This is the C-terminal domain of Enterobacteria phage T4 RNA ligase 2 (Rnl2). The C-terminal domain consists of a four-helix bundle. The C-terminal domain is thought to be critical for binding the Rnl2 to adenylylated nicked duplex. This is known as step 2 of the ligation pathway [1].

1: RNA ligase structures reveal the basis for RNA specificity and conformational changes that drive ligation forward. Nandakumar J, Shuman S, Lima CD; Cell. 2006;127:71-84. PMID:17018278