Yeast plasma membrane ghosts. An analysis of proteins by two-dimensional gel electrophoresis.

We have examined yeast cell ghost preparations to assess their value in obtaining plasma membrane proteins. Ghosts prepared by two methods involving stabilization of spheroplast envelopes had similar protein patterns by two-dimensional gel electrophoresis, and approximately 200 proteins were resolved. Spheroplasts were lactoperoxidase iodinated, and recovery of label in ghost preparations was greater than 60%. Spheroplasts appeared to be impermeable to the lactoperoxidase reagents as judged by an examination of two-dimensional gel electrophoretic patterns of ghost proteins that had been iodinated in spheroplasts or in unsealed ghosts. Spheroplasts were also impermeable to pronase proteases. Surface iodination and surface proteolysis allowed us to identify exposed ghost proteins; the major ghost glycoprotein was exposed in spheroplasts. Two-dimensional patterns of ghost proteins were not heavily contaminated (less than or equal to 25% of all proteins) by proteins present in soluble or promitochondrial fractions, and estimates of surface label and total cell protein recovery suggested that the ghost fraction represents a cell envelope enrichment of 8--10 fold over whole cells. Resolution of ghost proteins by two-dimensional gel electrophoresis appears to be a powerful aid toward identifying membrane proteins.     

Two distinct subfractions in isolated Saccharomyces cerevisiae plasma membranes.

The plasma membrane from Saccharomyces cerevisiae X2180-1A and a secretion-blocked mutant, secl (P. Novick and R. Schekman, Proc. Natl. Acad. Sci. U.S.A. 76:1858-1862, 1979) has been purified. Cell walls were digested by treatment with lyticase followed by concanavalin A coating of spheroplasts. alpha-Methylmannoside treatment after lysis, sonication at high salt concentration, and fractionation on a Renografin gradient resulted in two highly purified membrane fractions sedimenting at densities of 1.15 and 1.17 g/cm3. Yields determined by recovery of vanadate-sensitive ATPase activity were 11 to 18%, and those determined by recovery of the spheroplast surface label 125I were 17 to 29%. Iodinated cells have most of their label in sedimentable, nonspheroplast material. However, both membrane populations contain some 125I surface label and show ATPase activity with pH optima only at 5.5. The apparent Vmax of the plasma membrane ATPase equals 360 to 560 nmol of ATP hydrolyzed per min per mg of protein, with a Km for ATP of 0.7 mM. ATPase specific activity is not decreased in mutant plasma membrane. Analysis of 125I-labeled plasma membrane proteins by two-dimensional gel electrophoresis revealed seven major proteins on the plasma membrane surface.     

Asymmetric distribution of nitrate reductase subunits in the cytoplasmic membrane of Escherichia coli: evidence derived from surface labeling studies with transglutaminase.

The enzyme transglutaminase has been used to label surface proteins of Escherichia coli cytoplasmic membranes by covalently attaching to them a small fluorescent primary amine, dansyl cadaverine. Spheroplasts lacking outer membrane, osmotically lysed vesicles from the spheroplasts, and vesicles made by breaking cells in a French pressure cell were each labeled with transglutaminase and dansyl cadaverine. When the total cytoplasmic membrane proteins of each were examined on sodium dodecyl sulfate gels, three rather different labeling patterns were obtained. Labeling of the respiratory enzyme, nitrate reductase, in the membranes of each of these preparations was also examined. Membrane-bound nitrate reductase contains three subunits: A, B, and C. Dansyl cadaverine labeling of nitrate reductase in the presence of Triton X-100 indicated that subunits A and C could be labeled. When nitrate reductase was isolated from dansyl cadaverine-labeled spheroplasts, none of the subunits was labeled. When nitrate reductase was isolated from French press vesicles, subunit A was labeled and labeling was enhanced by the presence of nitrate during labeling. When nitrate reductase from osmotic vesicles was examined, subunit A was labeled in the presence of nitrate but no labeled subunits appeared when the vesicles were labeled in the absence of nitrate. It was concluded that (i) nitrate reductase is buried in the membrane with subunit A exposed only on the inner surface of the membrane, (ii) subunit C is sufficiently buried within the membrane so that it is inaccessible to transglutaminase, (iii) subunit B is not labeled under any condition, so its location is not known, and (iv) large osmotic vesicles are probably mosaics in which some protein components have been reoriented.     

