Artificial Metalloproteins with Dinuclear Iron-Hydroxido Centers

Dinuclear iron centers with a bridging hydroxido or oxido ligand form active sites within a variety of metalloproteins. A key feature of these sites is the ability of the protein to control the structures around the Fe centers, which leads to entatic states that are essential for function. To simulate this controlled environment, artificial proteins have been engineered using biotin-streptavidin (Sav) technology in which Fe complexes from adjacent subunits can assemble to form [Fe^III-(μ-OH)-Fe^III] cores. The assembly process is promoted by the site-specific localization of the Fe complexes within a subunit through the designed mutation of a tyrosinate side chain to coordinate the Fe centers. An important outcome is that the Sav host can regulate the Fe···Fe separation, which is known to be important for function in natural metalloproteins. Spectroscopic and structural studies from X-ray diffraction methods revealed uncommonly long Fe···Fe separations that change by less than 0.3 Å upon the binding of additional bridging ligands. The structural constraints imposed by the protein host on the di-Fe cores are unique and create examples of active sites having entatic states within engineered artificial metalloproteins.

Artificial Iron Proteins: Modeling the Active Sites in Non-Heme Dioxygenases

An important class of non-heme dioxygenases contains a conserved Fe binding site that consists of a 2-His-1-carboxylate facial triad. Results from structural biology show that, in the resting state, these proteins are six-coordinate with aqua ligands occupying the remaining three coordination sites. We have utilized biotin-streptavidin (Sav) technology to design new artificial Fe proteins (ArMs) that have many of the same structural features found within active sites of these non-heme dioxygenases. An Sav variant was isolated that contains the S112E mutation, which installed a carboxylate side chain in the appropriate position to bind to a synthetic FeII complex confined within Sav. Structural studies using X-ray diffraction (XRD) methods revealed a facial triad binding site that is composed of two N donors from the biotinylated ligand and the monodentate coordination of the carboxylate from S112E. Two aqua ligands complete the primary coordination sphere of the FeII center with both involved in hydrogen bond networks within Sav. The corresponding FeIII protein was also prepared and structurally characterized to show a six-coordinate complex with two exogenous acetato ligands. The FeIII protein was further shown to bind an exogenous azido ligand through replacement of one acetato ligand. Spectroscopic studies of the ArMs in solution support the results found by XRD.

Technical Aspects of Fecal Microbial Transplantation (FMT)

PURPOSE OF REVIEW

Fecal microbial transplantation (FMT) has become established as an effective therapeutic modality in the treatment of antibiotic-refractory recurrent Clostridium difficile colitis. A number of formulations and methods of delivery of FMT are currently available, each with distinct advantages. This review aims to review donor and patient selection for FMT as well as procedural aspects of FMT to help guide clinical practice.

RECENT FINDINGS

FMT can be obtained in fresh, frozen, lyophilized, and capsule-based formulations for delivery by oral ingestion, nasoenteric tube, colonoscopy, or enema (depending on the formulation used). Choosing the optimal method relies heavily on patient-related factors, including underlying pathology and severity of illness. As potential applications for FMT expand, careful donor screening and patient selection are critical to minimizing risk to patients and physicians. FMT represents an excellent therapeutic option for treatment of recurrent Clostridium difficile colitis and holds promise as a possible treatment modality in a variety of other conditions. The wide array of delivery methods allows for its application in various disease states in both the inpatient and outpatient setting.

Structure of a bacterial photosynthetic membrane

The internal photosynthetic membranes of a photosynthetic bacterium, Rhodopseudomonas viridis, have been studied with a variety of electron microscope techniques. The membranes are composed of a sheet of apparently identical subunits arranged in a hexagonal fashion. The individual subunits repeat at a distance of 110 A. Optical transforms have been used to enhance micrographs of this ordered membrane, and the images synthesized in this way show details of each subunit. The individual subunits are asymmetric, differing slightly in appearance at the outer and inner surfaces of the membrane, and these surface patterns seem to be combined in the image of the thylakoid membrane in negative stain. These studies fix a maximum size for the photosynthetic unit of R. viridis and suggest the suitability of this membrane for further diffraction analysis.

