A gel filtration approach to the study of ribosome-membrane interactions

Protein disulphide-isomerase of chick-embryo tendon

Protein disulphide-isomerase can be partially purified from the high-speed-supernatant fraction of extensively disrupted chick-embryo tendon tissue. The catalytic properties of the preparation resemble those of the enzyme from mammalian liver. Gel electrophoresis and isoelectric focusing show the enzyme to be very acidic, with pI 4.4 +/- 0.3. Gel filtration indicates an Mr for the active enzyme of 140 000. The enzyme can be partially purified by preparative gel filtration or isoelectric focusing, but its limited stability has prevented purification to homogeneity; active fractions from both gel filtration and isoelectric focusing show two major polypeptide components by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. The major polypeptides present in partially purified preparations have Mr 45 000 and 55 000; the latter band co-distributes with the enzyme activity in fractionations by both gel filtration and isoelectric focusing. The subcellular location of the enzyme cannot be established from work on homogenates of whole tissue, which are extensively disrupted. In homogenates from isolated tendon cells, the enzyme is located in a vesicle fraction that is excluded from Sepharose 2B but is of low density and can only be sedimented at very high speeds. This fraction is identified as deriving from the endoplasmic reticulum on the grounds of marker-enzyme studies and electron microscopy.

Dog pancreatic microsomal-membrane polypeptides analysed by two-dimensional gel electrophoresis

The two-dimensional polyacrylamide-gel electrophoresis technique of O'Farrell [(1975) J. Biol. Chem 250, 4007-4021] was applied to resolve and analyse the polypeptide composition of dog pancreatic rough microsomal membranes, which were shown to be active in co-translational processing of preprolactin synthesized from pituitary mRNA in a translation system in vitro. About 100 polypeptides are resolved. Treatment of rough microsomal membranes with EDTA and high KCl concentration yielded membranes stripped of their ribosomes with retention of activity for translocation and processing. Stripped microsomal membranes showed a selective concentration of approximately 25 polypeptides in the membranes when analysed by two-dimensional polyacrylamide-gel electrophoresis. The two-dimensional electrophoretic profile was catalogued into polypeptides that are glycoproteins, those that contain free thiol groups disposed at the cytosolic surface of microsomal vesicles and those that are of secretory origin but have been entrapped in the microsomal preparation. Several secretory components, including amylase, procarboxypeptidases, lipase and anionic trypsinogen, were tentatively identified among the microsomal polypeptides. The rough and stripped microsomal membranes from dog pancreas show a characteristic set of seven major acidic polypeptides, which are also identifiable in microsomal-membrane preparations isolated from dog liver and rat liver. One of these polypeptides was identified as protein disulphide-isomerase (EC 5.3.4.1).

Two-dimensional electrophoresis of membrane proteins. Factors affecting resolution of rat-liver microsomal proteins

A comparison has been made of published techniques for the resolution of rat liver microsomal proteins by two-dimensional electrophoresis. The method of Kaderbhai and Freedman (Biochim. Biophys. Acta 601 (1980) 21-20) gives good resolution of acidic proteins but excludes hydrophobic integral membrane proteins of pI greater than 7, including cytochrome P-450 apoproteins. The method of Vlasuk and Walz (Anal. Biochem. 105 (1980) 112-120) gives good resolution of proteins of pI 5-8, including cytochromes P-450, but fails to resolve a major acidic protein of pI less than 5. Isoelectric focusing of microsomal proteins is improved by the use of high concentrations of urea and low concentrations of sample proteins. Zwitterionic detergents of the general formula R . N+(CH3)2 . CH2CH2CH2SO3- are effective in solubilizing microsomal proteins, either alone or in presence of non-ionic detergent; compounds with a long alkyl chain (C14 or C16) are most effective. Isoelectric focusing of microsomal proteins solubilized by zwitterionic detergents did not give good resolution, probably because of incomplete dissociation and denaturation of the proteins. These detergents could not be used in the presence of high concentrations of urea. Although no single method of two-dimensional electrophoresis gives complete resolution of the whole range of microsomal proteins, conditions can be optimized for specific sets of proteins of interest. The technique can be used to monitor differences in microsomal composition between rat strains, or following induction, and for a variety of other studies.

