InterPro domain: IPR014751

XRCC4 is essential for non-homologous DNA end joining (NHDJ) in eukaryotes, which is required for double-strand break repair, and V(D)J recombination in immunoglobulin and T-cell receptor genes. XRCC4 forms a complex with DNA ligase IV, and acts as a regulatory element required for the stability and activity of the ligase. XRCC4 forms an elongated dumb-bell-like tetramer consisting of a C-terminal stalk that interacts with DNA ligase IV and an N-terminal globular head. The C-terminal oligomerisation domain consists of oligomers of short identical helices that form parallel coiled-coils [ 1 , 2 ].

This superfamily also matches the C-terminal of the coiled-coil myosin heavy chain tail region. Muscle contraction is caused by sliding between the thick and thin filaments of the myofibril. Myosin is a major component of thick filaments and exists as a hexamer of 2 heavy chains [ 3 ], 2 alkali light chains, and 2 regulatory light chains. The heavy chain can be subdivided into the N-terminal globular head and the C-terminal coiled-coil rod-like tail [ 4 ].

1. Crystal structure of the Xrcc4 DNA repair protein and implications for end joining. EMBO J. 19, 5962-70
2. Crystal structure of an Xrcc4-DNA ligase IV complex. Nat. Struct. Biol. 8, 1015-9
3. The primary structure of skeletal muscle myosin heavy chain: I. Sequence of the amino-terminal 23 kDa fragment. J. Biochem. 110, 54-9
4. Evolutionary implications of three novel members of the human sarcomeric myosin heavy chain gene family. Mol. Biol. Evol. 19, 375-93