The membrane-embedded segment of cytochrome b5 as studied by cross-linking with photoactivatable phospholipids. II. The nontransferable form

Distinct Splice Variants of Dynamin-related Protein 1 Differentially Utilize Mitochondrial Fission Factor as an Effector of Cooperative GTPase Activity

Multiple isoforms of the mitochondrial fission GTPase dynamin-related protein 1 (Drp1) arise from the alternative splicing of its single gene-encoded pre-mRNA transcript. Among these, the longer Drp1 isoforms, expressed selectively in neurons, bear unique polypeptide sequences within their GTPase and variable domains, known as the A-insert and the B-insert, respectively. Their functions remain unresolved. A comparison of the various biochemical and biophysical properties of the neuronally expressed isoforms with that of the ubiquitously expressed, and shortest, Drp1 isoform (Drp1-short) has revealed the effect of these inserts on Drp1 function. Utilizing various biochemical, biophysical, and cellular approaches, we find that the A- and B-inserts distinctly alter the oligomerization propensity of Drp1 in solution as well as the preferred curvature of helical Drp1 self-assembly on membranes. Consequently, these sequences also suppress Drp1 cooperative GTPase activity. Mitochondrial fission factor (Mff), a tail-anchored membrane protein of the mitochondrial outer membrane that recruits Drp1 to sites of ensuing fission, differentially stimulates the disparate Drp1 isoforms and alleviates the autoinhibitory effect imposed by these sequences on Drp1 function. Moreover, the differential stimulatory effects of Mff on Drp1 isoforms are dependent on the mitochondrial lipid, cardiolipin (CL). Although Mff stimulation of the intrinsically cooperative Drp1-short isoform is relatively modest, CL-independent, and even counter-productive at high CL concentrations, Mff stimulation of the much less cooperative longest Drp1 isoform (Drp1-long) is robust and occurs synergistically with increasing CL content. Thus, membrane-anchored Mff differentially regulates various Drp1 isoforms by functioning as an allosteric effector of cooperative GTPase activity.

Two tail-anchored protein variants, differing in transmembrane domain length and intracellular sorting, interact differently with lipids

C-tail-anchored (TA) proteins often require a transmembrane domain of moderate hydrophobicity to maintain their endoplasmic reticulum residence, but the suggested role of protein-lipid interactions in this phenomenon has not been established. Here, we studied the interaction of TA proteins with lipids by differential scanning calorimetry by using a model system consisting of liposomes embedding either of two forms of cytochrome b(5): the endoplasmic reticulum-resident wild-type (b(5)wt) and a mutant thereof (b(5)ext), that contains five extra nonpolar amino acids in its transmembrane domain and, therefore, reaches the plasma membrane. The proteins were incorporated into liposomes of palmitoyl-oleyl-phosphatidylcholine (POPC) or POPC mixed with either distearoyl-phosphatidylserine (DSPS), palmitoyl-oleyl-phosphatidylserine (POPS), distearoyl-phosphatidylcholine (DSPC), or C16-ceramide (CER). POPC liposomes displayed a single thermotropic transition centered at -3.4 degrees C. When present, the second lipid formed a domain within the POPC bilayer, as indicated by the appearance of an additional peak. This peak was centered at temperatures close to 0 degrees C in the case of liposomes containing 10% CER, DSPS, and POPS and at 23 degrees C in the case of DSPC, likely reflecting a higher degree of molecular packing for DSPC domains. In DSPS/POPC, POPS/POPC, or CER/POPC, but not in DSPC/POPC liposomes, the insertion of b(5)wt increased, whereas b(5)ext decreased, the relative contribution to the total enthalpy of the higher temperature, phase-separated component. These results were confirmed with fluorescence measurements by using pyrene-labeled phospholipids. The dissimilar interaction with lipids of these two differently localized TA proteins could have implications for their intracellular sorting.

