Proteins of ribosome-bearing and free-membrane domains in Bacillus subtilis

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Abstract
In lysates of B. subtilis, a free-membrane fraction without ribosomes can be separated from the denser membrane-ribosome complexes. As determined by 1-dimensional sodium dodecyl sulfate gel electrophoresis, these 2 fractions differ markedly in protein composition; at least 6 major bands (MW 130,000, 92,000, 68,000, 64,000, 45,000 and 31,000) are essentially unique to the complexed-membrane fraction (CM proteins) and 2 are unique to the free-membrane fraction. After growth was slowed, the proportion of the free-membrane fraction increased, but the composition of this fraction was the same; after puromycin treatment, which abruptly increased the proportion of the free-membrane fraction, this fraction contained CM proteins. Thus, the 2 fractions recovered from growing cells may represent topographically and functionally distinct domains. The effect of growth rate suggests that formation of the complexed domain is regulated at least roughly in parallel with the formation of ribosomes. The separation of these membrane fractions should facilitate the study of protein secretion, membrane topography and morphogenesis in bacteria.