Kinetics and specificity of homogeneous protein disulphide-isomerase in protein disulphide isomerization and in thiol-protein-disulphide oxidoreduction

The protein disulphide-bond isomerization activity of highly active homogeneous protein disulphide-isomerase (measured by re-activation of 'scrambled' ribonuclease) is enhanced by EDTA and by phosphate buffers. As shown for previous less-active preparations, the enzyme has a narrow pH optimum around pH 7.8 and requires the presence of either a dithiol or a thiol. The dithiol dithiothreitol is effective at concentrations 100-fold lower than the monothiols reduced glutathione and cysteamine. The enzyme follows Michaelis-Menten kinetics with respect to these substrates; Km values are 4,620 and 380 microM respectively. The enzyme shows apparent inhibition by high concentrations of thiol or dithiol compounds (greater than 10 X Km), but the effect is mainly on the extent of reaction, not the initial rate. This is interpreted as indicating the formation of significant amounts of reduced ribonuclease in these more reducing conditions. The purified enzyme will also catalyse net reduction of insulin disulphide bonds by reduced glutathione (i.e. it has thiol:protein-disulphide oxidoreductase or glutathione:insulin transhydrogenase activity), but this requires considerably higher concentrations of enzyme and reduced glutathione than does the disulphide-isomerization activity. The Km for reduced glutathione in this reaction is an order of magnitude greater than that for the disulphide-isomerization activity, and the turnover number is considerably lower than that of other enzymes that can catalyse thiol-disulphide oxidoreduction. Conventional two-substrate steady-state analysis of the thiol:protein-disulphide oxidoreductase activity indicates that it follows a ternary-complex mechanism. The protein disulphide-isomerase and thiol:protein-disulphide oxidoreductase activities co-purify quantitatively through the final stages of purification, implying that a single protein species is responsible for both activities. It is concluded that previous preparations, from various sources, that have been referred to as protein disulphide-isomerase, disulphide-interchange enzyme, thiol:protein-disulphide oxidoreductase or glutathione:insulin transhydrogenase are identical or homologous proteins. The assay, nomenclature and physiological role of this enzyme are discussed.

Translocation of proteins across the endoplasmic reticulum. I. Signal recognition protein (SRP) binds to in-vitro-assembled polysomes synthesizing secretory protein

An 11S protein composed of six polypeptide chains was previously purified from a salt extract of dog pancreas microsomal membranes and shown to be required for translocation of nascent secretory protein across the microsomal membrane (Wistar and Blobel 1980 Proc. Natl. Acad. Sci. U. S. A. 77:7112-7116). This 11S protein, termed signal recognition protein (SRP), has been shown here (a) to inhibit translation in the wheat germ cell-free system selectively of mRNA for secretory protein (bovine preprolactin) but not of mRNA for cytoplasmic protein (alpha and beta chain of rabbit globin); (b) to bind with relatively low affinity (apparent KD less than 5 x 10(-5)) to monomeric wheat germ ribosomes; and (c) to bind selectively and with 6,000-fold higher affinity (apparent KD less than 8 x 10(-9)) to wheat germ ribosomes engaged in the synthesis of secretory protein but not to those engaged in the synthesis of cytoplasmic protein. Low- and high-affinity binding as well as the selective translation-inhibitory effect were abolished after modification of SRP by N-ethyl maleimide. High-affinity binding and the selective translation-inhibitory effect of SRP were largely abolished when the leucine (Leu) analogue beta-hydroxy leucine was incorporated into the nascent secretory polypeptide.