Deconstructing design: a strategy for defending science

Despite its legal and scientific failings, the "intelligent design" (ID) movement has been a public relations success story in the United States. By first creating doubts about the adequacy of evolution to account for the complexity of life, the ID movement has invoked the values of "fairness" and "openness" to argue for inclusion in the classroom and curriculum. In this way, it has attempted to lay claim to the very principles of critical analysis and open discussion at the heart of the scientific enterprise, leaving many researchers in doubt as to how to respond to these challenges. Specific case studies, including the blood-clotting cascade and data from the human genome, show how scientists can have a leading role in deconstructing the arguments advanced in favor of ID. The key to this strategy is remarkably simple and was at the heart of the landmark 2005 Kitzmiller v. Dover trial on ID. It is for researchers to take the claims made by ID proponents seriously, and then to follow them to their logical scientific conclusions. When this is done effectively, the hypothesis of "design" can be publicly falsified in ways that are understandable to laypeople and decision makers in education.

Access to sign language interpreters in the criminal justice system

Historically, the provision of sign language interpreters to deaf suspects, defendants, and offenders has been a problematic issue in the criminal justice system. Inconsistency in the provision of interpreter services results largely from the ignorance of criminal justice professionals regarding deaf people's communication needs and accommodation options. Through analysis of 22 post-Americans with Disabilities Act cases and a survey of 46 professional sign language interpreters working in criminal justice settings, the present study considered access issues concerning sign language interpreters in law enforcement, courtrooms, and correctional settings. Recommendations to increase the accessibility of interpreting services include providing ongoing awareness training to criminal justice personnel, developing training programs for deaf legal advocates, and continuing access studies.

Patient with trisomy 6 mosaicism

Trisomy 6 and trisomy 6 mosaicism were found in chorionic villi cell culture and short term incubation in a prenatal diagnosis at 12 weeks of gestation in a pregnancy with a growth retarded fetus showing nuchal translucency. The child was born in the 25th gestational week with a number of malformations including heart defects, deep-set ears, cleft right hand, cutaneous syndactylies, and overlapping toes of irregular shape and length. Trisomy 6 was not found in peripheral blood lymphocytes but was confirmed in umbilical cord fibroblasts. Currently, at the age of 2-3/4 years, the development of the child is relatively normal despite considerable growth delay. At the age of two years, she developed a papular erythema clinically suggestive of epidermal nevi. Cytogenetic analysis of fibroblast cultures derived from skin from a right hand finger and the inguinal area confirmed the presence of a trisomy 6 mosaicism. This is the first observation of a liveborn with trisomy 6 mosaicism.

Linguistic diversity in deaf defendants and due process rights

Historically, ensuring the due process rights of deaf defendants has been a problematic issue in the criminal justice system (McAlister, 1994; Smith, 1994; Vernon & Coley, 1978; Vernon & Greenburg, 1996; Vernon & Miller, in press; Vernon & Raifman, 1997; Whalen, 1981; Wood, 1984). Inadequate communication can radically affect a deaf defendant's interactions in the courtroom. Pursuant to the concepts of fairness enshrined in the U.S. Constitution and the specific statutory language contained in federal and state laws, the courts must provide equal access for deaf defendants (Berko, 1992; Gallie & Smith, 2000; McCoy, 1992; Simon, 1994; Vernon & Raifman, 1997). It is the responsibility of the court to ensure that the appropriate accommodation is provided in the language most readily understood by the defendant.When adjudicating a deaf criminal defendant, courts must make certain that the defendant has equal access to various due process activities, such as assisting counsel in the development of a defense, deciding whether to testify, deciding which plea to enter, understanding the charges, understanding one's position as defendant, and comprehending the role of the defense and prosecuting attorneys, and judge (Berko, 1994; King, 1990; Simon, 1994; Smith, 1994; Vernon & Coley, 1978; Vernon & Miller, in press; Vernon, Raifman, & Greenberg, 1996).However, complex linguistic issues that impinge on adjudicative competence are present in some deaf defendants (Vernon & Miller, in press; Vernon & Raifman, 1997). Adjudicative competence refers to an individual's ability to adequately comprehend and participate in legal proceedings and due process activities. When diverse language use is an issue, a deaf defendant's ability to participate in proceedings can be established by the court using the modern test of adjudicative competence (Dusky v. U.S., 1960). This test examines a defendant's state of mind at the time of trial rather than at the time of the offense in terms of these factors: a defendant's capacity to participate, reasonable understanding of the proceedings, and level of cognitive functioning, irrespective of any mental disorder. This article will outline linguistic barriers to due process for deaf defendants.

