Cross-linker dynamics determine the mechanical properties of actin gels.
To evaluate the contributions of cross-linker dynamics and polymer deformation to the frequency-dependent stiffness of actin filament gels, we compared the rheological properties of actin gels with three types of cross-linkers: a weak one, Acanthamoeba alpha-actinin (dissociation rate constant 5.2 s-1, association rate constant 1.1 x 10(6) M-1 s-1); a strong one, chicken smooth muscle alpha-actinin (dissociation rate constant 0.66 s-1, association rate constant 1.20 x 10(6) M-1 s-1); and an extremely strong one, biotin/avidin (dissociation rate constant approximately zero). The biotin/avidin cross-linked gel, whose behavior is determined by polymer bending alone, behaves like a solid and shows no frequency dependence. The amoeba alpha-actinin cross-linked gel behaves like a viscoelastic fluid, and the frequency dependence of the stiffness can be explained by a mathematical model for dynamically cross-linked gels. The stiffness of the chicken alpha-actinin cross-linked gel is independent of frequency, and has viscoelastic properties intermediate between the two. The two alpha-actinins have similar association rate constants for binding to actin filaments, consistent with a diffusion-limited reaction. Rigid cross-links make the gel stiff, but make it elastic without the ability to deform permanently. Dynamically cross-linked actin filaments should allow the cell to react passively to various outside forces without any sort of signaling. Slower, signal-mediated pathways, such as severing filaments or changing the affinity of cross-linkers, could alter the nature of these passive reactions.
Department of Cell Biology and Anatomy, Johns Hopkins University, Baltimore, Maryland 21205.
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D000048:Acanthamoeba; D000185:Actinin / Q000737:chemistry; D000199:Actins / Q000737:chemistry* / Q000502:physiology; D000818:Animals; D001360:Avidin / Q000737:chemistry; D001696:Biomechanical Phenomena; D055592:Biophysical Phenomena; D001703:Biophysics; D001704:Biopolymers / Q000737:chemistry; D001710:Biotin / Q000737:chemistry; D002465:Cell Movement / Q000502:physiology; D048429:Cell Size / Q000502:physiology; D002645:Chickens; D003432:Cross-Linking Reagents; D003593:Cytoplasm / Q000502:physiology; D004548:Elasticity; D005782:Gels; D007700:Kinetics; D011485:Protein Binding; D012212:Rheology; D014783:Viscosity
D000199:Actins; D001704:Biopolymers; D003432:Cross-Linking Reagents; D005782:Gels; D000185:Actinin; D001360:Avidin; D001710:Biotin