The resolution and biochemical characterization of subcomplexes of the main light-harvesting chlorophyll a/b-protein complex of Photosystem II (LHC II)

Degradation of the main Photosystem II light-harvesting complex

Many factors trigger the degradation of proteins, including changes in environmental conditions, genetic mutations, and limitations in the availability of cofactors. Despite the importance for viability, still very little is known about protein degradation and its regulation. The degradation of the most abundant membrane protein on Earth, the light-harvesting complex of Photosystem II (LHC II), is highly regulated under different environmental conditions, e.g. light stress, to prevent photochemical damage of the reaction center. However, despite major effort to identify the protease/proteases involved in the degradation of the apoproteins of LHC II the molecular details of this important process remain obscure. LHC II belongs to the family of chlorophyll a/b binding proteins (CAB proteins) and is located in the thylakoid membrane of the plant chloroplast. The results of biochemical experiments to isolate and characterize the protease degrading LHC II are summarized here and compared to our own recent finding indicating that a metalloprotease of the FtsH family is involved in this process.

Photophysical properties ofProchlorococcus marinusSS120 divinyl chlorophylls and phycoerythrin in vitro and in vivo

Prochlorococcus marinus SS120 is an ecologically important and biochemically intriguing marine cyanobacterium. In addition to divinyl chlorophylls (DV-Chls) a and b it possesses a particular form of phycoerythrin (PE), but no other phycobilins and therefore no complete phycobilisomes. Here, a spectroscopic characterisation of these DV-Chls and PE is provided. Comparison of fluorescence quantum yields, excited state lifetimes and absorption characteristics indicate similar light-harvesting properties of the DV-Chls as their monovinyl counterparts. PE, which is present only in tiny amounts, was purified and considerably enriched. A phycourobilin to phycoerythrobilin ratio of 3:1 chromophores per (alphabeta) PE monomer is suggested. The in vitro fluorescence lifetime of PE is 1.74 ns. In vivo time-resolved fluorescence measurements with synchrotron radiation were used to investigate the possible role of PE in light-harvesting. The fluorescence decay time for PE is about 550 ps, indicating an unusually slow excitation energy transfer. The decay time slowed to 1 ns after addition of glycerol to cell cultures. The contribution of PE to total light-harvesting capacity was estimated to be about one (alphabeta) PE monomer per 330 DV-Chl b molecules. Thus, the capacity of PE to function primarily as a photosynthetic light-harvesting pigment in P. marinus SS120 is low.

The acclimative response of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) to elevated irradiances at the level of trimeric subunits

The changes in structural organization of the major light-harvesting chlorophyll a/b-protein complex of photosystem II (LHC II) at the level of trimeric subcomplexes were studied in spinach plants grown under low light conditions (50 micromol quanta m(-2) s(-1)) and then acclimated to elevated irradiances. By monitoring photochemical quenching of fluorescence yield (qP), photosystem II (PS II) functional status was assessed in leaves of plants acclimated to a range of elevated irradiances. Three separate acclimative irradiances were selected for the experiments, reflecting: limiting light conditions (150 micromol quanta m(-2) s(-1)), near to the inflexion point on the irradiance curve conditions (300 micromol quanta m(-2) s(-1)) and an excessive light, causing a moderate stress in the form of down regulation of PS II (450 micromol quanta m(-2) s(-1)). An immunoblot analysis showed that there was a clear decline in an abundance on chlorophyll basis of Lhcb1-3 apoproteins as an acclimative irradiance increased from 50 to 450 micromol quanta m(-2) s(-1), with Lhcb1 decreasing to a lesser extent than Lhcb2 and Lhcb3 (only at excessive irradiance). When analyzed by non-denaturing isoelectric focusing BBY membrane fragments (PSIIalpha-enriched, stacked thylakoid membranes) isolated from low light-grown plants were resolved into nine fractions, seven of which (labelled 3-9) were established by us previously [Jackowski and Pielucha, J. Photochem. Photobiol. B: Biol. 64 (2001) 45] to be LHC II subcomplexes representing mixed populations of closely similar trimers, comprising permutations of Lhcb1 and Lhcb2 (subcomplexes 3-7) or Lhcb1-3 (subcomplexes 8 and 9). A heterogeneity with regard to accumulation behaviour of LHC II subcomplexes in response to elevated irradiances was revealed. The subcomplexes 5 and 6 were accumulating at similar level, regardless of the light irradiance experienced. Another group consisting of the subcomplexes 3 and 4 (the most basic ones) showed a progressive increase in relative abundance with increasing an irradiance intensity whereas the subcomplexes 7-9 (the most acidic ones) exhibited a progressive decline in their relative abundance during an acclimation of spinach plants to elevated irradiances thus they may collectively represent an elevated irradiance-responsive subunit of LHCII.

Heterogeneity of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) at the level of trimeric subunits

To study organization of the main light-harvesting chlorophyll a/b-protein complex of photosystem II (LHCII) from spinach thylakoid membranes at the level of trimeric subcomplexes, we have applied non-denaturing isoelectric focusing (ndIEF) in vertical, slab polyacrylamide gels. When analyzed by two consecutive ndIEF/electroelution runs, spinach BBY membrane preparations (PSII(alpha)-enriched, stacked thylakoid membranes) were resolved into nine fractions of 100% purity, labelled 1-9 in order of decreasing pI values. Seven of these fractions (3-9) were shown by absorption spectroscopy to stand for LHCII subcomplexes. The subcomplexes were established - by monitoring their circular dichroism spectra and comparing them to the spectra of native LHCII trimers and monomers - to be structurally intact trimers. The analysis of polypeptide composition of the subcomplexes in terms of apparent molecular masses and Lhcb genes' products led us to the conclusion that each of the subcomplexes might be a mixed population of closely similar individual trimers, comprising of permutations of Lhcb1 and Lhcb2 (subcomplexes 3-7) or Lhcb1, Lhcb2 and Lhcb3 (subcomplexes 8 and 9).

Identification of Lhcb1/Lhcb2/Lhcb3 heterotrimers of the main light-harvesting chlorophyll a/b-protein complex of Photosystem II (LHC II)

Using non-denaturing isoelectric focusing in polyacrylamide vertical slab gel, we have purified to homogeneity three trimeric subcomplexes of LHC II from Arabidopsis thylakoid membranes. The polypeptide composition of the subcomplexes were studied by immunoblotting. Our results indicate the existence in vivo of LHC II heterotrimers containing Lhcb1, Lhcb2 and Lhcb3 gene products.