Characterization of the Porphyridium cruentum Chl a-binding LHC by in vitro reconstitution: LHCaR1 binds 8 Chl a molecules and proportionately more carotenoids than CAB proteins

The Porphyridium cruentum light harvesting complex (LHC) binds Chl a, zeaxa nthin and beta-carotene and comprises at least 6 polypeptides of a multigen e family. We describe the first in vitro reconstitution of a red algal ligh t-harvesting protein (LHCaR1) with Chl a/carotenoid extracts from P. cruent um. The reconstituted pigment complex (rLHCaR1) is spectrally similar to th e native LHC I, with an absorption maximum at 670 nm, a 77 K fluorescence e mission peak at 677 nm (ex. 440 nm), and similar circular dichroism spectra . Molar ratios of 4.0 zeaxanthin, 0.3 beta-carotene and 8.2 Chl a per polyp eptide for rLHCaR1 are similar to those of the native LHC I complex (3.1 ze axanthin, 0.5 beta-carotene, 8.5 Chl a). The binding of 8 Chl a molecules p er apoprotein is consistent with 8 putative Chl-binding sites in the predic ted transmembrane helices of LHCaR1. Two of the putative Chl a binding site s (helix 2) in LHCaR1 were assigned to Chl b in Chl a/b-binding (CAB) LHC I I [Kuhlbrandt et al. (1994) Nature 367: 614-21]. This suggests either that discrimination for binding of Chl a or Chl b is not very specific at these sites or that specificity of binding sites evolved separately in CAB protei ns. LHCaR1 can be reconstituted with varying ratios of carotenoids, consist ent with our previous observation that the carotenoid to Chl ratio is subst antially higher in P. cruentum grown under high irradiance. Also notable is that zeaxanthin does not act as an accessory light-harvesting pigment, eve n though it is highly likely that it occupies the position assigned to lute in in the CAB LHCs.