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
B. Grabowski et al., 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, PHOTOSYN R, 63(1), 2000, pp. 85-96
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.