Isolation and characterization of a subunit form of the light-harvesting complex of Rhodospirillum rubrum

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Abstract
A new method is described for the isolation of subunits of the light-harvesting complex from Rhodospirillum rubrum (wild type and the G-9 mutant) in yields that approach 100%. The procedure involved treating membrane vesicles with ethylenediaminetetraacetic acid-Triton X-100 to remove components other than the light-harvesting complex and reaction center. In the preparation from wild-type cells, a benzene extraction was then employed to remove carotenoid and ubiquinone. The next step involved a careful addition of the detergent n-octyl .beta.-D-glucopyranoside, which resulted in a quantitative shift of the long-wavelength absorbance maximum from 873 to 820 nm. This latter complex was then separated from reaction centers by gel filtration on Sephadex G-100. The pigment-protein complex, now absorbing at 820 nm, contained two polypeptides of about 6-kilodalton molecular mass (referred to as .alpha. and .beta.) in a 1:1 ratio and two molecules of bacteriochlorophyll (BChl) for each .alpha..beta. pair. This complex is much smaller in size than the orignal complex absorbing at 873 nm but probably is an associated form such as .alpha.2.beta.2 .cntdot. 4BChl or .alpha.3.beta.2 .cntdot. 4BChl. The 820-nm form could be completely shifted back to a form once again having a longer wavelength .lambda.max near 873 nm by decreasing the octyl glucoside concentration. Thus, the complex absorbing at 820 nm appears to be a subunit form of the original 873-nm complex.