Trypsin-induced changes in the orientation of latent ATPase in protoplast ghosts from Mycobacterium phlei.

Latent ATPase, located on the inner surface of protoplast ghosts of Mycobacterium phlei, was unmasked either by trypsin or an impermeable form of trypsin, ethylene maleic anhydride-trypsin. Density gradient experiments showed that the ghost preparations remained intact following trypsin treatment. Evidence was obtained that 125I-trypsin failed to penetrate the ghost membranes. Thus, attempts were made to determine whether the ATPase molecule in the ghost membranes is accessible from the outer surface. Treatment of protoplast ghosts and trypsin-treated ghosts with 125I by the lactoperoxidase method resulted in the labeling of ATPase only in the trypsin-treated ghost preparations. The antibody to latent ATPase inhibited ATPase activity in trypsin-treated ghosts. The changes in the fluorescence polarization of diphenyl hexatriene indicated that trypsin treatment of the ghost membranes resulted in an increase in membrane fluidity. These studies suggest that the latent ATPase moiety has undergone translocation to the outer surface or it became accessible to trypsin digestion from the outer surface of the membranes as a result of removal of some proteins covering ATPase molecule in the membranes.     

The nature and location of Acholeplasma laidlawii membrane proteins investigated by two-dimensional gel electrophoresis.

The high resolution, two-dimensional electrophoresis system for the separation of proteins described by O'Farrell, (O'Farrell, P.H. (1975) J. Biol. Chem. 250, 4007--4021) has been modified for the separation of Acholeplasma laidlawii proteins. Reproducible protein patterns have been obtained from A. laidlawii cell, membrane and soluble protein preparations. The isoelectric focusing of membrane proteins was greatly improved by removing the bulk of the membrane lipid before solubilizing the protein. A. laidlawii peripheral membrane proteins were removed from the membrane by low ionic strength washing and by treatment with EDTA. The effect of an exhaustive EDTA treatment and a rapid, warm EDTA treatment were compared. By comparing the protein patterns obtained in these ways it was possible to distinguish two separate groups of peripheral membrane proteins and one integral membrane protein group. The peripheral membrane proteins which were removed from the membrane at low ionic strength (group I) were also insoluble in Triton X-100, whereas additional peripheral membrane proteins extractable by subsequent EDTA treatment (group II) were soluble in Triton X-100. Exterior-facing membrane proteins were distinguished from the interior-facing ones by lactoperoxidase-catalyzed iodination of intact cells and membranes. Group I peripheral membrane proteins faced the cell interior whereas group II proteins faced the cell exterior. We counted approximately 320 individual whole cell proteins. Of these, about 140 were membrane associated and a maximum of 40 proteins were iodinated after iodinating intact cells. A. laidlawii was also grown in the presence of NaH232PO4 and whole cell proteins were separated by two-dimensional gel electrophoresis. One membrane protein and two soluble proteins were labelled.     

Proteins of the hepatoma tissue culture cell plasma membrane

The specificity of lactoperoxidase-catalyzed iodination for the proteins of the hepatoma tissue culture cell plasma membrane was examined by histochemical, biochemical, and cell fractionation techniques. Light microscope autoradiography of sectioned cells shows the incorporated label to be localized primarily at the periphery of the cell. Most of this label can be released from the cell by trypsin but not by collagenase or hyaluronidase. The label is recovered from the cells as either monoiodotyrosine or diiodotyrosine after hydrolysis of cell extracts with a mixture of proteolytic enzymes. The label co-purifies during cell fractionation with an authentic liver cell plasma membrane marker enzyme, 5′-nucleotidase. Thus, the incorporated iodide is itself a valid marker for those membrane polypeptides having tyrosine residues accessible to the lactoperoxidase. The polypeptide complexity of the purified plasma membrane was examined by high resolution dodecyl sulfate-polyacrylamide gel electrophoresis. At least 50 polypeptides in the membrane are accessible to iodination. These polypeptides probably represent the bulk of the protein mass of the membrane and iodinating them does not affect cell viability, growth rate, or cell function. Labeling experiments with fucose and glucosamine show that at least nine of the iodinated peptides may be glycoproteins.     

