The LHC superfamily of proteins includes not only the most common proteins involved in light harvesting, but also proteins that clearly have other functions . Most welhknown is the major LHC [ II complex of green plants, whose structure is now known at 2.5 A [7, 8] (see also Chapter 10). This protein has three transmembrane a-helices, binds chlorophyll a, chlorophyll b and the carotenoids lutein, violaxanthin, and neoxanthin, occurs in trimers in vivo, is the major light-harvesting protein in green plants and several types of eukaryotic algae, and usually harvests light for PSII [ 10]. In special conditions, however, LHC [II may also harvest light for PSI or play a direct role in photoprotection, and thus can be a switch between light harvesting and photoprotection . Most organisms contain a number of related, often monomeric, proteins, including proteins of this family that are bound to PSI, which are therefore denoted as light- harvesting complex I (LHC-I). In green plants, PSI binds four LHC-I proteins, and the structure of this supercomplex is known at 3.4 A resolution . Red algae, cryptophytes, dinoflagellates, and diatoms contain very similar proteins which bind chlorophyll c instead of chlorophyll b, or none of these at all, while they also bind different types of xanthophylls.
A special member of this family is PsbS. This protein has four transmembrane a-helices, occurs probably only in higher plants, and contains no pigments, or only a few. Therefore, this protein can not be regarded as a light-harvesting protein, but it is in a still unknown way involved in photoprotection, because mutants lacking this protein lack the rapidly reversible high- energy quenching (qE), a physiologically important mechanism by which excess excitation energy is harmlessly converted into heat [ 12, 13] . qE is a major contributor to the non-photochemical quenching (NPQ) of chlorophyll fluorescence. Other members of the LHC family are the early light- induced proteins (ELIPs) induced in many species in response to high light or other types of stress [ 14] . and a number of partial homologues with only one transmembrane a- helix that have been found in cyanobacteria  .
In this review, attention will be focused on the binding of light-harvesting proteins of the LHC family to PSI or PSII.
Was this article helpful?