Malleable conformation of the elastic PEVK segment of titin: non-co-operative interconversion of polyproline II helix, β-turn and unordered structures

To understand the structural basis of molecular elasticity and protein interaction of the elastic PEVK (Pro-Glu-Val-Lys) segment of the giant muscle protein titin, we carried out a detailed analysis of a representative PEVK module and a 16-module PEVK protein under various environmental conditions. Three conformational states, polyproline II (PPII) helix, beta-turn and unordered coil were identified by CD and NMR. These motifs interconvert without long-range co-operativity. As a general trend, the relative content of PPII increases with lower temperature and higher polarity, beta-turn increases with lower temperature and lower polarity, and unordered coil increases with higher temperature and higher polarity. NMR studies demonstrate that trans -proline residues are the predominant form at room temperature (22 degrees C), with little trans -to- cis isomerization below 35 degrees C. Ionic strength affects salt bridges between charged side chains, but not the backbone conformation. We conclude that titin PEVK conformation is malleable and responds to subtle environmental changes without co-operativity. This gradual conformational transition may represent a regulatory mechanism for fine-tuning protein interactions and elasticity.