A 64-kilodalton membrane protein of Bacillus subtilis covered by secreting ribosomes.

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Abstract
The complexed (ribosome-bearing) membrane fraction of B. subtilis contains several proteins (CM-proteins) that are virtually absent from the ribosome-free fraction and hence might be components of the apparatus of protein secretion. The accessibility of 4 of these proteins on the 2 surfaces of the membrane, as exposed in protoplasts or in inverted membrane vesicles, was determined by trypsin digestion and by labeling with a nonpenetrating reagent (diazoiodosulfanilic acid). The 68-kilodalton[kd] protein is a transmembrane protein and the 45-kd protein faces only the external surface; the 31-kd protein is inaccessible from either side. Of particular interest is the 64-kd protein; it can be digested by trypsin and can bind antibody, on the cytoplasmic surface, but only after the ribosomes are released. This protein is evidently a component of the apparatus of protein secretion, closely covered by secreting ribosomes. Whether the other CM-proteins are also involved in protein secretion is uncertain.

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