This family consists of epsilon subunits of the ATP synthase. The ATP synthase complex is composed of an oligomeric transmembrane sector (CF0), and a catalytic core (CF1). CF1 is composed of 5 subunits, of which the epsilon subunit functions as a potent inhibitor of ATPase activity in both soluble and bound CF1. Only when the epsilon inhibition is disabled is high ATPase activity detected in ATPase [1]

1: Aspects of Subunit Interactions in the Chloroplast ATP Synthase (II. Characterization of a Chloroplast Coupling Factor 1-Subunit III Complex from Spinach Thylakoids). Wetzel CM, McCarty RE; Plant Physiol 1993;102:251-259. PMID:12231816