The distribution of radioactive peptides synthesized by polysomes and ribosomal subunits combined in vitro with microsomal membranes

Demonstration of specific receptors of the rough endoplasmic membrane for the signal sequence of carp preproinsulin

1. Evidence is given for the cotranslational processing in a wheat germ cell-free system of carp preproinsulin to proinsulin by dog pancreatic rough endoplasmic membranes stripped of ribosomes by EDTA. Concomitantly with processing there is a translocation of the protein across the membrane as shown by the accessibility to proteases. Conversion of preproinsulin to proinsulin occurs post-translationally only in the presence of detergent. 2. Stripped rough endoplasmic membranes bind a significant proportion of the cell-free translation products synthesized in a wheat germ system in response to poly(A)-rich RNA from carp islets. Among the bound material there is completed preproinsulin. The bound material is accessible to proteases. It is located on the cytoplasmic side of the membranes. Peptides lacking a signal sequence, such as globin or proinsulin, are not bound. The binding can be abolished by pretreatment of the rough membranes with proteases. Neither smooth endoplasmic membranes from dog pancreas nor erythrocyte plasma membranes from rabbits are able to bind signal peptides. The binding sites on the rough membranes can be saturated with cell-free products coded by carpislet RNA and are then no longer able to process preproinsulin to proinsulin in a contranslational assay. The amount of membranes necessary to bind a certain amount of cell-free products post-translationally corresponds roughly to the amount required for complete conversion of preproinsulin to proinsulin cotranslationally. The binding phenomenon occurs independently of the presence of ribosomes and is not sensitive to high ionic strength. A hydrophobic peptide (Ac-Lys-Phe-Phe-Gly-Leu-Nle-NH2) did not compete with signal peptides. 3. The results show that stripped rough endoplasmic membranes possess specific protein-containing receptors for signal sequences on their cytoplasmic side. The recognition of the signal by the endoplasmic membranes occurs without participation of the ribosomes. The latter are only required for the translocation of the nascent peptide into the lumen of the membrane.

The binding of monoribosomes, oligoribosomes and polyribosomes to reticular membranes from rat liver

1. The activity of the enzyme protein disulphide-isomerase (EC 5.3.4.1) was used to measure the binding of ribosomal fractions to reticular membranes from the liver of unstarved male rats. 2. Membranes degranulated by treatment with KCl plus puromycin and washed by centrifugation through 0.5 M-KCl bound oligoribosomes but not monoribosomes. 3. Substitution of 0.25 M-KCl for 0.5 M-KCl in the wash solution produced membranes that bound monoribosomes and 60 S subunits in addition to oligoribosomes. 4. The binding of all classes of ribosomes to smooth and 'lightly granulated' rough membranes was activated by oestradiol. The hormone activation was much greater with polyribosomes from fed rats than for monoribosomes or oligoribosomes from starved rats. 5. Exposure of rough membranes to centrifugal forces caused them to undergo partial degranulation, as demonstrated by unmasking of protein disulphide-isomerase activity.

Isolation and characterization of membrane proteins responsible for attachment of polyribosomes to rough microsomal fraction of rat liver

A protein fraction which has a high affinity for polyribosomes was isolated from rough microsomal membranes of rat liver. The mode of polyribosome binding to this fraction (R-fraction) was studied by using CsCl equilibrium centrifugation and compared with that for stripped rough microsomal membranes. The following were found. (1) The polyribosome-binding cpacity of the R-fraction was heat-labile and sensitive to trypsin, and was suppressed by increasing KCl concentration and addition of 0.1 mM-aurintricarboxylic acid. (2) Of the four subfractions obtained by gel filtration of the R-fraction on a Sephadex G-200, only the R1-fraction, eluted at the void volume, showed a high affinity for polyribosomes. The polyribosome-binding capacity of the R1-fraction decreased with time on storage at 4 degrees C. (3) The R1-fraction contained three major proteins with mol. wts. 108,000, 99,000 and 65,000.