SDS-polyacrylamide gel electrophoresis of isolated cortices of Xenopus laevis eggs

Outer cytoplasmic membranes of Xenopus laevis eggs were isolated by manual dissection. Cortices of unfertilized eggs and membranes of fertilized stages were subjected to SDS-polyacrylamide gel electrophoresis. The Coomassie-blue stained gels of membranes from unfertilized eggs and fertilized stages showed 21 and 17 major bands, respectively, covering a molecular weight range of 27000–275000 Daltons. To demonstrate their purity, the preparations were examined by light and electron microscopy and the gel patterns were compared with those of gel electropherograms of isolated envelopes, yolk and pigment granules.To detect membrane proteins that reside in the outer membrane surface, the eggs were exposed to ¹³¹Iodine in the presence of lactoperoxidase. Approximately four proteins of molecular weights of 115000, 78000, 55000 and 27000 Daltons were labelled. In addition, all except one of the proteins of the vitelline envelope were strongly labelled, while the yolk platelets could only be iodinated after isolation.Changes of cortical protein patterns were studied in eggs and cortices: a) after fertilization; b) after activation by calcium-ionophore A 23187; c) after addition of calcium and distilled water to isolated cortices; and d) after incubation in the presence of the proteolytic enzyme papain. After fertilization, bands with molecular weights of 246000, 181000, 166000 and 34000 Daltons were missing. Ionophore and calcium treatment of unfertilized eggs produced a protein pattern similar to that observed after normal fertilization. After papain treatment, the band patterns of the membranes were not significantly different from those of cortices of unfertilized eggs.     

Two-dimensional electrophoresis of surface glycoproteins of normal BHK cells and ricin resistant mutants

The surface glycoproteins of baby hamster kidney (BHK) cells were iodinated by lactoperoxidase and submitted to a two-dimensional electrophoresis procedure involving isoelectric focusing in the first dimension and SDS gel electrophoresis in the second dimension. After autoradiography a complex but reproducible pattern was obtained. The technique was then applied to the study of three ricin-resistant mutant clones with reduced rates of cell-cell and/or cell-substratum adhesion. Abnormal patterns were observed in all three mutant clones indicating different mechanisms of ricin resistance and identifying glycoproteins which may be involved in cellular interactions.     

Electrophoretic analysis of the plasma membrane proteins of a mutant of Neurospora crassa which lacks a cell wall

The plasma membrane proteins of a mutant of Neurospora crassa (FGSC No. 326) which lacks a cell wall were analyzed by two-dimensional polyacrylamide gel electrophoresis. Approximately 180 different proteins were detected in purified plasma membrane preparations. Nonpermeating labeling experiments indicated that approximately 40% of these proteins were exposed on the extra-cytoplasmic surface of the plasma membranes of these cells. The studies demonstrate the complexity of the protein composition of N. crassa 326 plasma membranes to be greater than has been suggested by previous investigations.     

Lactoperoxidase-catalyzed iodination of surface membrane lipids

Lactoperoxidase-catalyzed iodination of intact cells, is known to label predominantly, if not exclusively, the exposed tyrosine residues of cell surface proteins. The present study demonstrates that during this iodination process surface membrane lipids are also iodinated through an enzyme-dependent step. Phosphatidylcholine, cholesterol-phosphatidylcholine liposomes and confluent secondary cultures of chick embryo cells were iodinated by the lactoperoxidase-glucose oxidase-glucose [¹²⁵I] procedure. Liposomes were efficiently labeled. In the cells, 20–30% of the radioactivity was found in proteins and 20–30% in the lipids. Both neutral and polar lipids were found to bind [¹²⁵I] covalently. Controls in which lactoperoxidase was omitted showed < 6% of the radioactivity found in liposomes or cells labeled with the enzyme.     

Radio-iodination of plasma membrane polypeptides from isolated mouse spermatogenic cells

