This domain is Psb31, an extrinsic protein found in photosystem II (PSII) present in Chaetoceros gracilis. Photosystem II (PSII) is a multisubunit, membrane protein complex located in the thylakoid membranes of oxygenic photosynthetic organisms from cyanobacteria to higher plants. The four helices in the N-terminal domain are arranged in an up-down-up-down fold and are similar in structure to PsbQ protein in Spinach, despite low sequence homology [1].

1: Crystal structure of Psb31, a novel extrinsic protein of photosystem II from a marine centric diatom and implications for its binding and function. Nagao R, Suga M, Niikura A, Okumura A, Koua FH, Suzuki T, Tomo T, Enami I, Shen JR; Biochemistry. 2013;52:6646-6652. PMID:23988112