RECONSTITUTION OF CORE LIGHT-HARVESTING COMPLEXES OF PHOTOSYNTHETIC BACTERIA USING CHEMICALLY SYNTHESIZED POLYPEPTIDES - 1 - MINIMAL REQUIREMENTS FOR SUBUNIT FORMATION
Ka. Meadows et al., RECONSTITUTION OF CORE LIGHT-HARVESTING COMPLEXES OF PHOTOSYNTHETIC BACTERIA USING CHEMICALLY SYNTHESIZED POLYPEPTIDES - 1 - MINIMAL REQUIREMENTS FOR SUBUNIT FORMATION, Biochemistry, 37(10), 1998, pp. 3411-3417
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.