Development and application of diazirines in biological and synthetic macromolecular systems

Many different reagents and methodologies have been utilised for the modification of synthetic and biological macromolecular systems. In addition, an area of intense research at present is the construction of hybrid biosynthetic polymers, comprised of biologically active species immobilised or complexed with synthetic polymers. One of the most useful and widely applicable techniques available for functionalisation of macromolecular systems involves indiscriminate carbene insertion processes. The highly reactive and non-specific nature of carbenes has enabled a multitude of macromolecular structures to be functionalised without the need for specialised reagents or additives. The use of diazirines as stable carbene precursors has increased dramatically over the past twenty years and these reagents are fast becoming the most popular photophors for photoaffinity labelling and biological applications in which covalent modification of macromolecular structures is the basis to understanding structure-activity relationships. This review reports the synthesis and application of a diverse range of diazirines in macromolecular systems.

Neuronal calcium sensor protein visinin-like protein-3 interacts with microsomal cytochrome b5 in a Ca2+-dependent manner

Visinin-like protein-3, which is one of the neuronal calcium sensors, has been shown to be mainly expressed in cerebellar Purkinje cells, but cellular function of this protein has not yet been elucidated. We examined the tissue distribution of murine visinin-like protein-3 transcripts using real-time reverse transcription-PCR. Visinin-like protein-3 mRNA was found to be expressed in peripheral tissues. Particularly, the expression of the transcript in the thymus was significantly higher than in other peripheral tissues. In addition, B6RVTC1 thymoma cells robustly expressed visinin-like protein-3 mRNA. To identify a target protein of visinin-like protein-3, we performed a pull-down experiment using glutathione S-transferase-tagged visinin-like protein-3 and two-dimensional electrophoresis. We demonstrated that microsomal cytochrome b(5) was a Ca(2+)-dependent binding partner of visinin-like protein-3. In a co-immunoprecipitation experiment, it was observed that hippocalcin, as well as visinin-like protein-3, could interact with cytochrome b(5). Furthermore, we confirmed that the sequence Val(114)-Tyr(127) at the C-terminal tail of cytochrome b(5) is the minimal structural requirement for binding to visinin-like protein-3. In addition, the loop His(19)-His(25) at the N terminus of visinin-like protein-3 is essential for binding to cytochrome b(5). Microsomal cytochrome b(5) was also shown to be a potential activator of cytochrome P450. The present findings raise the possibility that visinin-like protein-3 may link Ca(2+) signaling to the machinery of microsomal monooxygenase complex composed of cytochrome b(5), cytochrome P450, and some reductases. This report provides the first evidence of an interaction between visinin-like protein-3 and microsomal cytochrome b(5).

Synthesis of photoaffinity probes [2′-iodo-4′-(3′′-trifluoromethyldiazirinyl)phenoxy]-d-glucopyranoside and [(4′-benzoyl)phenoxy]-d-glucopyranoside for the identification of sugar-binding and phlorizin-binding sites in the sodium/d-glucose cotransporter protein

In this paper we describe the synthesis and photochemical and biochemical properties of two new photoaffinity probes designed for studies on the structure-function relationship of the sodium D-glucose cotransporter (SGLT1). The two probes are [2(')-iodo-4(')-(3(")-trifluoromethyldiazirinyl)phenoxy]-D-glucopyranoside (TIPDG), a mimic for the phenyl glucopyranoside arbutin which is transported by SGLT1 with a very high affinity, and [(4(')-benzoyl)phenoxy]-D-glucopyranoside (BzG), a model compound for phlorizin, the most potent competitive inhibitor of sugar translocation by SGLT1. Both photoaffinity probes TIPDG (lambda(max)=358 nm) and BzG (lambda(max)=293 nm) can be activated at 350-360 nm, avoiding protein-damaging wavelengths. In inhibitor studies on sodium-dependent D-glucose uptake into rabbit intestinal brush border membrane vesicles TIPDG and BzG showed a fully competitive inhibition with regard to the sugar with respective K(i) values of 22+/-5 microM for TIPDG and 12+/-2 microM for BzG. These K(i) values are comparable to those of their parent compounds arbutin (25+/-6 microM) and phlorizin (8+/-1 microM). To further test the potential of TIPDG and BzG as photoaffinity probes, truncated loop 13 protein, supposed to be part of the substrate recognition site of SGLT1, was exposed to TIPDG and BzG in solution. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry analysis demonstrated that TIPDG and BzG successfully labeled the protein. These preliminary results suggest that both photoaffinity probes are promising tools for the study of the structure-function relationship of SGLT1 and other SGLT1 family transporter proteins.

