Secondary and Tertiary Structure Elasticity of Titin Z1Z2 and a Titin Chain Model

Abstract

The giant protein titin, which is responsible for passive elasticity in muscle fibers, is built from approximately 300 regular immunoglobulin-like (Ig) domains and FN-III repeats. While the soft elasticity derived from its entropic regions, as well as the stiff mechanical resistance derived from the unfolding of the secondary structure elements of Ig- and FN-III domains have been studied extensively, less is known about the mechanical elasticity stemming from the orientation of neighboring domains relative to each other. Here we address the dynamics and energetics of interdomain arrangement of two adjacent Ig-domains of titin, Z1, and Z2, using molecular dynamics (MD) simulations. The simulations reveal conformational flexibility, due to the domain-domain geometry, that lends an intermediate force elasticity to titin. We employ adaptive biasing force MD simulations to calculate the energy required to bend the Z1Z2 tandem open to identify energetically feasible interdomain arrangements of the Z1 and Z2 domains. The finding is cast into a stochastic model for Z1Z2 interdomain elasticity that is generalized to a multiple domain chain replicating many Z1Z2-like units and representing a long titin segment. The elastic properties of this chain suggest that titin derives so-called tertiary structure elasticity from bending and twisting of its domains. Finally, we employ steered molecular dynamics simulations to stretch individual Z1 and Z2 domains and characterize the so-called secondary structure elasticity of the two domains. Our study suggests that titin's overall elastic response at weak force stems from a soft entropic spring behavior (not described here), from tertiary structure elasticity with an elastic spring constant of approximately 0.001-1 pN/A and, at strong forces, from secondary structure elasticity.

Structural advances on titin: towards an atomic understanding of multi-domain functions in myofilament mechanics and scaffolding

“…Biophysical characterization of constructs containing 2-6 domains revealed weak domain interactions through small interfaces, supporting the 'beads on a string' viewpoint. Prior to establishing the key role of the PEVK region in elasticity, Ig stretch-unfolding (later termed the secondary elasticity of titin; [8]) was thought to drive the mechanical response of titin. However, this is now a controversial concept and Ig stretch-unfolding is no longer regarded as a primary mechanism of titin elasticity in vivo [9,10].…”

“…Yet, a V-closure has only been observed in the crystal structure of Z1Z2, where it was stabilized by a crosslinking metal ion and co-existed with a conventional extended form ( Figure 1). An exhaustive study that used SAXS, NMR and MDS suggested that the closed conformation in Z1Z2 is rare and rapidly exchanges with the extended state [8,12]. The predominance of these extended arrangements probably stems from the fact that the integrating domains do not interact laterally with each other, so that V-conformations are not sustained in the absence of a stabilizing stimulus, e.g.…”

“…Titin's elasticity resulting from the straightening of its Iband springs is termed tertiary elasticity [8]. The Ig-tandem region lengthens during small stretch, but this does not result in measurable passive tension.…”

An energetic model for macromolecules unfolding in stretching experiments

“…These proteins are very long macromolecules with contour length larger than 1 mm [1], whose secondary structure is characterized by the presence of immunoglobulin (Ig) and fibronectin-type III (FNIII) domains, folded in forms of b-sheets, connected to the PEVK domain (rich in proline, glutamate, valine and lysine) [61]. At low forces, the elasticity is regulated by the tertiary structure and the (random coil) domain orientation, combined with the elasticity of PEVK domains [22,57,61]. At higher forces, the macromolecule response is dominated by an energetic competition of the entropic elasticity of the unfolded fraction of (Ig) and (FNIII) domains and by the enthalpic contribution of the folded !…”

Physical characterization of nanoparticle size and surface modification using particle scattering diffusometry

“…9,10 In these applications, the biomolecules that are conjugated onto nanoparticles can vary dramatically in size, with Stokes radii as small as 7 Å and as large as 1 lm, 11 and corresponding molecular weights ranging from <10 000 Da to >300 000 Da. Without a fluorescent label, it can be difficult to determine the tethering of these biomolecules to the particle surface, 12,13 and gold nanoparticles often quench fluorescence.…”