Actin isoform expression, cellular heterogeneity, and contractile function in smooth muscle.
Smooth muscles express four isoforms of actin: two smooth muscle specific and two cytoplasmic isoforms typically associated with the cytoskeleton of nonmuscle cells. The relative amounts of each isoform expressed and the total actin content vary with smooth muscle type, with development, in cell culture, pathologically, and potentially between cells within tissues. Our objective was to determine whether actin isoforms contribute to contractile diversity. Functional diversity may be the result of differences in the kinetics of the cross-bridge interaction with thin filaments consisting of different actin isoforms or of the fraction of cross-bridges developing force in series or in parallel resulting from thin filaments of different lengths. Our hypothesis was that functionally significant differences in actin isoform properties (i.e., myosin interactions or properties affecting thin filament lengths) would require isoform segregation into distinct populations of thin filaments within cellular domains (e.g., cytoskeletal and contractile) or in phenotypically different cells. We tested this hypothesis by determining the smooth muscle alpha- and gamma-actin and cytoplasmic beta-actin isoform composition of native thin filaments isolated from swine stomach using isoform-specific antibodies linked to colloidal gold beads with protein A (the cytoplasmic lambda-isoactin content was below the detection limit). The lengths of individual thin filaments were also estimated from electron micrographs. A statistically uniform population of thin filaments was observed consisting of randomly copolymerized isoactins in each filament with the same isoform proportions as the tissue. The average thin filament length was 1.35 +/- 0.06 (SEM) microns. These results, together with other studies, suggest that actin isoforms are functionally equivalent. The data imply that the high stress-generating and shortening capacities of smooth muscles are not primarily due to long thin filament to thick filament length ratios compared with striated muscles.
Drew
J S
JS
Department of Molecular Physiology and Biological Physics, University of Virginia, Charlottesville 22908, USA.
Murphy
R A
RA
D000199:Actins / Q000032:analysis / Q000378:metabolism*; D000818:Animals; D066298:In Vitro Techniques; D007382:Intermediate Filaments / Q000737:chemistry; D007536:Isomerism; D009119:Muscle Contraction / Q000502:physiology*; D009130:Muscle, Smooth / Q000378:metabolism* / Q000502:physiology* / Q000648:ultrastructure; D013552:Swine
D000199:Actins