The atypical chlorophyll a/b/c light-harvesting complex of Mantoniella squamata: molecular cloning and sequence analysis.

Abstract

cDNA species encoding precursor polypeptides of the chlorophyll a/b/c light-harvesting complex (LHC) of Mantoniella squamata were cloned and sequenced. The precursor polypeptides have molecular weights of 24.2 kDa and are related to the major chlorophyll a/b polypeptides of higher plants. Southern analysis showed that their genes belong to the nuclear encoded Lhc multigene family; the investigated genes most probably do not contain introns. The chlorophyll a/b/c polypeptides contain two highly conserved regions common to all LHC polypeptides and three hydrophobic alpha-helices, which span the thylakoid membrane. The first membrane-spanning helix, however, is not detected by predictive methods: its atypical hydrophilic domains may bind the chlorophyll c molecules within the hydrophobic membrane environment. Homology to LHC II of higher plants and green algae is specifically evident in the C-terminal region comprising helix III and the preceding stroma-exposed domain. The N-terminal region of 29 amino acids resembles the structure of a transit sequence, which shows only minor similarities to those of LHC II sequences. Strikingly, the mature light-harvesting polypeptides of M. squamata lack an N-terminal domain of 30 amino acids, which, in higher plants, contains the phosphorylation site of LHC II and simultaneously mediates membrane stacking. Therefore, the chlorophyll a/b/c polypeptides of M. squamata do not exhibit any light-dependent preference for photosystem I or II. The lack of this domain also indicates that the attractive forces between stacked thylakoids are weak.