Pigment-Protein Complexes of Thylakoid Membranes: Assembly, Supramolecular Organization

Organization of the photosynthetic units, and onset of electron transport and excitation energy distribution in greening leaves

“…When the rate of Chl formation is enhanced, the units increase in size by incorporation of LHC. This step-wise formation and growth of the units has been observed (a) in etiolated leaves exposed first to intermittent light (4,7,13), ms flashes (9, 52), far-red light (29,34,39), or vary low-light intensity (8,50) and then transferred to continuous light of normal intensity; and (b) in etiolated leaves transferred to darkness after a short preexposure to continuous light (1,2,7,17).…”

“…The absence of LHC-I and accordingly of CPIa, from protoehloroplasts'" and their formation after transfer of the intevmittent light plants to continuou~ light, is also evident from the 770K fluorescence emission spectra of the leaves and the isolated plastids (7,19,31). As shown in fig.…”

“…Recent~ilder aDS-PAGE procedures have been successful in separating more than 90% of the thylakoid Chl in association with proteins (II,15), allowing the separation of six major pigment-protein complexes from SDS-solubilizedmature thylakoids: CPIa, CPI. LHCPI, LHCP2, CPa and LHCP3 (7,15), with apparent Mr of 190, 110, 90, 65,50, and 35 kDa, respectively (7). These complexes are real entities existing in vivo, as shown by C14-1abelling experiments (18).…”

“…originate in the PSI unit, while the LHCPs and CPa in the P$II unit. CPI comes from the core of the PSI unit, and contains Ch!a, P700 (30 Chl/PT00), b-car and two polypeptides of about 69 kDa (7,381; its fluorescence emission peak at 77°K is at 725 nm (7,15,191. CPIa is the PSI unit in a solubilized form, as suggested from its identical polypeptide composition to that of the Triton X-1OO-extracted PSI particles (it contains the 69 kDa components of CPI, and polypeptides of 24, 23, 21, 20, 16-15, 10.5-9 kDa) (12,46); it contains Chla, Chlb, P700 (100 Chl/PT00), lutein and b-car and its emission band at 77OK is at 732 nm (12).…”

“…All these components are formed in parallel upon transfer of the plants to continuous light. Since there is a very good correlation between the formation of Chlb, LHC-II and grana, and the onset of the cation regulation of the excitation energy distribution between PSII and PSI, it has been suggested that LHC-II and grana are involved in this process (7,21,33). The rational for this involvement is that when grana are formed there is a spatial separation of PSII and PSI units, and accordingly, inhibition of excitation energy spillover from PSII to PSI.…”