The conformational behaviour of the light-harvesting chlorophyll &-binding protein (LHCP), the apoprotein of the major light-harvesting complex (LHCII) of photosystem I1 in plants, has been studied. According to the circular dichroism in the ultraviolet range measured with isolated LHCII, the protein in the complex adopts a folded structure with a high content of a helix (about 60%), whereas the non-pigmented, solubilized protein has a less ordered structure (about 20% a helix).LHCP-pigment complexes that have been reconstituted from the overexpressed protein and isolated pigments in the presence of detergents display a protein CD signal similar to that of authentic LHCII, indicating that LHCP folds into the native structure during the reconstitution procedure. Renaturation of LHCP in these experiments is dependent on the presence of pigments and the formation of stable LHCP-pigment complexes. Pigment-induced engagement of LHCP in a compact structure has also been shown by two additional experimental approaches. (a) Upon complex formation, LHCP or its precursor (pLHCP) becomes resistant to trypsin digestion with the exception of an N-terminal segment of the protein; the same protection of LHCP is known to occur in intact thylakoids. (b) Pigment binding renders a cysteine residue within the N-proximal hydrophobic domain of the protein as well as a newly introduced cysteine four amino acid positions from the C terminus inaccessible to modification with a sulfhydryl-specific label whereas the N terminus stays susceptible to specific labelling. These observations support the notion that only the N terminus protrudes from a compact protein-pigment structure in LHCII. The fact that the major part of LHCP is trypsin-resistant in pigmented complexes reconstituted in the absence of a membrane or even lipids justifies caution in using protection against trypsin as a criterion for the integration of LHCP into the thylakoid membrane.The efficiency of photosynthesis is greatly enhanced by antenna or light-harvesting complexes which collect light energy and transduce it to the photosynthetic reaction centers. The most prominent antenna in higher plants is the major light-harvesting complex of photosystem I1 (LHCII) comprising about half of the total chlorophyll in the thylakoid membrane. LHCII consists of an apoprotein (LHCP) of 25 -28 kDa and pigments, i.e. 14 or 15 molecules chlorophyll a and b and 2 -4 molecules xanthophyll bound/protein monomer (Bassi et al., 1990; Peter and Thornber, 1991). These pigments are non-covalently bound to the apoprotein. Little is known about the molecular interactions between protein and pigments. The light-harvesting function of LHCII requires that captured light quanta must be rapidly transferred between pigments within an antenna to be efficiently delivered at a reaction center (Sauer, 1986). Very rapid energy transfer between antenna chlorophylls has been measured by Abbreviations. LHCII, major light-harvesting complex of photosystem 11; LHCP, light-harvesting chlorophyll &-binding protei...