The lack of binding of methyl-n-amyl ketone (MAK) to rat liver DNA as demonstrated by direct binding measurements, and 32P-postlabeling techniques

It has been reported that 14C-labeled methyl-n-amyl ketone (MAK, 2-heptanone) is able to bind spontaneously, in vitro, to isolated rat liver DNA to the extent of 400 pmol/mg DNA; and that 14C-MAK, when given by gavage to female Fischer 344 rats, resulted in HPLC chromatograms of isolated, hydrolyzed liver DNA in which some radiolabel was not associated with the four normal DNA bases dA, dT, dC, and dG. The present studies were undertaken to re-examine the hypothesis that MAK is able to bind to rat liver DNA. In the in vitro study, liver nuclear DNA was incubated with [2-14C]-labeled MAK (25 mCi/mmol) in the absence, or in the presence of rat liver microsomes, precipitated, washed free of unbound MAK, and counted by scintillation spectrometry. No binding to DNA by MAK was detectable. In the in vivo study, groups of five female F344 rats were exposed by inhalation to 0, 80, 400, or 1000 ppm MAK for 6 h/day for 10 days. DNA was purified from the liver nuclei of the 0 and 1000 ppm dosed animals, and 32P-postlabeling techniques were used to assay for adducts. No DNA adducts were detected using these techniques. It was concluded that MAK lacks the ability to bind to rat liver DNA in vitro and in vivo.

Clinical significance of postlaser indocyanine green angiographic hot spots in age-related macular degeneration

OBJECTIVE

To report the natural history and clinical significance of indocyanine green (ICG) angiographic hot spots in age-related macular degeneration (ARMD) after laser photocoagulation.

DESIGN

Retrospective noncomparative case series.

PARTICIPANTS

Two hundred thirty consecutive patients with exudative ARMD who underwent krypton laser treatment between March 1993 and December 1996.

INTERVENTION

Krypton laser photocoagulation was performed on all patients. Digital videoangiograms, including both fluorescein angiography and ICG angiography, were obtained at 2 weeks, 4 weeks, 6 weeks, and 3 months postlaser treatment on each patient and repeated if clinical changes were noted.

MAIN OUTCOME MEASURES

Detection by ICG of hyperfluorescent spots (hot spots) within the hypofluorescent laser-treated area.

RESULTS

Forty patients (18%) developed ICG hot spots 2 weeks after laser treatment. The hot spots disappeared spontaneously without recurrent choroidal neovascular (CNV) membrane in 31 patients (78%). Recurrent CNV was discovered at the hot spot in four patients and away from the hot spot in five patients.

CONCLUSION

The postlaser ICG angiographic hot spot is not rare. These spots tend to disappear spontaneously and do not necessarily represent a recurrent CNV. The presence of a hot spot on ICG angiography is not an indication for immediate laser retreatment.

The bacterial SecY/E translocation complex forms channel-like structures similar to those of the eukaryotic Sec61p complex

The SecYEG complex is a major component of the protein translocation apparatus in the cytoplasmic membrane of bacteria. We have purified a translocationally active complex of the two subunits, SecY and SecE, from Bacillus subtilis. As demonstrated by electron microscopy, SecY/E forms ring structures in detergent solution and in intact lipid bilayers, often with a quasi-pentagonal appearance in projection. The particles represent oligomeric assemblies of the SecY/E complex and are similar to those formed by the eukaryotic Sec61p complex. We propose that these SecY/E rings represent protein-conducting channels and that the two essential membrane components SecY and SecE are sufficient for their formation.

