The Mechanism of LHCP Insertion Into Thylakoid Membranes

The light harvesting chlorophyll a/b-binding proteins (LHCPs) are the most abundant membrane proteins. LHCP is a nuclear encoded protein which is targeted to the thylakoid membranes by chloroplast signal recognition particles (cpSRP). Insertion into thylakoid membranes is facilitated by the cpSRP receptor cpFtsY and the Alb3 translocase. Work here focused on understanding the molecular events of LHCP insertion into the thylakoid membranes. Specifically, we sought to develop a tool to detect the insertion of the lumen-localized loop of LHCP into thylakoid membranes, which relies on cleavage of the loop by a thylakoid lumen processing protease. We also sought to understand effects of lumenal loop insertion mutations in LHCP on trimerization and association with photosystem II. Our data shows that insertion of the D1 processing site, a cleavage site in the D1 protein of photosystem II, in the lumenal loop of LHCP functions as a tool to detect the integration of LHCP into the thylakoid membranes. Cleavage of the D1 processing site is performed by lumen protease, C-terminal processing protease. Cleavage of the processing site is also independent of thermolysin treatment of the thylakoid membranes. Our data also shows that insertion of the OE33 signal peptide site or the D1 processing site in the lumenal loop of LHCP affects LHCP assembly into trimer. Insertion of the OE33 or the D1 cleavage site after amino acid 134 of LHCP results in formation of trimeric and monomeric LHCP upon integration into thylakoid membranes. Interestingly, this mutation also prevents LHCP assembly into photosystem II. The slow assembly of trimer and lack of photosystem II association appears to be unique to the mutation at position 134.