Light-harvesting complex phosphorylation

In order for PSIl and PSI to act in sequence during linear electron flow, the amount of light energy delivered to the two reaction centers must be controlled so that each center activates the same number of electrons. If the two photosystems are not equally excited, then cyclic electron flow occurs in PSI, and PSIl becomes less active. One mechanism for regulating the relative contribution of linear and cyclic electron flow in chloroplasts entails the reversible phosphorylation and dephosphorylation of the proteins associated with the PSIl light-harvesting complex (LHCII). 

Reversible phosphorylation and dephosphorylation of the PSII light-harvesting complex control the functional organization of the photosynthetic apparatus in thylakoid membranes. State I favors linear electron flow, whereas state II favors cyclic electron flow (see Figure 8-40). 

Fig. 18. Schematic model of the presumed mechanism for rapidly reversible nonphotochemical quenching (q ) and the proposed mechanism for sustained NPQ (q,). LHCs = light-harvesting complexes of PS II LHCII = major, peripheral light-harvesting complex of PS II LHCII-P = phosphorylated LHCII Z = zeaxanthin...

As seen in figure 1 mainly two polypeptides become phosphorylated under suitable conditions in the light, one at about 55 kD and one at 6 kD, the latter being identified as the subunit of the light-harvesting complex. As observed by Allen and Holmes [4] the phosphorylation of the B875 complex depends upon the redox state of the quinone pool. Furthermore the phosphorylation of B875 occurs only when the cytochrome b-cl complex is oxidized either by potassium ferricyanide or in the presence of DBMIB. 

The B875 kinase activity present in the supernatant fraction was then isolated by affinity chromatography on a Sepharose-ATP column and gel filtration. The purified enzyme was present in only low amounts but rapidly phosphorylated intact chromatophores in the presence of DBMIB or isolated B875 light harvesting complexes, as well as histone V-S (Fig. 5). Its molecular weight was in the range of 18 kD. 

Larrson UK, Sundby C and Andersson B. (1987). Characterization of two different subpopulations of spinach light-harvesting chlorophyll a-i-protein complex (LHC-II) polypeptide composition, phosphorylation pattern and association with photosystem-II. 

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