Chlorophyll Proteins of Photosystem I

Abstract

Data are presented which suggest the existence of a light-harvesting pigment-protein complex which is functionally and structurally associated with photosystem I (PSI) reaction centers. These observations are based on techniques which aflow isolation of PSI using minimal concentrations of Triton X-100. Properties of density and self aggregation allowed purification of a "native" PSI complex. The isolated PSI particles appear as 106 A spherical subunits when viewed by freeze fracture microscopy. When incorporated into phosphatidyl choline vesicles, the particles lose self-aggregation properties and disperse uniformly within the lipid membrane. The isolated PSI preparation contains 110 ± 10 chlorophylls/P700 (Chi a/b ratio greater than 18); this represents a recovery of 27% of the original chloroplast membrane Chi. These particles were enriched in Chi a forms absorbing at 701 to 710 nm. Chi fluorescence at room temperature exhibited a maximum at 690 nm with a pronounced shoulder at 710 nm. At 77 K, peak fluorescence emission was at 736 am; in the presence of dithionite an additional fluorescence maximum at 695 nm was obtained at 77 K. This dual fluorescence emission peak for the PSI particles is evidence for at least two Chl populations within the PSI membrane subunit. The fluorescence emission observed at 695 nm was identified as arising from the core of PSI which contains 40 Chl/P700 (PSI40). This core complex, derived from native PSI particles, was enriched in Chi a absorbing at 680 and 690 nm and fluorescing with maximal emission at 694 am at 77 K. PSI particles consisting of the PSI core complex plus 20 to 25 Chi antennae (65 Chl/P700) could also be derived from native PSI complexes. These preparations were enriched in Chl a forms absorbing at 697 nm and exhibited a 77 K fluorescence emission maximum at 722 am. A comparison of native PSI particles which contain 110 Chl/P700 (PSI-110) and PSI particles containing 65 Chl/P700 (PSI-65) provides evidence for the existence of a peripheral Chi-protein complex tightly associated in the native PSI complex. The native PSI subunits contain polypeptides of 22,500 to 24,500 daltons which are not found in the PSI-65 or PSI-40 subfractions. It is suggested that these polypeptides function to bind 40 to 45 Chi per structural complex, including the Chi which emits fluorescence at 736 am. A model for the organization of Chli forms is presented in which the native PSI membrane subunit consists of a reaction center core complex plus two regions of associated light-harvesting antennae. The presence of energy "sinks" within the antennae is discussed.

The effects of quinone analogues on cytochrome b6 reduction and oxidation in a reconstituted system

“…The cytochrome complex isolated in the absence of Triton [9] and the PS I preparation [15] are obtained from greenhouse spinach leaves by published methods. The reconstituted system containing PS I, plastocyanin and the be-f complex has been described [12].…”

The role of the light harvesting complex and photosystem II in thylakoid stacking in thechlorina-f2 barley mutant

“…The Chl~-P 1" complex from chlorina-f2 differed from that of wild-type due to the lack of the antennae complexes LHCI-680 and LHCI-730, and thus corresponded to the PSI-65 particle of maize and pea (4,19), consisting of Chlo-Pl and low molecular weight po[ypeptides. When thylakoids isolated from chlorina-f2 plants grown under continuous illumination were examined, an additional band (x) appeared with a mobility similar to Chlo/~,-P2 (Figure 9a).…”

“…Sucrose gradient ultracentrifugation of Triton X-100 solubilized thylakoids offers a one-step method for the separation of PSI and PSII. It is important to follow the recommendation of MULLET et al (19) to adjust the Triton concentration to obtain about 10% of the total chlorophyll in the initial 30,000xg pellet. The PSI preparation from chlorina-f2 has the same characteristics as the PSI-65 preparation derived from PSI-110 (19).…”

“…It is important to follow the recommendation of MULLET et al (19) to adjust the Triton concentration to obtain about 10% of the total chlorophyll in the initial 30,000xg pellet. The PSI preparation from chlorina-f2 has the same characteristics as the PSI-65 preparation derived from PSI-110 (19). The PSII from chlorina-f2 is similarly free of its chlorophyll a/b-containing LHCII.…”

Probing in vitro translation products with monoclonal antibodies to a 15.2 kD polypeptide subunit of photosystem I

“…In addition, photosynthetically active PSI particles have been isolated with polypeptide patterns partially complementary to those missing in the PSI mutants (4,5,24,27,31,42). These PSI particles contain from 3 (31) to 7 or 16 (27,29) Coomassie staining polypeptides depending on the method and species used.…”