Mapping of contact sites in complex formation between transducin and light-activated rhodopsin by covalent crosslinking: use of a photoactivatable reagent

Interaction of light-activated rhodopsin with transducin (T) is the first event in visual signal transduction. We use covalent crosslinking approaches to map the contact sites in interaction between the two proteins. Here we use a photoactivatable reagent, N-[(2-pyridyldithio)-ethyl], 4-azido salicylamide. The reagent is attached to the SH group of cytoplasmic monocysteine rhodopsin mutants by a disulfide-exchange reaction with the pyridylthio group, and the derivatized rhodopsin then is complexed with T by illumination at lambda >495 nm. Subsequent irradiation of the complex at lambda310 nm generates covalent crosslinks between the two proteins. Crosslinking was demonstrated between T and a number of single cysteine rhodopsin mutants. However, sites of crosslinks were investigated in detail only between T and the rhodopsin mutant S240C (cytoplasmic loop V-VI). Crosslinking occurred predominantly with T(alpha). For identification of the sites of crosslinks in T(alpha), the strategy used involved: (i) derivatization of all of the free cysteines in the crosslinked proteins with N-ethylmaleimide; (ii) reduction of the disulfide bond linking the two proteins and isolation of all of the T(alpha) species carrying the crosslinked moiety with a free SH group; (iii) adduct formation of the latter with the N-maleimide moiety of the reagent, maleimido-butyryl-biocytin, containing a biotinyl group; (iv) trypsin degradation of the resulting T(alpha) derivatives and isolation of T(alpha) peptides carrying maleimido-butyryl-biocytin by avidin-agarose chromatography; and (v) identification of the isolated peptides by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. We found that crosslinking occurred mainly to two C-terminal peptides in T(alpha) containing the amino acid sequences 310-313 and 342-345.

Identification of the endoplasmic reticulum targeting signal in vesicle-associated membrane proteins

The vesicle-associated membrane proteins (Vamp(s)) function as soluble N-ethylmaleimide-sensitive factor attachment receptor proteins in the intracellular trafficking of vesicles. The membrane attachment of Vamps requires a carboxyl-terminal hydrophobic sequence termed an insertion sequence. Unlike other insertion sequence-containing proteins, targeting of the highly homologous Vamp1 and Vamp2 to the endoplasmic reticulum requires ATP and a membrane-bound receptor. To determine if this mechanism of targeting to the endoplasmic reticulum extends to other Vamps, we compared the membrane binding of Vamp1 and Vamp2 with the distantly related Vamp8. Similar to the other Vamps, Vamp8 requires both ATP and a membrane component to target to the endoplasmic reticulum. Furthermore, binding curves for the three Vamps overlap, suggesting a common receptor-mediated process. We identified a minimal endoplasmic reticulum targeting domain that is both necessary and sufficient to confer receptor-mediated, ATP-dependent, binding of a heterologous protein to microsomes. Surprisingly, this conserved sequence includes four positively charged amino acids spaced along an amphipathic sequence, which unlike the carboxyl-terminal targeting sequence in mitochondrial Vamp isoforms, is amino-terminal to the insertion sequence. Because Vamps do not bind to phospholipid vesicles, it is likely that these residues mediate an interaction with a protein, rather than bind to acidic phospholipids. Therefore, we suggest that a bipartite motif is required for the specific targeting and integration of Vamps into the endoplasmic reticulum with receptor-mediated recognition of specifically configured positive residues leading to the insertion of the hydrophobic tail into the membrane.

