Leveraging the membrane - cytoskeleton interface with myosin-1

Abstract

Class 1 myosins are small motor proteins with the ability to simultaneously bind to actin filaments and cellular membranes. Given their ability to generate mechanical force, and their high prevalence in many cell types, these molecules are well positioned to carry out several important biological functions at the interface of membrane and the actin cytoskeleton. Indeed, recent studies implicate these motors in endocytosis, exocytosis, release of extracellular vesicles, and the regulation of tension between membrane and the cytoskeleton. Many class 1 myosins also exhibit a load-dependent mechano-chemical cycle that enables them to maintain tension for long periods of time without hydrolyzing ATP. These properties put myosins-1 in a unique position to regulate dynamic membrane-cytoskeleton interactions and respond to physical forces during these events.

Figure 1. An introduction to class 1 myosins

All myosins-1 share a common structural arrangement, with a globular N-terminal motor domain (blue) that is connected to a membrane-binding tail (red) by an α-helical neck region (yellow) that binds 1–6 CaM light chains. (A) Predicted structure of a prototypical class 1 myosin, myosin-1a, based on a Phyre server homology search [79,80] against the solved structures of other myosins [80]. (B) Domain structures of the eight vertebrate class 1 myosins. The most variable domain is the neck region, with different numbers of IQ motifs. Additional IQ motifs will not only bind more CaM light chains, they also increase the length of the neck region, which may increase the size of the working stroke [27]. The two long-tailed vertebrate class 1 myosins (-1e and -1f) are frequently referred to in the literature as ‘amoeboid’ myosins-1 because they share the extended C-terminal tail structure first observed in the Acanthamoeba myosin-1 sequences. These additional domains may mediate ATP independent interactions with actin (i.e. TH2/GPA) [81] and direct binding to other proteins (i.e. SH3) [12].

Figure 2. Regulation of class 1 myosin activity

(A) The myosin-1 mechanochemical cycle. Myosin-1 bound to actin in a nucleotide-free rigor state is released when it binds ATP (T). Hydrolysis of ATP to ADP (D) and phosphate (Pi) allows myosin to bind to the actin filament, which triggers Pi release and the initial portion of the working stroke. If the motor is unloaded, ADP is rapidly released and the cycle can repeat; however, if an opposing load is present, ADP release is inhibited, and the motor stalls in a strong actin-binding state. (B) Myosin-1 is mechanically regulated. In the context of a small ensemble of myosin-1 molecules, opposing mechanical load will raise the duty ratio and thus, the fraction of motors bound to actin at steady-state [25]. Reducing load will lower the duty ratio and reduce the number of myosin-1 molecules bound at any instant. (C) Myosin-1 is regulated by Ca²⁺/CaM interactions. Studies from a number of laboratories have shown that Ca²⁺ can reduce the affinity of CaM for IQ motifs in the neck region of myosins-1a, -1b, and -1c [–34]. CaM bound to the first IQ motif (proximal to the motor domain) is likely the first to dissociate [34]. The consequences of Ca²⁺ exposure are somewhat counter-intuitive: ATPase activity increases [,,,,–34], yet motility is dramatically inhibited [,,–34]. These results may indicate that CaM binding to the first IQ motif plays a role as a clutch, to control the transmission of conformational changes in the motor domain to the C-terminus of the molecule. It is important to note that myosin-1d appears to be divergent from the other characterized class 1 myosins, as it is not expected to be load-sensitive (it lacks a two-step working stroke)[24] and shows a different response to Ca²⁺ (ATPase is inhibited by Ca²⁺) [82].

Figure 3. Cellular functions of class 1 myosins

(A) A general function of class 1 myosins is to regulate membrane tension by dynamically linking the plasma membrane to cortical actin [6]. (B) Myosins-1 function in both endo- and exo-cytosis, where their motor activity may be used to actively deform the membrane or to regulate dynamics of the actin meshwork surrounding vesicles near the cell surface [,,,,–78]. (C) Myosin-1 molecules in microvilli generate tip-ward forces that contribute to the production of vesicles carrying specific cargos [7,8]. (D) In hair cell stereocilia, myosin-1 actively maintains a constant tension across tip-links, which optimizes the sensitivity of tip-link associated cation channels [9,63,65].