Abstract
The chlorophyll-protein complexes of purified maize photosystem II membranes were separated by a new mild gel electrophoresis system under conditions which maintained all of the major chlorophyll a/b-protein complex (LHCII) in the oligomeric form. This enabled the resolution of three chlorophyll a/b-proteins in the 26-31-kDa region which are normally obscured by monomeric LHCII. All chlorophyll a/b-proteins had unique polypeptide compositions and characteristic spectral properties. One of them (CP26) has not previously been described, and another (CP24) appeared to be identical to the connecting antenna of photosystem I (LHCI-680). Both CP24 and CP29 from maize had at least one epitope in common with the light-harvesting antennae of photosystem I, as shown by cross-reactivity with a monoclonal antibody raised against LHCI from barley thylakoids. A complex designated Chla.P2, which was capable of electron transport from diphenylcarbazide to 2,6-dichlorophenolindophenol, was isolated by nondenaturing gel electrophoresis. It lacked CP43, which therefore can be excluded as an essential component of the photosystem II reaction center core. Fractionation of octyl glucoside-solubilized photosystem II membranes in the presence and absence of Mg2+ enabled the isolation of the Chla . P2 complex and revealed the existence of a light-harvesting complex consisting of CP29, CP26, and CP24. This complex and the major light-harvesting system (LHCII) are postulated to transfer excitation energy independently to the photosystem II reaction center via CP43.
Highlights
26-31-kDa region which are normally obscured by monomeric LHCII
We show that carotenoids extend the spectral rangoef light absorption and CP24 of PSII is almost identical to the LHCI-6o8f0PSI and transfer energy tochlorophyll a
While there was absolutely no cross-reactivitwy ith the polypeptides of LHCII**, there was a very strong reaction with the 29-kDa apoprotein of CP29. These results show that CP29 and CP24 share at least one epitope
Summary
Bassi and co-workers [4, 35]. Gel Electrophoresis-Glycerol was added to a final concentration of40% toPSIImembranes,suspendedinstorage buffer (1 mg chlorophyll/ml), which were solubilized using a 10% stock so-. Tion a t 15,000 X g for 15 min and the supernatant was loaded onto acrylamide tube gels(4% stacking and 11% resolving). (14-20% acrylamide) containing 6 M urea and run a t 15 mA for 3 days using the buffer system described by Bassi et a/. Chlorophyll-proteins were prepared as described above, their apoproteins were resolved in urea-containing gels and transferred to a nitrocellulose filter (Millipore). Sucrose GradienUt ltracentrifugation-PSI1 membranes (1 mg chlorophyll/ml) in 5 mM Tricine, pH 8.0, were solubilized with octyl glucoside (octyl glucoside:chlorophylI, 401) for 5 minon ice and centrifuged a t 15,000 X g for 15 min. Afterillumination for 10min at 20 "C, thethylakoids were pelleted by centrifugation, solubilized in octylglucoside as described above, and resolved into component chlorophyll-proteins by nondenaturingSDS-PAGE.Thechlorophyll-proteins wereexcised and CP29. 680 680 680 reruninthepresence of 6 M urea (see above) and exposedfor autoradiography
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