Ribosome-Membrane Interaction in Eukaryotic Cells
Ribosomes in eukaryotic cells occur either free or bound to the membranes of the endoplasmic reticulum (ER). So far neither structural nor functional differences have been clearly revealed between these two classes of ribosomes. Thus, available evidence suggests that the ribosome-membrane interaction is not obligatory for the process of polypeptide synthesis per se in membrane-bound ribosomes. Rather this interaction is related to the fate of the nascent chain, providing the structural conditions for its transfer into the membrane-bounded compartment of the ER. Subsequently, the segregated chains may undergo the modifications (proteolytic cleavage, e.g. proinsulin; attachment of carbohydrate, e.g. immunoglobulins) required for secretion, storage, or disposal in the various intracellular membrane-bounded compartments (Golgi complex, condensing vacuoles, zymogen granules, peroxisomes, lysosomes). A role of the ribosome-membrane interaction in the vectorial discharge of proteins into the ER is indicated by (a) the close association of the nascent polypeptide chain with the ER membrane (1, 2); (b) the close association of the large ribosomal subunit with this membrane, detected both electron microscopically and biochemically (3); (c) the selective translation of specific mRNA’s on either free or membrane-bound ribosomes (4, 5, 6). This last observation implies that the information as to whether a particular mRNA is to be translated by free or membrane-bound ribosomes lies in mRNA itself. This and other features of protein biosynthesis (the cycle of ribosomal subunits (7)) can be incorporated into a tentative scheme such as the one illustrated below.
KeywordsMagnesium Hydrate Carbohydrate Codon Polypeptide
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