The light-harvesting complexes of higher plant Photosystem I: Lhca1/4 and Lhca2/3 form two red-emitting heterodimers
Résumé
The outer antenna of higher plants Photosystem I is composed of four complexes (Lhca1-4) belonging to the light-harvesting protein family. Difficulties in their purification have so far prevented the determination of their properties and most of the knowledge about Lhcas has been obtained from the study of the in vitro reconstituted antennas. In this work we were able to purify the native complexes, showing that Lhca2/3 and Lhca1/4 form two functional heterodimers. Both dimers show red-fluorescence emission with maxima around 730 nm, as in the intact PSI complex. This indicates that the dimers are in their native state and that LHCI-680, which was previously assumed to be part of the PSI antenna, does not represent the native state of the system. The data show that the light-harvesting properties of the two dimers are functionally identical, concerning absorption, long-wavelength emission and fluorescence quantum yield, while they differ in their high-light response. Implications of the present results for the understanding of the energy transfer process in PSI are discussed. Finally, the comparison of the properties of the native dimers with those of the reconstituted complexes demonstrates that all the major properties of the Lhcas are reproduced in the in vitro systems.
Origine : Fichiers produits par l'(les) auteur(s)