nebulin appears to be composed mainly of small repeats of approximately 35 amino acids. We have characterized in this study the conformational and functional properties of single repeats. Complete repeats were found to bind to F‐actin while a truncated one did not. One repeat is therefore the smallest unit for nebulin‐‐actin interaction. Circular dichroism and nuclear magnetic resonance spectra measured for the peptides in water indicated a transient helical conformation. The folded region is located for them all around the conserved sequence SDxxYK. The helical conformation is strongly stabilized by anionic detergents and trifluoroethanol while uncharged or positively charged detergents have no effect. Since the surface of the actin filament is known to contain clusters of negative charges, anionic detergents may mimic the effect of an actin environment. 3D structures were calculated for three representative peptides in SDS. In vivo, the nebulin helices should form a complex with the actin filament. Based on the assumed importance of charge interactions between nebulin and actin, we propose a model for the structure of the F‐actin‐nebulin complex in vivo. According to that, two nebulin molecules occupy symmetrical positions along the central cleft of the actin filament bridging the two strands of the actin two‐start helix. The consistency of this model with experimental data is discussed.

The thick filament protein myosin-binding protein-C shows a highly modular architecture, with the C-terminal region responsible for tethering to the myosin and titin backbone of the thick filament. The N-terminal region shows the most significant differences between cardiac and skeletal muscle isogenes: an entire Ig-domain (C0) is added, together with highly regulated phosphorylation sites between Ig domains C1 and C2. These structural and functional differences at the N-terminus reflect important functions in cardiac muscle regulation in health and disease. Alternative interactions of this part of MyBP-C with the head-tail (S1-S2) junction of myosin or to actin filaments have been proposed, but with conflicting experimental evidence. The regulation of myosin or actin interaction by phosphorylation of the cardiac MyBP-C N-terminus may play an additional role in length-dependent contraction regulation. We discuss here the evidence for these proposed interactions, considering the required properties of MyBP-C, the way in which they may be regulated in muscle contraction and the way they might be related to heart disease. We also attempt to shed some light on experimental pitfalls and future strategies.

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The titin domains and several of the immunoglobulin-like domains from other modular muscle proteins are highly conserved at the positions corresponding to the hydrophobic core of M5. Our results indicate that it may be possible to use the structure of M5 as a molecular template to model most of the other immunoglobulin-like domains in muscle titin.