Formation and properties of smooth muscle myosin 20-kDa light chain-skeletal muscle myosin hybrids and photocrosslinking from the maleimidylbenzophenone-labeled light chain to the heavy chain

Single-molecule analysis reveals that regulatory light chains fine-tune skeletal myosin II function

Myosin II is the main force-generating motor during muscle contraction. Myosin II exists as different isoforms that are involved in diverse physiological functions. One outstanding question is whether the myosin heavy chain (MHC) isoforms alone account for these distinct physiological properties. Unique sets of essential and regulatory light chains (RLCs) are known to assemble with specific MHCs, raising the intriguing possibility that light chains contribute to specialized myosin functions. Here, we asked whether different RLCs contribute to this functional diversification. To this end, we generated chimeric motors by reconstituting the MHC fast isoform (MyHC-IId) and slow isoform (MHC-I) with different light-chain variants. As a result of the RLC swapping, actin filament sliding velocity increased by ∼10-fold for the slow myosin and decreased by >3-fold for the fast myosin. Results from ensemble molecule solution kinetics and single-molecule optical trapping measurements provided in-depth insights into altered chemo-mechanical properties of the myosin motors that affect the sliding speed. Notably, we found that the mechanical output of both slow and fast myosins is sensitive to the RLC isoform. We therefore propose that RLCs are crucial for fine-tuning the myosin function.

Regulatory and catalytic domain dynamics of smooth muscle myosin filaments

Domain dynamics of the chicken gizzard smooth muscle myosin catalytic domain (heavy chain Cys-717) and regulatory domain (regulatory light chain Cys-108) were determined in the absence of nucleotides using saturation-transfer electron paramagnetic resonance. In unphosphorylated synthetic filaments, the effective rotational correlation times, tau(r), were 24 +/- 6 micros and 441 +/- 79 micros for the catalytic and regulatory domains, respectively. The corresponding amplitudes of motion were 42 +/- 4 degrees and 24 +/- 9 degrees as determined from steady-state phosphorescence anisotropy. These results suggest that the two domains have independent mobility due to a hinge between the two domains. Although a similar hinge was observed for skeletal myosin (Adhikari and Fajer (1997) Proc. Natl. Acad. Sci. U.S.A. 94, 9643-9647. Brown et al. (2001) Biochemistry 40, 8283-8291), the latter displayed higher regulatory domain mobility, tau(r)= 40 +/- 3 micros, suggesting a smooth muscle specific mechanism of constraining regulatory domain dynamics. In the myosin monomers the correlation times for both domains were the same (approximately 4 micros) for both smooth and skeletal myosin, suggesting that the motional difference between the two isoforms in the filaments was not due to intrinsic variation of hinge stiffness. Heavy chain/regulatory light chain chimeras of smooth and skeletal myosin pinpointed the origin of the restriction to the heavy chain and established correlation between the regulatory domain dynamics with the ability of myosin to switch off but not to switch on the ATPase and the actin sliding velocity. Phosphorylation of smooth muscle myosin filaments caused a small increase in the amplitude of motion of the regulatory domain (from 24 +/- 4 degrees to 36 +/- 7 degrees ) but did not significantly affect the rotational correlation time of the regulatory domain (441 to 408 micros) or the catalytic domain (24 to 17 micros). These data are not consistent with a stable interaction between the two catalytic domains in unphosphorylated smooth muscle myosin filaments in the absence of nucleotides.

Expression of chicken gizzard RLC complements the cytokinesis and developmental defects of Dictyostelium RLC null cells

Dictyostelium RLC null cells have defects in cytokinesis and development that can be rescued by expression of either the wild type Dictyostelium RLC or an RLC mutant that cannot be phosphorylated by MLCK (S13A) (Ostrow et al., 1994). The wild type and S13A mutant LCs rescued the cells equally well, despite the fact that RLC phosphorylation increases purified Dictyostelium myosin's activity 5-fold. In this report, we assess the ability of foreign RLCs to rescue the RLC null phenotype. The RLC from smooth muscle myosin, whose activity is tightly controlled by phosphorylation, rescued the null cell phenotype. The purified hybrid myosin had an activity and motility comparable to phosphorylated Dictyostelium myosin. In contrast, cells expressing skeletal muscle RLC were deficient in cytokinesis and development, despite having an activity and motility similar to that of myosin with the unposphorylatable S13A mutant RLC. Neither foreign LC was phosphorylated when expressed in Dictyostelium. These results suggest that the level of actin-activated ATPase activity and motility is not the sole determinant of proper myosin function in vivo. Other heavy chain/light chain interactions, which occur only with the native RLC and smooth muscle RLC, appear to be necessary for optimal function.

Single molecule imaging of fluorophores and enzymatic reactions achieved by objective-type total internal reflection fluorescence microscopy

Imaging of single fluorescent molecules has been achieved in a relatively simple manner using objective-type total internal reflection fluorescence microscopy (TIRFM). Switching from epi-fluorescence microscopy to objective-type TIRFM was achieved by translation of a single mirror in the system. Clear images of single molecules of an orange fluorescent dye, Cy3, were obtained with a fluorescence-to-background ratio of 12, using a conventional high aperture objective (PlanApo, 100 x, NA 1.4) with ordinary coverslips and immersion oil. This method allowed visualization of single molecules under scanning probe microscopes. Taking advantage of the technique of single molecule imaging, individual ATP turnovers have been visualized with a fluorescent ATP analogue, Cy3-ATP, using a simple experimental strategy. Clear on/off signals were obtained that correspond to the association and dissociation of single Cy3-ATP/ADP molecules with a single myosin head molecule. This method will allow a variety of single-molecular assays of biomolecular functions to be performed using fluorescently labeled substrates, ligands, messengers, and biologically active molecules. Thus, the present technique provides a simple yet powerful and universal tool for researchers to probe the events of single molecules.

Myosin subfragment-1 is fully equipped with factors essential for motor function

The sliding velocity of actin filaments propelled by chicken skeletal myosin subfragment-1 (S1) was measured when the tail end of S1 was specifically bound to the glass surface. To achieve the specific binding, a regulatory light chain was replaced by a recombinant fusion protein of biotin-dependent transcarboxylase (BDTC) and chicken gizzard smooth muscle regulatory light chain (cgmRLC). The BDTC-cgmRLC of S1 was then attached to the glass surface using a biotin-avidin system. The velocity of actin filaments caused by S1 bound to the surface in this manner was 6.8 +/- 0.6 microm/sec at 29 degrees C, which was 3.5-fold greater than that (1.9 +/- 0.3 microm/sec) when bound directly to the surface as in previous studies, but similar to that caused by native chicken skeletal myosin (6.5 +/- 0.6 microm/sec). The actin-activated Mg-ATPase activity was similar to that of S1 before the RLC of S1 was exchanged for BDTC-cgmRLC. The results indicate that S1 can produce a normal fast movement of actin filaments as well as hydrolyse ATP and generate force.