Small-Angle Neutron Scattering with Contrast Variation Reveals Spatial Relationships between the Three Subunits in the Ternary Cardiac Troponin Complex and the Effects of Troponin I Phosphorylation

Small-angle neutron scattering with contrast variation has been used to determine the shapes and dispositions of the three subunits of cardiac troponin and to study the influence of phosphorylation on the structure. Three contrast variation series were collected on three different isotopically labeled variants of the cTnC/cTnI/cTnT(198-298) complex, one of which contained deuterated and bisphosphorylated cTnI. Analysis of the scattering data shows cTnT(198-298) interacting with a single lobe of a somewhat compacted cTnC that sits at one end of an elongated rodlike cTnI, covering about one-third of its length. The cTnT(198-298) sits near the center of the long cTnI axis. The components undergo significant conformational changes and reorientations in response to protein kinase A phosphorylation of cTnI. The rodlike cTnI bends sharply at the end interacting with the cTnC/cTnT(198-298) component, which reorients so as to maintain its contacts with cTnI while undergoing only a relatively small change in shape.