FIGURE 12-46 Phosphorylation of LHCII and the regulation of linear versus cyclic electron flow. (a) (Top) In normal sunlight, PSI and PSII are equally activated, and the photosystems are organized in state 1. In this arrangement, light-harvesting complex II (LHCII) is not phosphorylated, and six copies of LHCII trimers, together with several other light-harvesting proteins, encircle a dimeric PSII reaction center in a tightly associated supercomplex in the grana (for clarity, molecular details of the supercomplexes are not shown). As a result, PSII and PSI can function in parallel in linear electron flow. (Bottom) When light excitation of the two photosystems is unbalanced (e.g., too much light via PSII), LHCII becomes phosphorylated, dissociates from PSII, and diffuses into the unstacked membranes, where it associates with PSI and its permanently associated LHCI. In this alternative supramolecular organization (state 2), most of the absorbed light energy is transferred to PSI, supporting cyclic electron flow and ATP production, but no formation of NADPH and thus no CO₂ fixation. (b) Model of a PSI “super-supercomplex” involved with PGRL1-dependent cyclic electron flow that was isolated from green algae in stage 2. The super-supercomplex contains multiple complexes, including the integral membrane protein PGRL1 (but not PGR5). See F. A. Wollman, 2001, EMBO J. 20:3623; and M. Iwai, et al., 2010, Nature 464:1210.