Intracellular trafficking and localization of the pseudorabies virus Us9 type II envelope protein to host and viral membranes

The Us9 protein is a phosphorylated membrane protein present in the lipid envelope of pseudorabies virus (PRV) particles in a unique tail-anchored type II membrane topology. In this report, we demonstrate that the steady-state residence of the Us9 protein is in a cellular compartment in or near the trans-Golgi network (TGN). Through internalization assays with an enhanced green fluorescent protein epitope-tagged Us9 protein, we demonstrate that the maintenance of Us9 to the TGN region is a dynamic process involving retrieval of molecules from the cell surface. Deletion analysis of the cytoplasmic tail reveals that an acidic cluster containing putative phosphorylation sites is necessary for the recycling of Us9 from the plasma membrane. The absence of this cluster results in the relocalization of Us9 to the plasma membrane due to a defect in endocytosis. The acidic motif, however, does not contain signals needed to direct the incorporation of Us9 into viral envelopes. In this study, we also investigate the role of a dileucine endocytosis signal in the Us9 cytoplasmic tail in the recycling and retention of Us9 to the TGN region. Site-directed mutagenesis of the dileucine motif results in an increase in Us9 plasma membrane staining and a partial internalization defect.

The Us9 gene product of pseudorabies virus, an alphaherpesvirus, is a phosphorylated, tail-anchored type II membrane protein

The Us9 gene is highly conserved among the alphaherpesviruses sequenced to date, yet its function remains unknown. In this report, we demonstrate that the pseudorabies virus (PRV) Us9 protein is present in infected cell lysates as several phosphorylated polypeptides ranging from 17 to 20 kDa. Synthesis is first detected at 6 h postinfection and is sensitive to the DNA synthesis inhibitor phosphonoacetic acid. Unlike the herpes simplex virus type 1 Us9 homolog, which was reported to be associated with nucleocapsids in the nuclei of infected cells (M. C. Frame, D. J. McGeoch, F. J. Rixon, A. C. Orr, and H. S. Marsden, Virology 150:321-332, 1986), PRV Us9 localizes to the secretory pathway (predominately to the Golgi apparatus) and not to the nucleus. By fusing the enhanced green fluorescent protein (EGFP) reporter molecule to the carboxy terminus of Us9, we demonstrated that Us9 not only is capable of targeting a Us9-EGFP fusion protein to the Golgi compartment but also is able to direct efficient incorporation of such chimeric molecules into infectious viral particles. Moreover, through protease digestion experiments with Us9-EGFP-containing viral particles, we demonstrated that the Us9 protein is inserted into the viral envelope as a type II, tail-anchored membrane protein.

Effect of cholesterol on the tight insertion of cytochrome b5 into large unilamellar vesicles

When cytochrome b5 is added to large unilamellar vesicles (LUVs) of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), it binds predominantly in a 'loose,' or transferable form. Prolonged incubation of 30 degrees C leads to insertion in the physiological 'tight,' nontransferable form, with a halftime for the loose --> tight conversion of approx. 9 days. In this study, the effect of cholesterol on the rate of tight insertion was determined. Tight binding was assayed by depleting the LUVs of loose cytochrome b5 with an excess of SUV acceptors and then separating the liposome populations by gel-filtration or velocity sedimentation. Incorporation of cholesterol into the LUVs was found to markedly increase the rate of tight insertion, even though cholesterol decreases the equilibrium binding constant and saturation level of protein binding. The effect is not a continuously increasing function of cholesterol content, but attains a maximum at 20-25% mol%, where the rate enhancement is approx. 10-fold over baseline. At higher cholesterol levels, the rate decreases, returning to baseline at 40 mol% cholesterol. These observations are highly unusual in that cholesterol generally decreases the membrane binding affinity and the permeability of solutes, and does so as a monotonic function of cholesterol concentration (above the liquid-crystalline phase transition of the phospholipids). It is suggested that tight insertion is enhanced by lipid-protein packing mismatches and by bilayer fluidity; the former increases monotonically with increasing cholesterol whereas the latter decreases monotonically. At 20-25 mol% cholesterol the optimum balance of these physical properties is obtained for tight insertion.

