Attachment of ribosome crystals to intracellular membranes

Abstract

The attachment to membranes of ribosome crystals formed by cooling lizard oocytes and chick embryos has been investigated by electron microscopy of whole cells and by biochemical and structural experiments, using the cross-linking reagent glutaraldehyde. It was found that the crystalline ribosomes in both animals form only on the rough endoplasmic reticulum and nuclear envelope, that they bind to these membranes through one unique site on the large ribosomal subunit, that the bond between the large subunit and the site on the membrane is sensitive to the concentration of K +, but not of Mg 2+, and that this bond is selectively stabilized by mild treatment with glutaraldehyde. These results closely match those obtained from ribosomes in secretory cells, suggesting that there may be no difference between the two sets of ribosomes in their direct interaction with membranes. The glutaraldehyde reaction was used to obtain crystals and components from which the small subunits had been preferentially released. A comparison between small subunit depleted and normal crystals led to an estimate for the positions of the subunits over the membrane surface. The side-by-side subunit assignments, “S” and “L”, suggested previously (Unwin & Taddei, 1977; Unwin, 1977), were confirmed. It was deduced further that the crystalline ribosomes have the long axis of their small subunit approximately parallel to the membrane surface, and appear raised up from this surface because of interaction between their large subunits.