Translocation of proteins across the endoplasmic reticulum. II. Signal recognition protein (SRP) mediates the selective binding to microsomal membranes of in-vitro-assembled polysomes synthesizing secretory protein

Translocation-competent microsomal membrane vesicles of dog pancreas were shown to selectively bind nascent, in vitro assembled polysomes synthesizing secretory protein (bovine prolactin) but not those synthesizing cytoplasmic protein (alpha and beta chain of rabbit globin). This selective polysome binding capacity was abolished when the microsomal vesicles were salt-extracted but was restored by an 11S protein (SRP, Signal Recognition Protein) previously purified from the salt-extract of microsomal vesicles (Walter and Blobel, 1980. Proc. Natl. Acad. Sci. U. S. A. 77:7112-7116). SRP-dependent polysome recognition and binding to the microsomal membrane was shown to be a prerequisite for chain translocation. Modification of SRP by N-ethyl maleimide abolished its ability to mediate nascent polysome binding to the microsomal vesicles. Likewise, polysome binding to the microsomal membrane was largely abolished when beta-hydroxy leucine, a Leu analogue, was incorporated into nascent secretory polypeptides. The data in this and the preceding paper provide conclusive experimental evidence that chain translocation across the endoplasmic reticulum membrane is a receptor-mediated event and thus rule out proposals that chain translocation occurs spontaneously and without the mediation by proteins. Moreover, our data here demonstrate conclusively that the initial events that lead to translocation and provide for its specificity are protein-protein (signal sequence plus ribosome with SRP) and not protein-lipid (signal sequence with lipid bilayer) interactions.

Resolution of microsomal membranes into fractions differing in polypeptide composition

Microsomes from rat liver, prepared by gel filtration, were subjected to centrifugation in a continuous sucrose density gradient containing a low concentration of deoxycholate. The membranes were subfractionated into five bands differing in appearance and equilibrium density. Each band, when analysed by sodium dodecyl sulphate polyacrylamide gel electrophoresis, displayed a characteristic population of membrane proteins.

Purification of a membrane-associated protein complex required for protein translocation across the endoplasmic reticulum

The capacity of microsomal membranes to translocate nascent presecretory proteins across their lipid bilayer can be largely abolished by extracting them with high ionic strength buffers. It can be reconstituted by adding the salt extract back to the depleted membranes [Warren, G. & Doberstein, B. (1978) Nature (London) 273, 569-571]. Utilizing hydrophobic chromatography, we purified to homogeneity a protein component of the salt extract that reconstitutes the translocation activity of the extracted membranes. This component behaves as a homogeneous species upon gel filtration, ion-exchange chromatography, adsorption chromatography, and sucrose-gradient centrifugation. When examined by polyacrylamide gel electrophoresis in NaDodSO4, six polypeptides with apparent Mr of 72,000, 68,000, 54,000, 19,000, 14,000, and 9000 are observed in about equal and constant stoichiometry, suggesting that they are subunits of a complex. The sedimentation coefficient of 11S is in good agreement with the sum of the Mr of the subunits. The Mr 68,000 and 9000 subunits label intensely with N-[3H]ethylmaleimide. Thus, the reported sulfhydryl group requirement of the translocation activity in the unfractionated extract [Jackson, R. C., Walter, P. & Blobel, G. (1980) Nature (London), 286, 174-176] may be localized to either or both the Mr 68,000 and 9000 subunits of the purified complex.

Two-dimensional gel electrophoresis of rat liver microsomal membrane proteins

Total rat liver microsomal proteins are not suitable for isoelectric focusing in polyacrylamide gels, even in the presence of sodium dodecyl sulphate and excess non-ionic detergent; considerable quantities of protein form an aggregate in the isoelectric focusing gel. This prevents resolution of microsomal proteins by the increasingly popular two-dimensional electrophoresis technique employing isoelectric focusing followed by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. The problem is caused by the extreme insolubility of some microsomal proteins, especially cytochrome P-450 species, which precipitate during isoelectric focusing. A selective extraction of microsomes with sodium deoxycholate excludes these poorly soluble proteins. The extracted proteins can then be resolved without difficulty by isoelectric focusing, and give excellent two-dimensional gel patterns showing more than 100 proteins, mainly in the pI range 5--7. The technique should be useful in studies on microsome protein topology and on changes in microsome composition.