The carboxyl terminus of the membrane-binding domain of cytochrome b5 spans the bilayer of the endoplasmic reticulum

Preliminary studies (Vergères, G., and Waskell, L. (1992) J. Biol. Chem. 267, 12583-12591) have suggested that the carboxyl-terminal membrane-binding domain of cytochrome b5 traverses the membrane and that the carboxyl terminus is in the lumen of the endoplasmic reticulum. In order to confirm and extend these studies, additional experiments were conducted. The gene coding for rat cytochrome b5 was transcribed and the resulting mRNA was translated in vitro in a rabbit reticulocyte lysate in the presence of microsomes. The binding and topology of cytochrome b5 were investigated by treating microsomes containing the newly incorporated cytochrome b5 with carboxypeptidase Y and trypsin. Our studies indicate that cytochrome b5 is inserted both co- and post-translationally into microsomes in a topology in which the membrane-binding domain spans the bilayer with its COOH terminus in the lumen. Cytochrome b5 is also incorporated into microsomes pretreated with trypsin in a topology indistinguishable from the one resulting from the insertion of the protein into untreated microsomes, reconfirming that cytochrome b5 does not use the signal recognition particle-dependent translocation machinery. Our results do not allow a distinction to be made between a spontaneous insertion mode or some other trypsin-resistant receptor-mediated mechanism. A role for Pro115 in the middle of the membrane-binding domain of cytochrome b5 was also examined by mutating it to an alanine and subsequently characterizing the ability of the mutant protein to be incorporated into membranes. The mutant protein inserted more slowly in vitro into microsomes as well as into pure lipid bilayers by a factor of 2 to 3.

Cytochrome b5, its functions, structure and membrane topology

The first part of the present communication reviews recent advances in our understanding of the known physiological functions of cytochrome b5. In addition, one section is devoted to a description of a recently discovered function of cytochrome b5, namely its involvement in the synthesis of the oncofetal antigen N-glycolylneuraminic acid. The second part of the article summarizes site-directed mutagenesis studies, primarily conducted in the author's laboratory, in both the catalytic heme-binding and membrane-binding domain of cytochrome b5. These studies have shown that: 1) the membrane binding domain of cytochrome b5 spans the bilayer; 2) cytochrome b5 lacking 19 COOH-terminal amino acids does not bind to membrane bilayers; and 3) specific amino acids in the membrane binding domain have been mutated and shown not to be essential for the function of cytochrome b5 with its redox partners.

Prostaglandin H synthase: implications for membrane structure

The crystal structure of the membrane protein prostaglandin H synthase (PGHS) provides strong evidence for the existence of monotopic membrane proteins: PGHS seems to interact with the membrane via a motif of amphipathic helices positioned parallel to the plane of the membrane. The orientation of this unique membrane binding motif is fixed in space by an epidermal growth factor(EGF)-like module on its amino-terminal end and by the catalytic domain at its carboxy-terminal end. The catalytic domain of PGHS has a high structural homology to other mammalian heme peroxidases.

Tight insertion of cytochrome b5 into large unilamellar vesicles

Cytochrome b5 spontaneously binds to liposomes in a 'loose', or transferable form, whereas in vivo b5 binds post-translationally to the ER in the 'tight' or nontransferable form. The mechanism of tight insertion is unknown, except that it does not require SRP or energy input. The present study shows that prolonged incubation of b5 with large unilamellar vesicles (LUVs) of phosphatidylcholine results in slow conversion of the loose to the tight form, with a halftime of days. However, the process is complex. When the b5-LUVs are depleted of loose b5, by transfer of b5 to sonicated vesicles, the tight b5 is found to be concentrated to near saturating levels in a small fraction of the LUVs. If the LUVs devoid of tight b5 are recovered and then reincubated with fresh b5, the same slow transformation recurs. Apparently, a new population of vesicles, containing tight b5, is generated during the prolonged incubation with the protein. The b5-enriched LUVs contain about the same level of trapped sucrose as does the original vesicle preparation, indicating that vesicle integrity is maintained throughout the process. When fresh b5 is added to these tight b5-containing LUVs, all the freshly bound protein rapidly inserts (< 2 h) into the tight configuration. Apparently, the newly formed tight-b5/LUV vesicle population is 'insertion-active'. A model for these complex transformations is proposed.