Cell surface polypeptides of mouse pachytene spermatocytes and round spermatids (steps 1–8) have been iodinated using 1,2,3,6,tetracholoro-3α, 6α-diphenylglycouril (IODOGEN). Labeled proteins have been assayed using two-dimensional polyacrylamide electrophoresis and radioautography. Purified plasma membranes, prepared from both spermatocytes and spermatids after the iodination of intact cells, exhibit 25–30 polypeptides which label reproducibly. No significant qualitative differences are noted in the labeled polypeptide map obtained from each of the purified cell types. Iodinated proteins range in molecular weight from greater than 100k daltons to approximately 40k daltons. The isoelectric points of labeled constituents range from pI 5.7 to 7.2. Three polypeptides represent the major iodinated species: p 94/5.8, p 75/5.9, and p 53/7.1. Comparison with total plasma membrane constituents assayed using Coomassie brilliant blue indicates that many of the radioactively labeled proteins are not present in quantities sufficient to allow ready detection without isotopic techniques. As a result, many of the proteins identified autoradiographically represent newly described surface components of mouse pachytene spermatocytes and round spermatids. The preparation of purified plasma membrane fractions prior to electrophoresis ensures that all iodinated species are in fact cell surface components. Furthermore, experiments designed to assess the vectorial nature of the IODOGEN-catalyzed labeling procedure suggest that most, if not all, of the iodinated species are exposed on the external side of the cell plasma membrane. Therefore, these studies have (1) identified hitherto unrecognized plasma membrane components of mouse pachytene spermatocytes and round spermatids and (2) provided the first available biochemical data concerning the molecular orientation of particular proteins in the surface membranes of developing mouse spermatogenic cells.     

Membrane proteins of the vacuolar system. III. Further studies on the composition and recycling of endocytic vacuole membrane in cultured macrophages

In previous publications (Muller, W.A., R.M. Steinman, Z.A. Cohn. 1980, J.Cell Biol. 86:292-314), we found that the membrane of macrophage phagolysosomes could be selectively radioiodinated in living cells, The technique required phagocytosis of lactoperoxidase covalently coupled to latex spheres (LPO-latex), followed by iodination on ice with Na(125)I and hydrogen peroxide. In this paper, we use the LPO-latex system to further analyze the composition and recycling of phagocytic vacuole membrane. Three approaches were employed to examine the polypeptide composition of the phagolysosome (PL) and plasma membranes (PM). (a) The efficiency of intracellular iodination was increased by increasing lysosomal pH with chloroquine. By one-dimensional SDS PAGE, the heavily labeled chloroquine-treated PL exhibited the same labeled polypeptides as PM iodinated extracellularly with LPO-latex. (b) Iodinated PL and PM were compared by two-dimensional gel electrophoresis. No differences in the isoelectric point and molecular weight of the major iodinated species were detected. (c) Quantitative immune precipitation was performed with five specific antibodies directed against cell surface antigens. Four antibodies precipitated similar relative amounts of labeled antigen on the cell surface and endocytic vacuole. One antibody, secreted by hybridoma 2.6, detected a 21-kdalton polypeptide that was enriched sevenfold in PL membrane. This enrichment was cell surface-derived, since the amount of labeled 2.6 was increased sevenfold when iodinated PM was driven into the cell during latex uptake. Therefore, intracellular iodination primarily detects PL proteins that are identical to their PM counterparts. Additional studies employed electron microscope autoradiography to monitor the centrifugal flow of radiolabeled polypeptides from PL to PM. Cells were iodinated intralysosomally and returned to culture for only 5-10 min at 37 degrees C. Most of the cell-associated label then redistributed to the cell surface or its adjacent area. Significant movement out of the lysosome compartment occurred even at 2 degrees C and 22 degrees C. Extensive and rapid membrane flow through the secondary lysosome presumably contributes to the great similarity between PM and PL membrane polypeptides.     

Surface Structure of Intact Cells and Spheroplasts of Pseudomonas aeruginosa

This report describes the ultrastructural features of Pseudomonas aeruginosa after freeze-etching of intact cells and enzymatically prepared spheroplasts. Freeze-etching of intact cells revealed two convex layers of the cell wall and particles within the hydrophobic interior of the cell membrane. Areas of the membrane free of particles were sometimes elevated in the form of rather large dome-shaped structures. Spheroplasts were formed from intact cells by the addition of trypsin to a reaction mixture of lysozyme and ethylenediaminetetraacetic acid. Spheroplasts contained the outer lipoid layer of the cell wall. It was possible to observe this cell wall layer in freeze-etch preparations of spheroplasts. The spheroplast membrane like that of intact cells was cleaved along a central plane to expose particles and particle-free areas.     

