Characterization of binding interaction between S100A1 and titin's UN2A region.

Abstract

Titin, the giant sarcomere protein, consists of multiple domains and regions of repeated structural elements that are essential for sarcomeric integrity of the myofibril. The N2A region within titin has been identified as a signaling hub in skeletal muscle that is responsible for various mechanics and interactions in the muscle. The N2A region is located at the junction between the proximal tandem Ig region and the PEVK region and is comprised of four Ig domains and a unique 117 amino acid insertion sequence (UN2A). The insertion sequence resides between the Ig80 and Ig81 domains, and exists in skeletal muscle, as well as the N2AB isoform of cardiac muscle. An analysis of the insertion region sequence revealed a potential binding site for the Ca2+-binding protein S100A1. A combination of circular dichroism and FRET experiments demonstrated the conformational change in UN2A that is induced by S100A1, and can be affected differently due to pH changes. Surface plasmon resonance was used to study the binding kinetics and affinities of this interaction at different pH and pCa values. S100A1's binding to UN2A is calcium-dependent, and the strength of this interaction is affected by pH. The S100A1/UN2A binding interaction has been shown to perturb the structure of UN2A potentially at its stable helical core. The results will provide insight on the nature of this interaction and how it may have a functional role in the muscle.