The roles of Lhcb1 och Lhcb2 in regulation of photosynthetic light harvesting

Abstract: Photosynthesis in higher plants relies upon collection of light by chlorophyll molecules associated with light harvesting chlorophyll a/b-binding (LHC) proteins. The two most abundant of these are Lhcb1 and Lhcb2, which make up light harvesting complex (LHC) II trimers. They are also involved in facilitating state transitions, a process during which energy balancing between photosystem (PS) II and I is achieved. Overexcitation of PSII reduces the plastoquinone pool which activates STN7, a kinase, that phosphorylates a threonine residue on Lhcb1 and Lhcb2. In order to studythe kinetics of this we developed antibodies capable of recognizingphosphorylated forms of each of these proteins. This showed that Lhcb2 is more rapidly phosphorylated than Lhcb1, that there are no differences in the migration of phosphorylated and non-phosphorylated forms of Lhcb1 and Lhcb2 and that the majority of phosphorylated LHCII (P-Lhcb1 and PLhcb2) are associated with super- and megacomplexes. Furthermore, a state 2-specific LHCII-PSI-LHCI band contains P-Lhcb2 but almost no P-Lhcb1, and a band corresponding to M trimers (band 4 from sucrose gradients, composed of LHCII, CP24 and CP29), contains only P-Lhcb1 but no P-Lhcb2. We also developed artificial microRNA lines specifically depleted in either Lhcb1 or Lhcb2, amiLhcb1 and amiLhcb2 respectively. We show that the roles of Lhcb1 and Lhcb2 in state transitions are complementary. Lhcb1 modulate the size of grana stacks. In the absence of Lhcb1 only a few LHC trimers are formed, while in the absence of Lhcb2, the antenna looks like in the wild type although the plants cannot perform state transitions normally. Trimers containing P-Lhcb2 functionally detach from PSII and connect to PSI to balance the relative excitation pressure. State transitions only occur when both Lhcb1 and Lhcb2 are present, presumably in a (Lhcb1)2 Lhcb2 heterotrimer. In absence of Lhcb2, the LHCII-PSI-LHCI supercomplex is not formed indicting that P-Lhcb2 mediates attachment of LHCII to PSI.We tried complementing amiLhcb2 with modified Lhcb2 genes coding for proteins with altered amino acids, Arg2 to Lys or the phosphorylatable Thr3 residue to Asn or Ser. Introduction of the additional gene often causes loss of amiRNA-inhibition, however we could confirm that substitution of the Thr3 with Asn led to the absence of Lhcb2 phosphorylation and thus no state transition.