Does the Actin Network Architecture Leverage Myosin-I Functions?

Conserved Phosphorylation of the Myosin1e TH1 Domain Impacts Membrane Association and Function in Yeast and Worms

Cells have an intrinsic ability to rapidly respond to environmental change to regulate cell cycle progression and membrane organisation, thereby affecting cell growth and division. The actin cytoskeleton is a highly dynamic complex of proteins that can rapidly reorganise to change the growth pattern of a cell. Class I myosins are monomeric actin-associated motor proteins that play key roles in diverse cellular functions such as tension sensing and membrane reorganisation, as well as promoting actin polymer nucleation at sites of cell growth. We have analysed the localisation and function of both C. elegans class 1 myosins, HUM-1 (Myo1e) and HUM-5 (Myo1d). Both motors are non-essential. While HUM-1 is expressed in diverse cells and tissues, HUM-5 localises exclusively to a subset of cells in the nervous system. While animals lacking hum-1 displayed a reduced maximal brood size and a delay in embryo release, deleting both hum-1 and hum-5 together shortened C. elegans lifespan. Moreover, we identified that phosphorylation of a conserved serine residue within the Myo1e TH1 domain had an impact on the localisation and function of the motor protein in both C. elegans and the fission yeast, S. pombe, indicating this modification modulates the ability of Myo1e/HUM-1 to interact with phospholipids at the plasma membrane. We conclude that TH1 domain phosphorylation plays a key role in regulating the cellular distribution and function of Myo1e motors across all eukaryotes.

MLC2: Physiological Functions and Potential Roles in Tumorigenesis

The myosin regulatory light chain 2 (MLC2) is a crucial regulator of myosin activity. Its phosphorylation, mediated by various kinases, plays a vital role in maintaining normal physiological functions in skeletal muscle, myocardium, smooth muscle, and nonmuscle cells. Moreover, MLC2 has been implicated in the development of many cancers through its phosphorylation. An increasing number of studies have shown that MLC2 may influence tumor progression by modulating cancer cell growth, migration, invasion, apoptosis, and autophagy. In this paper, we provide a concise overview of the phosphorylation regulatory mechanisms of MLC2 and its roles in both physiology and tumorigenesis. Furthermore, this study proposes potential directions for future research.

Actin-dependent α-catenin oligomerization contributes to adherens junction assembly

Classic cadherins, specifically E-cadherin in most epithelial cells, are transmembrane adhesion receptors, whose intracellular region interacts with proteins, termed catenins, forming the cadherin-catenin complex (CCC). The cadherin ectodomain generates 2D adhesive clusters (E-clusters) through cooperative trans and cis interactions, while catenins anchor the E-clusters to the actin cytoskeleton. How these two types of interactions are coordinated in the formation of specialized cell-cell adhesions, adherens junctions (AJ), remains unclear. Here, we focus on the role of the actin-binding domain of α-catenin (αABD) by showing that the interaction of the αABD with actin generates actin-bound linear CCC oligomers (CCC/actin strands) incorporating up to six CCCs. This actin-driven CCC oligomerization, which is cadherin ectodomain independent, preferentially occurs along the actin cortex enriched with key basolateral proteins, myosin-1c, scribble, and DLG1. In cell-cell contacts, the CCC/actin strands integrate with the E-clusters giving rise to the composite oligomers, E/actin clusters. Targeted inactivation of strand formation by point mutations emphasizes the importance of this oligomerization process for blocking intercellular protrusive membrane activity and for coupling AJs with the actomyosin-derived tensional forces.

Actin-membrane linkers: Insights from synthetic reconstituted systems

At the cell surface, the actin cytoskeleton and the plasma membrane interact reciprocally in a variety of processes related to the remodeling of the cell surface. The actin cytoskeleton has been known to modulate membrane organization and reshape the membrane. To this end, actin-membrane linking molecules play a major role in regulating actin assembly and spatially direct the interaction between the actin cytoskeleton and the membrane. While studies in cells have provided a wealth of knowledge on the molecular composition and interactions of the actin-membrane interface, the complex molecular interactions make it challenging to elucidate the precise actions of the actin-membrane linkers at the interface. Synthetic reconstituted systems, consisting of model membranes and purified proteins, have been a powerful approach to elucidate how actin-membrane linkers direct actin assembly to drive membrane shape changes. In this review, we will focus only on several actin-membrane linkers that have been studied by using reconstitution systems. We will discuss the design principles of these reconstitution systems and how they have contributed to the understanding of the cellular functions of actin-membrane linkers. Finally, we will provide a perspective on future research directions in understanding the intricate actin-membrane interaction.

Discovery of the first unconventional myosin: Acanthamoeba myosin-I

Having characterized actin from Acanthamoeba castellanii (Weihing and Korn, Biochemistry, 1971, 10, 590-600) and knowing that myosin had been isolated from the slime mold Physarum (Hatano and Tazawa, Biochim. Biophys. Acta, 1968, 154, 507-519; Adelman and Taylor, Biochemistry, 1969, 8, 4976-4988), we set out in 1969 to find myosin in Acanthamoeba. We used K-EDTA-ATPase activity to assay myosin, because it is a unique feature of muscle myosins. After slightly less than 3 years, we purified a K-EDTA ATPase that interacted with actin. Actin filaments stimulated the Mg-ATPase activity of the crude enzyme, but this was lost with further purification. Recombining fractions from the column where this activity was lost revealed a "cofactor" that allowed actin filaments to stimulate the Mg-ATPase of the purified enzyme. The small size of the heavy chain and physical properties of the purified myosin were unprecedented, so many were skeptical, assuming that our myosin was a proteolytic fragment of a larger myosin similar to muscle or Physarum myosin. Subsequently our laboratories confirmed that Acanthamoeba myosin-I is a novel unconventional myosin that interacts with membrane lipids (Adams and Pollard, Nature, 1989, 340 (6234), 565-568) and that the cofactor is a myosin heavy chain kinase (Maruta and Korn, J. Biol. Chem., 1977, 252, 8329-8332). Phylogenetic analysis (Odronitz and Kollmar, Genome Biology, 2007, 8, R196) later established that class I myosin was the first myosin to appear during the evolution of eukaryotes.