Chemically synthesized polypeptides have been utilized with a reconstitution assay to determine the role of specific amino acid side chains in stabilizing the core light-harvesting complex (LH1) of photosynthetic bacteria and its subunit complex. In the preceding paper [Meadows, K. A., Parkes-Loach, P.S., Kehoe, J. W., and Loach, P. A. (1998) Biochemistry 37, 3411-3417], it was demonstrated that 31-residue polypeptides (compared to 48 and 54 amino acids in the native polypeptides) having the same sequence as the core region of the β-polypeptide of Rhodobacter sphaeroides (sphβ31) or Rhodospirillum rubrum (rrβ31) could form subunit-type complexes. However, neither polypeptide interacted with the native α-polypeptides to form a native LH1 complex. In this paper, it is demonstrated that larger segments of the native Rb. sphaeroides β-polypeptide possess native behavior in LH1 formation. Polypeptides were synthesized that were six (sphβ37) and ten amino acids (sphβ41) longer than sphβ31. Although sphβ37 exhibited behavior nearly identical to that of sphβ31, sphβ41 behaved more like the native polypeptide. In the case of rrβ31, a polypeptide with four additional amino acids toward the C terminus was synthesized (rrβ35). Because LH1- forming behavior was not recovered with this longer polypeptide, one or more of the three remaining amino acids at the C-terminal end of the native β- polypeptide seem to play an important role in LH1 stabilization in Rs. rubrum. Three analogues of the core region of the Rb. sphaeroides β- polypeptide were synthesized, in each of which one highly conserved amino acid was changed. Evidence was obtained that the penultimate amino acid, a Trp residue, is especially important for subunit formation. When it was changed to Phe, the λ(Max) of the subunit shifted from 823 to 811 nm and the association constant decreased about 500-fold. Changing each of two other amino acids had smaller effects on subunit formation. Changing Trp to Phe at the location six amino acid residues toward the C terminus from the His coordinated to Bch1 resulted in an approximately 10-fold decrease in the association constant for subunit formation but did not affect the formation of a LH1-type complex compared to sphβ31. Finally, changing Arg to Leu at the location seven amino acid residues toward the C terminus from the His coordinated to Bchl decreased the association constant for subunit formation by about 30fold. In this case, no LH1-type complex could be formed. On the basis of these results, in comparison with the crystal structure of the LH2 β-polypeptide of Rhodospirillum molischianum, two possible structures for the subunit complex are suggested.
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