Titin PEVK segment: charge-driven elasticity of the open and flexible polyampholyte

Abstract

The giant protein titin spans half of the sarcomere length and anchors the myosin thick filament to the Z-line of skeletal and cardiac muscles. The passive elasticity of muscle at a physiological range of stretch arises primarily from the extension of the PEVK segment, which is a polyampholyte with dense and alternating-charged clusters. Force spectroscopy studies of a 51 kDa fragment of the human fetal titin PEVK domain (TP1) revealed that when charge interactions were reduced by raising the ionic strength from 35 to 560 mM, its mean persistence length increased from 0.30±0.04 nm to 0.60±0.07 nm. In contrast, when the secondary structure of TP1 was altered drastically by the presence of 40 and 80% (v/v) of trifluoroethanol, its force-extension behavior showed no significant shift in the mean persistence length of ~0.18±0.03 nm at the ionic strength of 15 mM. Additionally, the mean persistence length also increased from 0.29 to 0.41 nm with increasing calcium concentration from pCa 5-8 to pCa 3-4. We propose that PEVK is not a simple entropic spring as is commonly assumed, but a highly evolved, gel-like enthalpic spring with its elasticity dominated by the sequence-specific charge interactions. A single polyampholyte chain may be fine-tuned to generate a broad range of molecular elasticity by varying charge pairing schemes and chain configurations.

Titin‐based mechanosensing modulates muscle hypertrophy

“…However, these studies were performed in intact, contracting muscle. Consequently, the results might be confounded by the presence of calcium cycling: calcium binds to the PEVK spring‐domain of titin, changing its stiffness,15, 16, 17 and titin‐based stiffness modulates contractility20, 41, 42, 43, 44 which in turn might affect muscle growth. The importance of the current findings is that they show that the effects of titin‐based mechanosensing occur independent of contractile activity, and can be ascribed to titin's elastic properties.…”

Phosphorylation and calcium antagonistically tune myosin-binding protein C’s structure and function

“…Phosphorylation sites are commonly found in intrinsically disordered regions of proteins, with phosphorylation having both local effects on the order of secondary structures and global effects on the orientation of tertiary states (32)(33)(34). Such structural rearrangements can result in changes in the affinity for binding partners (33,35) and/or the mechanics of the phosphoprotein in the bound state (32,36).…”

Titin as a Giant Scaffold for Integrating Stress and Src Homology Domain 3-mediated Signaling Pathways

“…Elsewhere, we have proposed that the PEVK segment is a polyampholyte that wraps into a nanogel via ion pair interactions and that the mechanically induced unraveling and wrapping forms the basis of PEVK elasticity (26). In this context of ligand/receptor interaction, the accessibility of SH3-containing protein to the ligand is perhaps controlled by how the PEVK nanogel alters the pore size distribution in response to mechanical stress.…”

“…The facile interconversion of these conformational states under subtle environmental conditions such as temperature, ionic strength, and solvent polarity has revealed that the titin PEVK segment is conformationally malleable, without involving proline trans/cis isomerization. The elasticity of PEVK also varies with ionic strength, where the elasticity has been measured to either decrease (26,27) or increase (28) with increasing ionic strength. This behavior implies that ionic interactions within the PEVK segment play an important role in the ensemble average of the polypeptide configuration and therefore the relative entropic and enthalpic contributions to molecular elasticity.…”

“…This behavior implies that ionic interactions within the PEVK segment play an important role in the ensemble average of the polypeptide configuration and therefore the relative entropic and enthalpic contributions to molecular elasticity. We have proposed that the titin PEVK segment is an open and flexible polyampholyte with its elasticity driven and modulated by the number and location of internal salt bridges (26). The elasticity of adult soleus titin PEVK appears to vary along its length, with the amino-terminal region being more rigid than its carboxyl-terminal region (28).…”