Refined carotenoid analysis of the major light harvesting complex of Mantoniella squamata

Abstract

The major light-harvesting complex (LHC) of the prasinophycean alga Mantoniella squamata is unique compared to other chlorophyll (Chl) a/b-binding LHC with respect to the primary protein structure and the pigmentation. Although the presence of Chl a, Chl b, a Chl c-type pigment and the xanthophylls neoxanthin, violaxanthin and prasinoxanthin was clearly determined, several carotenoids remained unidentified or were described controversially. We re-analysed the carotenoid composition and identified a new set of xanthophylls present in the LHC: uriolide, micromonol, micromonal and dihydrolutein. Additionally, one hydrophobic component was detected, presumably a xanthophyll. The pigment analysis in combination with quantitative protein determinations revealed a pigment-protein stoichiometry of 6 Chl a, 6 Chl b, 2 Chl c* and about 2 prasinoxanthin molecules per polypeptide. The other xanthophylls were present in sub-stoichiometric amounts. A comparison of results from LHC isolated either by sucrose density centrifugation or SDS-polyacryl gel electrophoresis revealed a decline in the amount of prasinoxanthin and a loss of violaxanthin using the latter preparation procedure, while the stoichiometric ratios of the other 6 xanthophylls remained constant. The fact that 8 different xanthophylls were found in the LHC of M. squamata can be explained best in terms of an oligomeric, presumably trimeric LHC organisation with subunits of heterogeneous pigmentation. Especially, the very stable assembly of most of the minor xanthophylls led to the assumption that these components play an important role in stabilisation and probably also in trimerisation of the LHC in vivo.