The major antenna complex of photosystem II has a xanthophyll binding sitenot involved in light harvesting

We have characterized a xanthophyll binding site, called V1, in the major l ight harvesting complex of photosystem II, distinct from the three tightly binding sites previously described as L1, L2, and N1. Xanthophyll binding t o the V1 site can be preserved upon solubilization of the chloroplast membr anes with the mild detergent dodecyl-a-D-maltoside, while an IEF purificati on step completely removes the ligand. Surprisingly, spectroscopic analysis showed that when bound in this site, xanthophylls are unable to transfer a bsorbed light energy to chlorophyll a. Pigments bound to sites L1, L2, and N1, in contrast, readily transfer energy to chlorophyll a. This result sugg ests that this binding site is not directly involved in light harvesting fu nction. When violaxanthin, which in normal conditions is the main carotenoi d in this site, is depleted by the de-epoxidation in strong light, the site binds other xanthophyll species, including newly synthesized zeaxanthin, w hich does not induce detectable changes in the properties of the complex. I t is proposed that this xanthophyll binding site represents a reservoir of readily available violaxanthin for the operation of the xanthophyll cycle i n excess light conditions.