GTP is required for the integration of a fragment of the Neurospora crassa H(+)-ATPase into homologous microsomal vesicles

The integration of a fragment of the Neurospora crassa plasma membrane H(+)-ATPase was examined to determine if insertion of the fragment into homologous microsomal vesicles is obligatorily dependent on a nucleoside triphosphate. RNA transcripts that encoded the amino terminal 344 amino acids of the Neurospora crassa plasma membrane H(+)-ATPase(pma(344)+) were translated in a N. crassa in vitro system. The pma(344)+ integrated post-translationally into homologous microsomal vesicles independent of the associated ribosomes and dependent on the presence of GTP or guanylyl imidodiphosphate, a nonhydrolyzable analogue of GTP. ATP or analogues thereof did not support the integration of pma(344)+ into nRM post-translationally. These results were interpreted to suggest that a GTPase plays an essential role in the integration of the amino terminal portion of the pma+ into the endoplasmic reticulum.

Major Myelin proteolipid: the 4-alpha-helix topology

Several conflicting models have been proposed for the membrane arrangement of the major myelin proteolipid (PLP). We have compared features of the sequence of PLP with those of other eukaryotic integral membrane proteins, with the view of identifying the most likely transmembrane topology. A new, simple model is suggested, which features four hydrophobic alpha-helices spanning the whole thickness of the lipid bilayer. Its orientation may be such that both the N- and C-termini face the cytosol. None of the biochemical, biophysical or immunological experiments hitherto reported provides incontrovertible evidence against the model. The effect or absence thereof of various PLP mutations is discussed in the frame of the proposed 4-helix topology.

Beta-galactosidase fused to the hydrophobic domain of cytochrome b5 spontaneously associates with liposomes

Since liver microsomal cytochrome b5 spontaneously associates with liposomes and membranes by means of its C-terminal hydrophobic domain (HP), chimeric proteins containing HP prepared by genetic fusion might also spontaneously associate with liposomes or cellular membranes. Synthetic DNA corresponding to the hydrophobic domain of cytochrome b5 was enzymatically fused in-frame to cloned DNA corresponding to the C-terminus of the Escherichia coli enzyme, beta-galactosidase. This protein, LacZ:HP, synthesized in E. coli and purified from a crude E. coli membrane extract, was shown to spontaneously associated with liposomes, as does cytochrome b5. Association is rapid and stable in the presence of salt and high pH and the fusion protein behaves as an integral membrane protein. LacZ:HP can be readily and extensively purified from crude extracts by association with liposomes and this procedure may provide a convenient purification scheme for proteins not otherwise readily purified, for example polypeptides from cloned gene fragments to be used for antibody production. These hybrid proteins may represent a new potentially useful class of polypeptides capable of hydrophobic interactions with membranes.

Structure of cytochrome b5 and its topology in the microsomal membrane

The complete amino acid sequence of human and chicken liver microsomal cytochrome b5 was determined. The amino termini of cytochrome b5 from four other mammalian species were examined in order to determine their complete covalent structure. As in the rat species, cytochrome b5 preparations from man, rabbit, calf and horse had an acetylated alanine as the first residue. In contrast, the pig cytochrome had alanine at the amino terminus. The amino terminus of the chicken cytochrome b5 was also unmodified, and extended three residues absent in the mammalian species. In order to investigate whether the carboxy-terminal segment of cytochrome b5 is located on the cytosolic or the luminal side of the microsomal membrane, rabbit liver microsomes were treated with trypsin and subjected to gel filtration and high-pressure liquid chromatography. The nonpolar peptide isolated from these microsomes lacked the terminal hexapeptide, indicating that when cytochrome b5 is bound to intact microsomes, the carboxy terminus is located on the cytosolic side of the membrane and does not extend in the lumen of the endoplasmic reticulum.

