Myosin light chains are key regulators of class 1 myosins and typically comprise two domains, with calmodulin being the archetypal example. They bind IQ motifs within the myosin neck region and amplify conformational changes in the motor domain. A single lobe light chain, myosin light chain C (MlcC), was recently identified and shown to specifically bind to two sequentially divergent IQ motifs of the Dictyostelium myosin-1C. To provide a molecular basis of this interaction, the structures of apo-MlcC and a 2:1 MlcCâ
myosin-1C neck complex were determined. The two non-functional EF-hand motifs of MlcC pack together to form a globular four-helix bundle that opens up to expose a central hydrophobic groove, which interacts with the N-terminal portion of the divergent IQ1 and IQ2 motifs. The N-and C-terminal regions of MlcC make critical contacts that contribute to its specific interactions with the myosin-1C divergent IQ motifs, which are contacts that deviate from the traditional mode of calmodulin-IQ recognition.The class 1 myosins (myosin-1) are a widely expressed family of single-headed, non-filament-forming, membrane-binding myosins (1-3). The myosin-1 heavy chain consists of a highly conserved N-terminal motor domain with sites for binding actin and for ATP hydrolysis, a light chain-binding domain (LCBD) 5 that acts as the motor's lever arm, and a C-terminal tail. The myosin-1 tail contains a tail homology 1 (TH1) domain that interacts with acidic phospholipids and, in some cases, also contains a glycine-and proline-rich TH2 domain that binds filamentous actin and an Src homology 3 domain that forms complexes with proteins that stimulate actin filament assembly (4 -10). Myosin-1 is able to cross-link the plasma membrane to the underlying actin cytoskeleton and plays a direct role in the control of cortical and membrane tension (11,12). These motor proteins can also drive vesicle trafficking, pseudopod retraction, and the uptake of particles and fluids by phagocytosis and micropinocytosis (13-16).The myosin-1 LCBD includes one or more IQ motifs, which are âŁ-helical sequences between 18 and 25 residues in length that terminate with a loosely conserved IQXXXRGXXXR consensus sequence (17, 18). IQ motifs bind calmodulin (CaM) or CaM-related light chains (LCs) in the absence of Ca 2Ï© . The LCs stabilize the âŁ-helical structure of the LCBD, allowing it to function as a rigid swinging lever arm to amplify ATP-dependent conformational changes that originate within the motor domain (19). The LCs are also important regulatory sites that interact with the motor domain to influence mechanochemical properties such as step size, motility rate, and force sensing (1,20,21).The social amoeba Dictyostelium discoideum has been used extensively to study myosin-1 structure, function, and regulation. The seven Dictyostelium myosin-1 isozymes include three with short tails that contain only a TH1 domain (myosin-1A, -1E, and -1F), three with long tails that have TH1, TH2, and Src homology 3 domains (myosin-1B, -1C, and -1D), and one ...