Iodination of Myxococcus xanthus during development

Intact cells of Myxococcus xanthus were iodinated with [125I]lactoperoxidase to permit examination of the surface components accessible to labeling during cell development. Vegetative cells, starved on a defined solid medium, aggregated, formed fruiting bodies, and produced myxospores. Cells collected at different stages were iodinated, and their proteins were analyzed by one- and two-dimensional electrophoresis and autoradiography. One-dimensional electrophoresis revealed six iodinated bands in vegetative cell extracts. During development, 10 radioactive bands were detected, 4 of which migrated to the same positions as those of vegetative cells. Only six bands were detected in purified, labeled myxospores. Of these, one band possessed mobility similar to that of labeled vegetative cell proteins, whereas the other bands possessed mobility similar to that detected in developing cells. Analysis of two-dimensional gels indicated that at least 14 proteins were iodinated in vegetative cells, one of which was intensely labeled (protein b). Another of the proteins (protein a) was labeled throughout development. During development, about 30 proteins were iodinated and the prominently labeled ones were designated c, d, e, f, and g. The latter two (proteins f and g) were not detected in purified, iodinated myxospores. The data indicated a pronounced change in surface structure during development; some of the change may be involved in cellular interaction during aggregation.     

The organization of the major protein of the human erythrocyte membrane

The enzyme lactoperoxidase was used to catalyse the radioiodination of membrane proteins in intact human erythrocytes and in erythrocyte `ghosts'. Two major proteins of the erythrocyte membrane were isolated after iodination of these two preparations, and the peptide `maps' of each protein so labelled were compared. Peptides from both proteins are labelled in the intact cell. In addition, further mobile peptides derived from one of the proteins are labelled only in the `ghost' preparation. Various sealed `ghost' preparations were also iodinated, lactoperoxidase being present only at either the cytoplasmic or extra-cellular surface of the membrane. The peptide `maps' of protein E (the major membrane protein) labelled in each case were compared. Two discrete sets of labelled peptides were consistently found. One group is obtained when lactoperoxidase is present at the extra-cellular surface and the other group is found when the enzyme is accessible only to the cytoplasmic surface of the membrane. The results support the assumption that the organization of protein E in the membrane of the intact erythrocyte is unaltered on making erythrocyte `ghosts'. They also confirm previous suggestions that both the sialoglycoprotein and protein E extend through the human erythrocyte membrane.     

Outer membrane of Escherichia coli: properties of the F sex factor traT protein which is involved in surface exclusion.

The traT protein (TraTp) of the F sex factor is the product of one of the two genes involved in surface exclusion. Several detergents were examined under different conditions in order to determine their ability to solubilize TraTp from membrane vesicles. These experiments showed that TraTp behaved similar to a number of peptidoglycan-associated outer membrane proteins and that it existed in multimeric aggregates within the membrane. However, unlike other major outer membrane proteins, the amount of TraTp incorporated into the membrane was not affected by lipopolysaccharide-deficient mutants, even when mutants totally lacking the neutral sugars in their lipopolysaccharide backbone were used. TraTp wqs also examined by two-dimensional gel electrophoresis, where it ran as a discrete spot with a very basic isoelectric point. By coupling cyanogen bromide-activated dextran onto whole cells and by labeling whole cells with 125I (via lactoperoxidase), it was shown that TraTp was exposed on the cell surface. TraTp in a membrane environment was also insensitive to proteolytic attack by trypsin.     

Internal and external membrane proteins of the cyanobacterium, Synechococcus cedrorum

The protein composition and architecture of the photosynthetic membranes from the cyanobacterium, Synechococcus cedrorum, were analyzed with the aid of site-specific labels. Using membranes labeled with ³⁵S, about 50 membrane proteins can be detected by sodium dodecyl sulfate acrylamide gel electrophoresis. Approximately half of the proteins are accessible to modification by the impermeant probe, lactoperoxidase, indicating that they have surface-exposed domains. At least six of these external proteins can be removed by EDTA washing; the correspondence in molecular weights between five of these EDTA-extractable proteins and those of typical chloroplast coupling factor preparations may indicate that they are subunits of a membrane-bound ATPase. The photoactive, lipophilic compound, [¹²⁵I]iodonaphthyl azide, was used to label protein domains in contact with the lipid bilayer. Iodonaphthyl azide modification led to a labeling pattern significantly different from that seen with lactoperoxidase. In particular, proteins in the 13 000–20 000 dalton range that were labeled poorly or not at all by lactoperoxidase were heavily modified by iodonaphthyl azide.Photosystem I and II particles, extracted from the membrane by digitonin treatment, were iodinated by lactoperoxidase after isolation. The PS I particles acted as a relatively tight complex, with most of the proteins remaining inaccessible to surface modification. The PS II particles, on the other hand, responded as a more open structure, with most of the subunits yielding to lactoperoxidase iodination. Similar studies on a highly fluorescent, temperature-sensitive mutant of S. cedrorum revealed a different organization of the PS II complex. This mutant, when grown at 40°C, inserts a 51 kdalton polypeptide in place of a 53 kdalton protein. This protein also replaces the 53 kdalton species in the PS II complex of the mutant after 40°C growth. The structure of this complex is altered in that more sites become accessible to lactoperoxidase. This is particularly true of the 51 kdalton protein, which is barely labeled in wild-type PS II complexes.     

