Structure of the Single-lobe Myosin Light Chain C in Complex with the Light Chain-binding Domains of Myosin-1C Provides Insights into Divergent IQ Motif Recognition

Abstract

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 MlcC⅐myosin-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 ␣-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 2ϩ . The LCs stabilize the ␣-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 ...

Calmodulin and Calmodulin Binding Proteins in Dictyostelium: A Primer

“…The two IQ motifs present in both MyoA and MyoE both bind CaM, while MyoC does not bind CaM but instead binds the light chain MlcC [34]. The non-traditional binding between MyoC and MlcC is mainly due to specific hydrophobic residues (Phe-718, Tyr-736) that have replaced arginine found in traditional IQ motifs [35]. Although myosin-I (e.g., myosin-1A) isozymes have been shown to use CaM as a light chain in vertebrates, MyoA and MyoE are the first Dictyostelium forms of myosin-I to be shown to use CaM in this manner.…”

Calmodulin‐mediated events during the life cycle of the amoebozoan Dictyostelium discoideum

“…Myosin light chain C (MlcC), a regulator of myosin-1C (Myo1C), is one of the best characterized CaMBPs in D. discoideum (e.g. Langelaan et al, 2016). It binds to apo-CaM via the Ca 2+ -independent IQ motif (summarized in Catalano & O'Day, 2008).…”

Class I myosins: Highly versatile proteins with specific functions in the immune system

“…The area between the head and the tail contains a domain known as “neck,” which is formed by a variable number of IQ motifs and is characterized by the IQXXXRGXXXR sequence (I‐isoleucine; Q‐glutamine) . The IQ motif, functions as a binding site for calmodulin (CaM), and CaM‐like proteins …”

“…is characterized by the IQXXXRGXXXR sequence (I-isoleucine; Qglutamine). 12,13 The IQ motif, functions as a binding site for calmodulin (CaM), and CaM-like proteins. 14…”