Use of pyrene-labelled actin to probe actin-myosin interactions: kinetic and equilibrium studies

Mechanistic analysis of actin-binding compounds that affect the kinetics of cardiac myosin-actin interaction

Actin-myosin mediated contractile forces are crucial for many cellular functions, including cell motility, cytokinesis, and muscle contraction. We determined the effects of ten actin-binding compounds on the interaction of cardiac myosin subfragment 1 (S1) with pyrene-labeled F-actin (PFA). These compounds, previously identified from a small-molecule high-throughput screen (HTS), perturb the structural dynamics of actin and the steady-state actin-activated myosin ATPase activity. However, the mechanisms underpinning these perturbations remain unclear. Here we further characterize them by measuring their effects on PFA fluorescence, which is decreased specifically by the strong binding of myosin to actin. We measured these effects under equilibrium and steady-state conditions, and under transient conditions, in stopped-flow experiments following addition of ATP to S1-bound PFA. We observed that these compounds affect early steps of the myosin ATPase cycle to different extents. They increased the association equilibrium constant K₁ for the formation of the strongly bound collision complex, indicating increased ATP affinity for actin-bound myosin, and decreased the rate constant k₊₂ for subsequent isomerization to the weakly bound ternary complex, thus slowing the strong-to-weak transition that actin-myosin interaction undergoes early in the ATPase cycle. The compounds' effects on actin structure allosterically inhibit the kinetics of the actin-myosin interaction in ways that may be desirable for treatment of hypercontractile forms of cardiomyopathy. This work helps to elucidate the mechanisms of action for these compounds, several of which are currently used therapeutically, and sets the stage for future HTS campaigns that aim to discover new drugs for treatment of heart failure.

N-terminal splicing extensions of the human MYO1C gene fine-tune the kinetics of the three full-length myosin IC isoforms

The MYO1C gene produces three alternatively spliced isoforms, differing only in their N-terminal regions (NTRs). These isoforms, which exhibit both specific and overlapping nuclear and cytoplasmic functions, have different expression levels and nuclear-cytoplasmic partitioning. To investigate the effect of NTR extensions on the enzymatic behavior of individual isoforms, we overexpressed and purified the three full-length human isoforms from suspension-adapted HEK cells. MYO1C^C favored the actomyosin closed state (AM^C), MYO1C¹⁶ populated the actomyosin open state (AM^O) and AM^C equally, and MYO1C³⁵ favored the AM^O state. Moreover, the full-length constructs isomerized before ADP release, which has not been observed previously in truncated MYO1C^C constructs. Furthermore, global numerical simulation analysis predicted that MYO1C³⁵ populated the actomyosin·ADP closed state (AMD^C) 5-fold more than the actomyosin·ADP open state (AMD^O) and to a greater degree than MYO1C^C and MYO1C¹⁶ (4- and 2-fold, respectively). On the basis of a homology model of the 35-amino acid NTR of MYO1C³⁵ (NTR³⁵) docked to the X-ray structure of MYO1C^C, we predicted that MYO1C³⁵ NTR residue Arg-21 would engage in a specific interaction with post-relay helix residue Glu-469, which affects the mechanics of the myosin power stroke. In addition, we found that adding the NTR³⁵ peptide to MYO1C^C yielded a protein that transiently mimics MYO1C³⁵ kinetic behavior. By contrast, NTR³⁵, which harbors the R21G mutation, was unable to confer MYO1C³⁵-like kinetic behavior. Thus, the NTRs affect the specific nucleotide-binding properties of MYO1C isoforms, adding to their kinetic diversity. We propose that this level of fine-tuning within MYO1C broadens its adaptability within cells.

Kinetic signatures of myosin-5B, the motor involved in microvillus inclusion disease

Myosin-5B is a ubiquitous molecular motor that transports cargo vesicles of the endomembrane system in intracellular recycling pathways. Myosin-5B malfunction causes the congenital enteropathy microvillus inclusion disease, underlining its importance in cellular homeostasis. Here we describe the interaction of myosin-5B with F-actin, nucleotides, and the pyrazolopyrimidine compound myoVin-1. We show that single-headed myosin-5B is an intermediate duty ratio motor with a kinetic ATPase cycle that is rate-limited by the release of phosphate. The presence of a second head generates strain and gating in the myosin-5B dimer that alters the kinetic signature by reducing the actin-activated ADP release rate to become rate-limiting. This kinetic transition into a high-duty ratio motor is a prerequisite for the proposed transport function of myosin-5B in cellular recycling pathways. Moreover, we show that the small molecule compound myoVin-1 inhibits the enzymatic and functional activity of myosin-5B in vitro Partial inhibition of the actin-activated steady-state ATPase activity and sliding velocity suggests that caution should be used when probing the effect of myoVin-1 on myosin-5-dependent transport processes in cells.

Direct measurements of the coordination of lever arm swing and the catalytic cycle in myosin V

Myosins use a conserved structural mechanism to convert the energy from ATP hydrolysis into a large swing of the force-generating lever arm. The precise timing of the lever arm movement with respect to the steps in the actomyosin ATPase cycle has not been determined. We have developed a FRET system in myosin V that uses three donor-acceptor pairs to examine the kinetics of lever arm swing during the recovery and power stroke phases of the ATPase cycle. During the recovery stroke the lever arm swing is tightly coupled to priming the active site for ATP hydrolysis. The lever arm swing during the power stroke occurs in two steps, a fast step that occurs before phosphate release and a slow step that occurs before ADP release. Time-resolved FRET demonstrates a 20-Å change in distance between the pre- and postpower stroke states and shows that the lever arm is more dynamic in the postpower stroke state. Our results suggest myosin binding to actin in the ADP.Pi complex triggers a rapid power stroke that gates the release of phosphate, whereas a second slower power stroke may be important for mediating strain sensitivity.

Analytical methods for kinetic studies of biological interactions: A review

The rates at which biological interactions occur can provide important information concerning the mechanism and behavior of these processes in living systems. This review discusses several analytical methods that can be used to examine the kinetics of biological interactions. These techniques include common or traditional methods such as stopped-flow analysis and surface plasmon resonance spectroscopy, as well as alternative methods based on affinity chromatography and capillary electrophoresis. The general principles and theory behind these approaches are examined, and it is shown how each technique can be utilized to provide information on the kinetics of biological interactions. Examples of applications are also given for each method. In addition, a discussion is provided on the relative advantages or potential limitations of each technique regarding its use in kinetic studies.