Troponin-Tropomyosin: An Allosteric Switch or a Steric Blocker?

Abstract

The interaction of myosin subfragment 1 (S1) with actin-tropomyosin-troponin (regulated actin) is highly nucleotide dependent. The binding of S1 or S1-ADP (but not S1-ATP nor N,N'-rho-phenylenedimaleimide-modified S1-ATP) to regulated actin activates ATP hydrolysis even in the absence of Ca(2+). Investigations with S1 and S1-ADP have led to the idea that some actin sites are directly blocked toward the binding of S1 either by tropomyosin or troponin. The blocked state is thought to occur only at ionic strengths greater than 50 mM. The question is whether nonactivating S1 binding is blocked under the same conditions. We show that troponin inhibits binding of the nonactivating state, N,N'-rho-phenylenedimaleimide-S1-ATP, to actin but only when tropomyosin is absent. A lag in the rate of binding of activating S1 to actin (an indicator of the blocked state) occurs only in the presence of tropomyosin. Thus, tropomyosin inhibits binding of rigor S1 but not S1-ATP-like states. No evidence for an ionic strength-dependent change in the mechanism of regulation was observed either from measurements of the rate of activating S1 binding or from the equilibrium binding of nonactivating S1 to actin. At all conditions examined, N,N'-rho-phenylenedimaleimide-S1-ATP bound to regulated actin in the absence of Ca(2+). These results support the view of regulation in which tropomyosin movement is an allosteric switch that is modulated by activating myosin binding but that does not function solely by regulating myosin binding.

Tropomyosin Exon 6b Is Troponin-specific and Required for Correct Acto-myosin Regulation

“…Tobacman (54) invokes actin-actin communication that is propagated along the filament and is linked to the effect of Tm and S1 to the conformation of the actin monomer. The Smith et al (55,56) (57,58). In the work presented here, we do not measure any cooperativity parameter or provide data that further defines switching between regulatory states.…”

Insights into the kinetics of Ca2+-regulated contraction and relaxation from myofibril studies

“…There are alternative regulatory mechanisms. Instead of directly regulating actin-myosin interaction by blocking or unblocking binding sites, the state of the thin filament may allosterically control the transition of crossbridges to force states [23,24,43,121]. To control the transition instead of the binding would be advantageous for muscles, especially for the myocardium, enabling a fast reversible relaxation that is not counteracted by the crossbridges formed during contraction.…”

Aberrant movement of β-tropomyosin associated with congenital myopathy causes defective response of myosin heads and actin during the ATPase cycle

“…Unlike the switched off monomers, the switched on monomers are able to activate myosin ATPase, i.e. actin monomers may be in the so-called http ''active/inactive'' functional states ( [16] for a review, see [1]). Polarized fluorimetry has shown that in muscle fibers the switched on and switched off monomers differ in filament flexibility and orientation relative to the filament axis [13][14][15][17][18][19].…”