RECONSTITUTION OF CORE LIGHT-HARVESTING COMPLEXES OF PHOTOSYNTHETIC BACTERIA USING CHEMICALLY SYNTHESIZED POLYPEPTIDES - 1 - MINIMAL REQUIREMENTS FOR SUBUNIT FORMATION

Described are the chemical synthesis, isolation, and characterization of each of three polypeptides whose amino acid sequences reproduce por tions of the amino acid sequence of the beta-polypeptides of the core light-harvesting complex (LH1) of Rhodobacter sphaeroides or Rhodospir illum rubrum. The native beta-polypeptides of LH1 of these organisms c ontain 48 and 54 amino acids, respectively. The smallest synthetic pol ypeptide had an amino acid sequence identical to that of the last 16 a mino acids of the beta-polypeptide of Rb. sphaeroides (sph beta 16) bu t failed to form either a subunit-alpha LH1-type complex under reconst itution conditions. Also, this polypeptide, lengthened on the N termin us by adding the sequence Lys-Ile-Ser-Lys to enhance solubility, faile d to form a subunit-alpha LH1-type complex. In contrast, polypeptides containing either the 31 amino acids at the C terminus of the beta-pol ypeptide of Rb. sphaeroides (sph beta 31) or the equivalent 31 amino a cids of the beta-polypeptide of Rs. rubrum (rr beta 31) were fully com petent in forming a subunit-type complex and exhibited association con stants for complex formation comparable to or exceeding those of the n ative beta-polypeptides. The absorption and CD spectra of these subuni t-type complexes were nearly identical to those of subunit complexes f ormed with native beta-polypeptides. It may be concluded that all stru ctural features required to make the subunit complex are present in th e well-defined, chemically synthesized polypeptides. Neither polypepti de appeared to interact with the native alpha-polypeptides to form a L H1-type complex. However, sph beta 31 formed a LH1-type complex absorb ing at 849 nm without an alpha-polypeptide. Although chemical synthese s of polypeptides of this size are common, the purification of membran e-spanning segments is much more challenging because the polypeptides lack solubility in water. The chemical syntheses reported here represe nt the first such syntheses of membrane-spanning polypeptides which di splay native activity upon reconstitution.