A photo-chemically induced dynamic nuclear polarization NMR study on rabbit and bovine cytochrome b5

Although it has been indicated that proteins with chromophoric groups are not suitable for photo-chemically induced dynamic nuclear polarization (photo-CIDNP) measurements, we have successfully obtained these spectra for a heme protein, cytochrome b5. The characteristics of the spectra differed in some points from those so far reported. The intensities of the signals in the aromatic region were very weak, while those of the beta-methylene protons of one histidine and one tryptophan were extremely strong in comparison with the aromatic protons. It was demonstrated, on the basis of the photo-CIDNP spectrum, that one of seven histidines, all three tyrosines and a single tryptophan of the rabbit soluble cytochrome b5 are exposed on the surface of the protein. The results of comparison of the photo-CIDNP spectra for the rabbit soluble and intact, and bovine intact, cytochrome b5 led us to the conclusion that the conformation of the hydrophilic, catalytic part of cytochrome b5 is quite similar among these three proteins. In the presence of Chaps micelles, bovine intact cytochrome b5 was in monomeric form and the histidine signals disappeared from its photo-CIDNP spectrum. When bovine intact cytochrome b5 was reconstituted into egg yolk phosphatidylcholine liposomes, although separate signals due to the protein part were observed in the normal 1H-NMR spectrum, no photo-CIDNP signal could be detected. The normal spectrum suggests that the conformation of the protein embedded in liposomes is similar to that of the oligomeric form without lipids or a detergent.

Incorporation of cytochrome b5 into endoplasmic reticulum vesicles as protein-lysophospholipid micelles

Cytochrome b5 is incorporated into vesicles of the endoplasmic reticulum as protein-lysophosphatidylcholine micelles. Cytochrome b5 becomes firmly bound to the membrane and at the same time lysophosphatidylcholine is acylated by acyltransferases of the endoplasmic reticulum and converted into the membrane component phosphatidylcholine. The possibility of an insertion of cytochrome b5 into the endoplasmic reticulum in vivo by this mechanism is discussed.

Bovine opsin has more than one signal sequence

By deletion of selected segments from a bovine opsin complementary DNA clone and subsequent analysis of transcripts in a cell-free translation-translocation system, we have localized two out of four theoretically conceivable signal sequences required for the integration of opsin into microsomal membranes.

Multiple mechanisms of protein insertion into and across membranes

Protein localization in cells is initiated by the binding of characteristic leader (signal) peptides to specific receptors on the membranes of mitochondria or endoplasmic reticulum or, in bacteria, to the plasma membrane. There are differences in the timing of protein synthesis and translocation into or across the bilayer and in the requirement for a transmembrane electrochemical potential. Comparisons of protein localization in these different membranes suggest underlying common mechanisms.

Ultrastructure of reconstituted rat liver microsomal membranes and cytochrome b5- or P-450-containing proteoliposomes

Thin sectioning and freeze-fracture electron microscopy have been used to show that it is possible to obtain topologically closed vesicles by means of reconstitution of rat liver microsomal membrane "ghosts." The reconstitution by 15 hr dialysis resulted in the formation of vesicles with intramembrane particles (IMP) while after 40 hr dialysis no IMP were observed in the membranes. The protein/lipid ratio and functional activity of NADPH- and NADH-linked enzyme systems were similar in both cases. Cytochrome P-450 (LM2) was incorporated into liposomes of different composition (protein: lipid ratio--1:200). IMP were observed only when the incorporation of cytochrome P-450 was performed in the presence of detergent Emulgen 913 as specific additive to the initial protein-lipid-sodium cholate mixture or in the course of incubation of proteoliposomal suspensions at 37 degrees C. After the incorporation of cytochrome b5 into azolectin liposomes vesicular membranes contain IMP if the incorporated membrane protein: lipid ratio is at least 1:50. Pronase-induced splitting off of a 11 kDa heme-containing fragment of cytochrome b5 did not affect IMP content. The conditions of IMP formation in reconstituted membranes and in microsomal ghosts are discussed.

