Supercomplexes with Proteins from the LHC Superfamily

The LHC superfamily of proteins includes not only the most common proteins involved in light harvesting, but also proteins that clearly have other functions [9]. 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 [11]. 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 [2]. 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 [15] .

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?

0 0

Post a comment