CHARACTERIZATION OF 2 TIGHT-HARVESTING SUBUNITS ISOLATED FROM THE BROWN ALGA PELVETIA-CANALICULATA - HETEROGENEITY OF XANTHOPHYLL DISTRIBUTION

The main light-harvesting fraction from Pelvetia canaliculata was isol ated on a sucrose density gradient from digitonin-solubilized chloropl asts. After further solubilization by dodecyl maltoside, the bulk frac tion was separated into two subunits by preparative isoelectric focusi ng, The more acidic brown fraction was mainly composed of 22 kDa polyp eptides having an apparent pi of 4.55. Its pigment composition was ver y simple, containing chlorophyll (Chi) a, Chi c and fucoxanthin. The i n vivo spectral properties of fucoxanthin, namely a shift in light abs orption to the preen and efficient energy transmission to Chi a, were conserved in this subunit. No xanthophyll associated with photoprotect ion was found in this band, even when obtained from photoinhibited tha lli. The less acidic green band contained predominantly 22 kDa polypep tides that were resolved into numerous components by denaturing isoele ctric focusing, Its pigment composition was more complex, containing, in addition, pigments of the so-called xanthophyll cycle. In photoinhi bited thalli, about half of the violaxanthin was converted into anther axanthin and zeaxanthin. All the pigments of the xanthophyll cycle wer e specifically associated with this subunit, and it may thus have a ce ntral role in the thermal dissipation of the absorbed light energy as postulated for light-harvesting complex II isolated from green plants.