The Poly-E motif in Titin's PEVK region undergoes pH dependent conformational changes

The muscle protein titin plays a crucial role in passive elasticity and the disordered PEVK region within titin is central to that function. The PEVK region is so named due to its high proline, glutamate, valine and lysine content and the high charge density in this region results in a lack of organized structure within this domain. The PEVK region is highly extensible but the molecular interactions that contribute to the elastic nature of the PEVK still remain poorly described. The PEVK region is formed by two unique sequence motifs. The PPAK motif is a 26 to 28 amino acid sequence that contains a mixture of charged and hydrophobic residues and is the primary building block for the PEVK region. Poly-E sequence motifs vary in length and contain clusters of 3-4 glutamic acids distributed throughout the motif. In this study, we derived two 28-residue peptides from the human titin protein sequence and measured their structural characteristics over a range of pHs. Our results demonstrate that the poly-E peptide undergoes a shift from a more rigid and elongated state to a more collapsed state as pH decreases with the midpoint of this transition being at pH ~5.5. Interestingly, a similar conformational shift is not observed in the PPAK peptide. These results suggest that the poly-E motif might provide a nucleating site for the PEVK when the muscle is not in an extended state.