F-actin serves as a track for myosin's motor functions and activates its ATPase activity by several orders of magnitude, enabling actomyosin to produce effective force against load. Although actin activation is a ubiquitous property of all myosin isoforms, the molecular mechanism and physiological role of this activation are unclear. Here we describe a conserved actin-binding region of myosin named the 'activation loop', which interacts with the N-terminal segment of actin. We demonstrate by biochemical, biophysical and in vivo approaches using transgenic Caenorhabditis elegans strains that the interaction between the activation loop and actin accelerates the movement of the relay, stimulating myosin's ATPase activity. This interaction results in efficient force generation, but it is not essential for the unloaded motility. We conclude that the binding of actin to myosin's activation loop specifically increases the ratio of mechanically productive to futile myosin heads, leading to efficient muscle contraction.
The effect of trehalose on the interaction of human serum albumin (HSA) with neutral and negatively charged small unilamellar vesicles (SUVs) composed of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) or of mixtures of DMPC (19:1 w/w) with 1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol (DMPG) was studied by time-resolved fluorescence and dynamic light scattering measurements. The results are interpreted with supporting nonbond calculations describing the nonbond domains most likely to be involved in the protein-SUV interaction. In the absence of trehalose, lifetime measurements of the single Trp of HSA are indicative of two different SUV-HSA associative mechanisms depending on the [lipid]/[HSA] concentration ratios. At low ratios, depletion of phospholipid molecules from vesicles by HSA occurs independently of the lipid composition of the vesicles via favorable hydrophobic contacts. At higher ratios, vesicle-HSA assocation is favored by electrostatic interactions for the negatively charged SUVs. For neutral SUVs, hydrophobically driven penetration of HSA is proposed. All association mechanisms are damped in the presence of trehalose, due to its capacity to coat the interacting surfaces. The results of dynamic light scattering experiments clearly show that the aging of the liposomes is dependent on the lipid composition. The aging of DMPC vesicles is faster and not affected by the presence of either HSA or trehalose. The aging of DMPC/DMPG liposomes is more pronounced in the presence of HSA. These SUVs are stabilized by trehalose through different mechanisms depending on whether they are covered by HSA or not.
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