Chlorophyll binding to peptide maquettes containing a retention motif

The motif Glu-X-X-His/Asn-X-Arg is conserved in the first and third membran e-spanning domains of all light-harvesting chlorophyll a/b- and ale-binding proteins in chloroplasts. Molecular modeling of synthetic peptides contain ing the sequence Glu-Ile-Val-His-Ser-Arg, a motif found in the apoprotein o f the major light-harvesting complex in plants, generated a loop structure formed by intrapeptide, electrostatic attraction between Glu and Arg. His, Asn, and charge-compensated Glu-Arg pairs are known ligands of the magnesiu m atom in chlorophyll, The prediction that this structure should bind two m olecules of chlorophyll was confirmed experimentally with an assay based on fluorescence resonance energy transfer between peptides and chlorophyll a. Motifs with both potential ligands bound approximately two times the amoun t of chlorophyll as one in which His was replaced by Ale. These results sup port the conclusion that formation of this intermediate, within membranes o f the envelope, is a crucial step in assembly of light-harvesting complexes and a mechanism that regulates import of the apoproteins into the chloropl ast.