The 64-kilodalton membrane protein of Bacillus subtilis is also present as a multiprotein complex on membrane-free ribosomes.

The 64-kDa membrane protein of Bacillus subtilis is evidently involved in the attachment of secreting ribosomes to membrane. On immunoprecipitation with antibody to this protein, the solubilized particulate fraction, with or without prior chemical cross-linking, yields a complex of four proteins (64, 60, 41, and 36 kDa). This "S complex" was found to be associated with membrane-free ribosomes rather than with membrane, but the 64-kDa protein is also present, without the other proteins of the S complex, in the membrane-ribosome fraction and in the cytosol. Only the form present in the membrane-ribosome fraction is protected from protease. These findings suggest a cycle in which the complex participates in initiation of secretion but not in the later stages. It is not yet clear whether the 64-kDa protein found in the membrane-ribosome complexes is retained from the S complex after initiation and later recycled via the cytosol or whether it is a separate pool.