1. Laboratory for Structural Biochemistry, RIKEN Harima Institute at SPring-8, 1-1-1 Kouto, Mikazuki, Sayo, Hyogo 679-5148, Japan
2. PRESTO, Japan Science and Technology Corporation (JST), Kawaguchi, Saitama 332-0012, Japan
3. Present address: Department of Cardiac Physiology, National Cardiovascular Center Research Institute, 5-7-1 Fujishiro-dai, Suita, Osaka 565-8565, Japan.

Correspondence to: Soichi Takeda^1,2,3Yuichiro Maéda¹ Correspondence and requests for materials should be addressed to S.T. (Email: stakeda@ri.ncvc.go.jp) or Y.M. (Email: ymaeda@spring8.or.jp). Atomic coordinates and structure factors have been deposited in the Protein Data Bank under accession codes 1J1D for Tn46K and 1J1E for Tn52K.

Abstract

Troponin is essential in Ca²⁺ regulation of skeletal and cardiac muscle contraction. It consists of three subunits (TnT, TnC and TnI) and, together with tropomyosin, is located on the actin filament. Here we present crystal structures of the core domains (relative molecular mass of 46,000 and 52,000) of human cardiac troponin in the Ca²⁺-saturated form. Analysis of the four-molecule structures reveals that the core domain is further divided into structurally distinct subdomains that are connected by flexible linkers, making the entire molecule highly flexible. The -helical coiled-coil formed between TnT and TnI is integrated in a rigid and asymmetric structure (about 80 Å long), the IT arm, which bridges putative tropomyosin-anchoring regions. The structures of the troponin ternary complex imply that Ca²⁺ binding to the regulatory site of TnC removes the carboxy-terminal portion of TnI from actin, thereby altering the mobility and/or flexibility of troponin and tropomyosin on the actin filament.