Structure of rough, smooth, stripped and reconstituted rough membranes derived from rat liver as visualized by the freeze fracture technique

The structure of purified fractions of rough, smooth, stripped rough and reconstituted rough membranes have been investigated by the freeze etching technique. Preparations of rough and reconstituted rough membranes, active in protein synthesis, show vesicles whose outer surface is covered with ribosome-like particles. The inner surface of these vesicles contains also numerous particles of the same size. The particles located on the outer surface are largely absent in the stripped rough membrane preparations which, however, retain the particles located on the inner face. Particles were not seen either on the outer nor on the inner face of the smooth membranes. The possibility is considered that the particles located on the inner face are specific to the rough membranes and might play a role in the specific binding of ribosomes to the membranes.

Transfer of proteins across membranes. I. Presence of proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-bound ribosomes of murine myeloma

Fractionation of MOPC 41 DL-1 tumors revealed that the mRNA for the light chain of immunoglobulin is localized exclusively in membrane-bound ribosomes. It was shown that the translation product of isolated light chain mRNA in a heterologous protein-synthesizing system in vitro is larger than the authentic secreted light chain; this confirms similar results from several laboratories. The synthesis in vitro of a precursor protein of the light chain is not an artifact of translation in a heterologous system, because it was shown that detached polysomes, isolated from detergent-treated rough microsomes, not only contain nascent light chains which have already been proteolytically processed in vivo but also contain unprocessed nascent light chains. In vitro completion of these nascent light chains thus resulted in the synthesis of some chains having the same mol wt as the authentic secreted light chains, because of completion of in vivo proteolytically processed chains and of other chains which, due to the completion of unprocessed chains, have the same mol wt as the precursor of the light chain. In contrast, completion of the nascent light chains contained in rough microsomes resulted in the synthesis of only processed light chains. Taken together, these results indicate that the processing activity is present in isolated rough microsomes, that it is localized in the membrane moiety of rough microsomes, and, therefore, that it was most likely solubilized during detergent treatment used for the isolation of detached polysomes. Furthermore, these results established that processing in vivo takes place before completion of the nascent chain. The data also indicate that in vitro processing of nascent chains by rough microsomes is dependent on ribosome binding to the membrane. If the latter process is interfered with by aurintricarboxylic acid, rough microsomes also synthesize some unprocessed chains. The data presented in this paper have been interpreted in the light of a recently proposed hypothesis. This hypothesis, referred to as the signal hypothesis, is described in greater detail in the Discussion section.

Interactions of liver encoplasmic reticulum membranes and polysomes in vitro

The interactions of various preparations of endoplasmic reticulum membranes and polysomes have been studied by means of a sandwich sucrose gradient that clearly isolates free ribosomes, smooth endoplasmic reticulum (S.E.R.) and rough endoplasmic reticulum (R.E.R.) from the microsomal fraction of rat liver homogenates. Reconstructed rough membranes separate well from the native R.E.R. but occupy the same position along the gradient as the S.E.R. and the rough membranes, stripped of their ribosomes by means of LiCl. Native R.E.R. and S.E.R. do not bind any added labeled polysomes at 0 degree C; previous treatment with LiCl does not modify the behavior of S.E.R. The presence of cell sap during the binding reaction does not increase polysome fixation by stripped-rough membranes but protects in some way the polysomes and preserves all their original functional capacity of amino acid incorporation into protein.

The in vitro reconstitution of a functional rough membrane active in protein synthesis

Rough endoplasmic reticulum was reconstituted from free polyribosomes and rough membrane stripped from its ribosomes by KCl and puromycin. The reconstituted rough membrane resembled the native rough membrane in the following aspects: RNA/protein ratio, buoyant density in a continuous sucrose gradient, amino acid incorporation capacity and sensitivity towards protein synthesis inhibitors. When the reconstitution was done with double labelled polyribosomes ([32-P] polyribosomes, [3-H] leucine labelling of nascent peptide chain before or after the attachment of the polyribosomes to the membrane) both labels banded together with the reconstituted rough membrane band. Hybrid rough membrane could be formed from rat liver stripped rough membrane and wheat germ ribosomes. This hybrid membrane could translate globin mRNA.