The Ig Doublet Z1Z2: A Model System for the Hybrid Analysis of Conformational Dynamics in Ig Tandems from Titin

Abstract

Titin is a gigantic elastic filament that determines sarcomere ultrastructure and stretch response in vertebrate muscle. It folds into numerous Ig and FnIII domains connected in tandem. Data on interdomain arrangements and dynamics are essential for understanding the function of this filament. Here, we report a mechanistic analysis of the conformational dynamics of two Ig domains from the N terminus of titin, Z1Z2, by using X-ray crystallography, SAXS, NMR relaxation data, and residual dipolar couplings in combination. Z1Z2 preferentially adopts semiextended conformations in solution, with close-hinge arrangements representing low-probability states. Although interdomain contacts are not observed, the linker appears to acquire moderate rigidity via small, local hydrophobic interactions. Thus, Z1Z2 constitutes an adaptable modular system with restricted dynamics. We speculate that its preexistent conformation contributes to the selective recruitment of the binding partner telethonin onto the repetitive surface of the filament. The structural interconversion of four Z1Z2 conformers is analyzed.

A regular pattern of Ig super-motifs defines segmental flexibility as the elastic mechanism of the titin chain

“…These findings suggest that the elasticity of the skeletal I-band tandem is mediated by a regular pattern of domain pairs with two different bending stiffnesses, namely pairs that resist bending and have a tendency to retain an extended conformation and pairs that permit complex interdomain motions of large amplitude as observed in the doublet Z1Z2 (6). The stretch and recoil of the titin chain could then be expected to first induce conformational changes in the long permissive linkers, likely to act as weak ''flexor'' points.…”

“…Values in parentheses correspond to the highest resolution shell. 6. § Distance between the centers of mass of individual domains.…”

“…¶ Subscripts 6IG and 3IG refer to the crystal structure of I65-I70 and I67-I69, respectively. ʈ Domain arrangement of Z1Z2 in solution as calculated from SAXS data and NMR residual dipolar couplings (6). It should be noted that Ig-doublets containing long linkers exhibit similar opening and torsion angles but differ in their swing angle.…”

Spontaneous Dimerization of Titin Protein Z1Z2 Domains Induces Strong Nanomechanical Anchoring

“…It is admittedly intriguing that the isolated Ig Z1 and Z2 tandem crystallizes with no indication of tandem formation (36). It is therefore likely that the (I27) 4 adaptor promotes the dimerization process.…”

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

“…But even those have proven a challenge for NMR and X-ray crystallography methods, which are troubled by their intrinsic flexibility. Nonetheless, several multi-domain 3D-structures are now available, namely: Z1Z2 (Z-disk) [12]; I65-I70 (I-band) [13]; A77-A78 [14], A164-A165 [PDB 3LCY] (A-band); and A168-A170 (M-line) [15] (an extended list of fragments, subfragments and mutated variants is given in Supplementary Table S1). Multi-domain structures reveal three primary types of domain interfaces ( Figure 2): (i) loose connections, where domains are joined by hydrophilic three-residue linkers that, being free from interactions, impose low steric restrictions on dynamics and allow large modular motions between neighbouring domains (representative pairs are Z1Z2, I65-I66 and I66-I67); (ii) restrained connections, where domains are joined through ultra-short linkers (0-2 residues) and sustain a small number of direct domain-domain contacts.…”