Electrophoretic analysis of the plasma membrane proteins of a mutant of Neurospora crassa which lacks a cell wall

The plasma membrane proteins of a mutant of Neurospora crassa (FGSC No. 326) which lacks a cell wall were analyzed by two-dimensional polyacrylamide gel electrophoresis. Approximately 180 different proteins were detected in purified plasma membrane preparations. Nonpermeant labeling experiments indicated that approximately 40% of these proteins were exposed on the extracytoplasmic surface of the plasma membranes of these cells. The studies demostrate the complexity of the protein composition of N. crassa 326 plasma membranes to be greater than has been suggested by previous investigations.     

Cell surface constituents of sarcoma 180 ascites tumor cells

The cell surface protein components of Sarcoma 180 ascites tumor cells have been investigated by a combination of plasma membrane isolation techniques and lactoperoxidase iodination. For plasma membrane isolation cells were homogenized in the presence or absence of Zn²⁺ and fractionated by sucrose density gradient centrifugation or a two-phase partition to give large membrane fragments or membrane envelopes. Membrane purification was monitored by phase contrast microscopy and chemical and enzyme marker assays. The membrane preparations were analyzed by acrylamide gel electrophoresis in sodium dodecylsulfate. Each preparation showed a common protein pattern of about 15 bands ranging in molecular weights from 33 000 to >300000. Two carbohydrate-containing bands were also present in all preparations. Membranes prepared with Zn²⁺ were much less fragmented and showed much greater amounts of three high molecular weight components than those prepared in the absence of Zn²⁺. This might suggest a role for these components in membrane stabilization.The tumor cells were also subjected to iodination with lactoperoxidase, followed by membrane isolation and acrylamide gel electrophoresis in sodium dodecylsulfate in order to identify polypeptides accessible to the cell surface. The major radioactive band coincided with the major carbohydrate-containing band, presumably a surface glycoprotein. A second carbohydrate-containing band showed variable labeling behavior between different cell preparations. This material had a high molecular weight, as indicated by both acrylamide gel electrophoresis and gel permeation chromatography in dodecylsulfate. Several other components are labeled to a lesser extent in the intact cell.     

The nature and location of Acholeplasma laidlawii membrane proteins investigated by two-dimensional gel electrophoresis

The high resolution, two-dimensional electrophoresis system for the separation of proteins described by O'Farrell, (O'Farrell, P.H. (1975) J. Biol. Chem. 250, 4007–4021) has been modified for the separation of Acholeplasma laidlawii proteins.Reproducible protein patterns have been obtained from A. laidlawii cell, membrane and soluble protein preparations. The isoelectric focusing of membrane proteins was greatly improved by removing the bulk of the membrane lipid before solubilizing the protein.A. laidlawii peripheral membrane proteins were removed from the membrane by low ionic strength washing and by treatment with EDTA. The effect of an exhaustive EDTA treatment and a rapid, warm EDTA treatment were compared. By comparing the protein patterns obtained in these ways it was possible to distinguish two separate groups of peripheral membrane proteins and one integral membrane protein group. The peripheral membrane proteins which were removed from the membrane at low ionic strength (group I) were also insoluble in Triton X-100, whereas additional peripheral membrane proteins extractable by subsequent EDTA treatment (group II) were soluble in Triton X-100.Exterior-facing membrane proteins were distinguished from the interiorfacing ones by lactoperoxidase-catalyzed iodination of intact cells and membranes. Group I peripheral membrane proteins faced the cell interior whereas group II proteins faced the cell exterior. We counted approximately 320 individual whole cell proteins. Of these, about 140 were membrane associated and a maximum of 40 proteins were iodinated after iodinationg intact cells.A. laidlawii was also grown in the presence of NaH₂³²PO₄ and whole cell proteins were separated by two-dimensional gel electrophoresis. One membrane protein and two soluble proteins were labelled.