Thylakoid-determinant subunit of high molecular weight LFNRs-containing protein complexes. Component of high molecular weight thylakoid LFNRs-containing protein complexes containing LIR1, LFNR1, LFNR2, TIC62 and TROL proteins. Interacts directly with LFNR1 and LFNR2; LIR1 increases the affinity of LFNR1 and LFNR2 for TIC62 and subsequent thylakoid relocalization. Expression is controlled by light and by a circadian clock. Differentially regulated at the level of mRNA stability at different times of day, being a target of the degradation pathway mediated by the downstream (DST) instability determinant and thus following a circadian clock rhythm with highest levels in early afternoon (PubMed:16055688). Rapidly degraded upon illumination; this degradation coincides with the release of the LFNR from the thylakoid membrane (PubMed:26941088). Repressed by calmodulin (CaM) antagonists such as trifluoperazine (TFP) and N-(6-aminohexyl)-5-chloro-1-naphthelene-sulfonamid-hydrochloride (W7); this repression is alleviated by lanthanum (e.g. LaCl(3)) (PubMed:16980540). May form interchain disulfide bonds with LFNR1 and LFNR2. Slight increase in nonphotochemical quenching (NPQ). Marked decrease in the accumulation of LFNR1 and LFNR2-containing thylakoid protein complexes.