Assembly of Light-Harvesting Complexes of Photosystem II and the Role of Chlorophyll b

Charge separation within a reaction center, whether in Photosystem I or II, can occur on a time scale of picoseconds. Yet it would take much longer—50–100 ms—between such events if a photosystem depended on only the 6 chlorophyll (Chi) molecules in a reaction center for direct absorption of a photon, even at full sunlight. An increase in the absorption bed to 100 Chls would reduce the time between photon capture to about 1–2 ms per reaction center. In fact, the number of Chi molecules in a photosystem in plants is generally several-fold greater. Photosynthetic organisms have the ability to increase absorption of light by assembly of light-harvesting complexes (LHCs), a major factor in the evolutionary development of efficient photosynthesis. Most of the Chi in green algae and plants resides in LHCs that transfer excitation energy on a pico- to nano-second time scale to the photosystem reaction centers that they surround. The apoproteins of these complexes, encoded in nuclear genes, are synthesized outside of the chloroplast and, after import into the chloroplast, reside in thylakoid membranes. Chi b in chlorophytes and Chi c in chromophytes play an important role in retention of the apoproteins in the chloroplast and subsequent assembly of the complexes. Energy transfer from Chi b to Chi a has been a powerful tool to monitor LHC II assembly in vivo during chloroplast development. Ultrafast kinetic analysis of Forster fluorescence resonance energy transfer, coupled with reconstitution of complexes with mutagenized proteins in which Chi-binding amino acids were replaced with non-liganding residues, identified the Chi species in most of the 12 sites in LHC II. In this chapter, the extensive amount of experimental data on LHC II accumulation in Chi Mess plants, much of it based on the use of fluorescence in experimental measurements, is summarized to provide a biological basis for evaluating the in vitro reconstitution results. Although the evidence supports processing of LHCP precursors before import into the chloroplast is completed, and thus assembly of LHC II within the inner membrane of the chloroplast envelope-a process that requires Chi and is facilitated by Chi b, the details of the assembly mechanism are not known. A model is proposed according to which assembled LHC II is transported via invagination and vesiculation of the envelope inner membrane, whereas unassembled LHCPs, in the absence of Chi, are directed to cytoplasmic vacuoles for degradation.