Use of carboxypeptidase Y in studies of the mode of insertion of cytochrome b5 into lipid vesicles

Carboxypeptidase Y preparations from baker's yeast have been found to exhibit endopeptidase activity when cytochrome b5 was used as substrate. As the susceptibility of cytochrome b5 to attack by carboxypeptidase Y has been used to distinguish between two modes of insertion of cytochrome b5 into lipid bilayer, one which has the C terminal buried in the lipid bilayer and one which has a free C terminal, caution should be taken when employing carboxypeptidase Y preparations for this type of studies.

Reconstitution of cytochrome b5 into lipid vesicles in a form which is nonsusceptible to attack by carboxypeptidase Y

Pig liver cytochrome b5 is reconstituted into lipid vesicles by a method whereby cytochrome b5-lysophospholipid micelles are fused with liposomes. The reconstitution method inserts cytochrome b5 into the lipid bilayer in a mode which renders the C-terminal part of cytochrome b5 nonsusceptible to attack by carboxypeptidase Y. The effect of the lipid composition of the vesicles on the mode of insertion has been examined, when cytochrome b5 is reconstituted using the presently described method and methods previously used to reconstitute this protein, namely the direct incorporation method and the cholate dialysis method.

Synthesis of phospholipids containing photoactivatable carbene precursors in the headgroups and their crosslinking with membrane proteins

Many integral membrane enzymes require for their activity interactions with the polar headgroups of phospholipids, in addition to the hydrophobic interactions within the lipid bilayer. The interactions with the polar headgroups may have preferential or absolute specificity. To study such interactions, phospholipids have been synthesized which carry photoactivable moieties in their headgroups. Three types of phospholipids, PL-I, PL-II and PL-III, were synthesized. The synthetic phospholipids, PL-I and PL-II were able to reconstitute enzymatic activity of the membrane proteins which were studied. Covalent crosslinking between these phospholipids and the membrane proteins was demonstrated after photolysis of the reconstituted phospholipid-protein complexes.

Phospholipids involved in specific binding of 12-O-(5-azido-2-nitrobenzoyl)phorbol-13-acetate to epidermal microsomes, a photolabeling study

The preparation of 12-O-(5-azido-2-nitro)benzoylphorbol-13-acetate (NABPA) is described. It is used as a photoaffinity probe to study the biochemical components involved in the specific binding of phorbol esters to an epidermal particulate fraction (microsomes) from NMRI mice: without irradiation NABPA binds in a saturable and high affinity manner (KD = 12 nM; Rt = 2.6 pmol/mg protein) to microsomes; after irradiation (at 350 nm) specific photolabeling (representing specific binding of NABPA) is found of phospholipids (phosphatidyl-serine (PS) and -ethanolamine(PE)), but not of protein. The results are discussed in the context of protein kinase C being a receptor for phorbol esters.

Structure of the membrane-embedded F0 part of F1F0 ATP synthase from Escherichia coli as inferred from labeling with 3-(Trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine

3-(Trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine [( 125I]TID) is a photoactivatable carbene precursor designed to label selectively the hydrophobic core of membranes. We have used this reagent to obtain information on the topological organization of the membrane-embedded subunits of F1F0 ATP synthase from Escherichia coli. The study included [125I]TID labeling of F0 subunits in different structural (conformational) states and Edman degradations of the labeled polypeptides in order to assign the covalently bound radioactivity to individual amino acid residues. Released phenylthiohydantoin amino acids were analyzed by thin-layer chromatography, and the radioactive derivatives were visualized by autoradiography. The data suggest that labeling patterns can be correlated in a meaningful manner with reagent accessibility and hence with protein-lipid contact. Subunit b appears to be anchored to the membrane by a short N-terminal segment. As almost all of the amino acids of this part are accessible to the reagent, it is inferred that this segment has little interaction with the other subunits. In contrast, in the two segments of subunit c that were labeled with [125I]TID, only certain amino acids reacted with the label. The pattern of these labeled residues is compatible with that of tightly packed alpha-helices.