Large crystalline sheets of ribosomes (‘ribosomal bodies’) grow in the oocytes of the lizard, Lacerta sicula, during winter hibernation. Individual ribosomes in the sheets attach to endoplasmic reticulum membranes via a salt-sensitive linkage indistinguishable from that associated with membrane-bound ribosomes in secretory cells. A three-dimensional map obtained from the isolated sheets showed both ribosomal subunits to be adjacent to the membrane surface, attached to it by a part protruding from the large subunit. This configuration places the end of the exit channel for nascent protein close to the membrane attachment site, so that the emerging chain is optimally located to interact with components of the translocation apparatus.
Unwin, P.N.T. and Taddei, C. Packing of ribosomes in crystals from the lizard, Lacerta Sicula. J. Mol. Biol. 114, 491-506 (1977). (pdf)
Unwin, P.N.T. Three-dimensional model of membrane-bound ribosomes obtained by electron microscopy. Nature 269, 118-122 (1977). (pdf)
Unwin, P.N.T. Attachment of ribosome crystals to intracellular membranes. J. Mol. Biol. 132, 69-84 (1979). (pdf)
Milligan, R.A. and Unwin, P.N.T. Location of exit channel for nascent protein in 80S ribosome. Nature 319, 693-695 (1986). (pdf)