This family consists of several photosystem II 12 kDa extrinsic protein (PsbU) proteins from cyanobacteria and algae. PsbU is an extrinsic protein of the photosystem II complex of cyanobacteria and red algae. PsbU is known to stabilise the oxygen-evolving machinery of the photosystem II complex against heat-induced inactivation [1]. This family appears to be related to the Helix-hairpin-helix domain.

1: PsbU, a protein associated with photosystem II, is required for the acquisition of cellular thermotolerance in synechococcus species PCC 7002 Nishiyama Y, Los DA, Murata N; Plant Physiol 1999;120:301-308. PMID:10318707