Higher plant light-harvesting complexes LHCIIa and LHCIIc are bound by dicyclohexylcarbodiimide during inhibition of energy dissipation.

We have investigated the binding to proteins of the photosynthetic apparatus of the carboxy-modifying agent dicyclohexylcarbodiimide, (cHxN)2C; this inhibits the protective dissipation of excess absorbed light energy (qE) by the light-harvesting apparatus of photosystem II (LHCII), suggesting that carboxyl amino-acid side chains within hydrophobic protein domains may be involved in qE. (cHxN)2(14)C was used to label thylakoids and photosystem II particles, so as to identify proteins which may be involved in the detection of lumen pH during qE induction. Of six thylakoid proteins labelled with (cHxN)2C under conditions where qE is efficiently induced, three are associated with photosystem I, and none with the bulk LHCII. PSII-associated label is bound to three minor components of LHCII, identified as LHCIIa (two species) and LHCIIc, as shown by protein sequencing of tryptic fragments of purified complexes. pH titration of qE formation and protein labelling in coupled thylakoids showed that both qE and labelling of LHCIIa increased at pH 7-8.