The multisubunit IkappaB kinase complex shows random sequential kinetics and is activated by the C-terminal domain of IkappaB alpha

The multisubunit IkappaB kinase (IKK) catalyzes the signal-inducible phosphorylation of N-terminal serines of IkappaB. This phosphorylation is the key step in regulating the subsequent ubiquitination and proteolysis of IkappaB, which then releases NF-kappaB to promote gene transcription. As measured by 33P incorporation into a GST-IkappaB alpha fusion protein, varying both the concentration of GST-IkappaB alpha and [gamma-33P]ATP resulted in a kinetic pattern consistent with a random, sequential binding mechanism. Values of 55 nM and 7 microM were obtained for the dissociation constants of GST-IkappaB alpha and ATP, respectively. The value of alpha, a factor by which binding of one substrate changes the dissociation constant for the other substrate, was determined to be 0.11. This indicates that the two substrates bind in a cooperative fashion. Peptides corresponding to either amino acids 26-42 (N-terminal peptide) or amino acids 279-303 (C-terminal peptide) of IkappaB alpha inhibited the IKK-catalyzed phosphorylation of GST-IkappaB alpha; the C-terminal peptide, unexpectedly, was more potent. The inhibition by the C-terminal peptide was competitive with respect to GST-IkappaB alpha and mixed with respect to ATP, which verified the sequential binding mechanism. The C-terminal peptide was also a substrate for the enzyme, and a dissociation constant of 2.9-6.2 microM was obtained. Additionally, the N-terminal peptide was a substrate (Km = 140 microM). Competitive inhibition of the IKK-catalyzed phosphorylation of the C-terminal peptide by the N-terminal peptide indicated that the peptides are phosphorylated by the same active site. Surprisingly, the presence of the C-terminal peptide greatly accelerated the rate of phosphorylation of the N-terminal peptide as represented by a 160-fold increase in the apparent second-order rate constant (kcat/Km). These results are consistent with an allosteric site present within IKK that recognizes the C terminus of IkappaB alpha and activates the enzyme. This previously unobserved interaction with the C terminus may represent an important mechanism by which the enzyme recognizes and phosphorylates IkappaB.

Influence of vesicle surface composition on the interfacial binding of lecithin:cholesterol acyltransferase and apolipoprotein A-I

Interfacial binding affinities and capacities of lecithin:cholesterol acyltransferase (LCAT) and apolipoprotein A-I (apoA-I) for surfaces of different phosphatidylcholine (PC) composition, cholesterol content, and apolipoprotein content were measured with a vesicle model system. Native polyacrylamide gel electrophoresis was used to separate free protein from vesicle-bound protein. ApoA-I was isolated from human plasma and radiolabeled with iodine, whereas radiolabeled LCAT was purified from the media of Chinese hamster ovary cells that were transfected with human LCAT cDNA and incubated in the presence of [35S] cysteine and methionine. Bound and free radiolabeled LCAT and apoA-I were quantified by phosphorimage analysis. ApoA-I binding was not influenced by cholesterol content (14 mole%) but was influenced by the PC fatty acyl composition of the vesicle. PC species containing long chain, polyunsaturated fatty acids (PUFA) in the sn-2 position resulted in increased binding affinity (Kd = 75-177 nM) but reduced capacity (0.1-0.3 apoA-I/ 1000 PC) in comparison to sn-1 palmitoyl, sn-2 oleoyl PC (POPC, 750 nM and 1.4 apoA-I/1000 PC). LCAT binding affinity to POPC (2190 nM) was stronger in the presence of cholesterol (530 nM), and LCAT binding capacity was reduced (2.63 and 0.6 molecules LCAT/1000 PC, respectively). In comparison to POPC, LCAT binding affinity to sn-1 palmitoyl, sn-2 arachidonyl PC was stronger (611 nM) and binding capacity was reduced (0.7 LCAT/1000 PC). LCAT binding affinity and capacity to sn-1 palmitoyl, sn-2 eicosapentaneoyl PC (2041 nM, and 2.5 LCAT/1000 PC) were similar to those observed for POPC. We conclude that vesicle surface PC fatty acyl composition and cholesterol content significantly influence LCAT and apoA-I interfacial binding and therefore may alter LCAT enzymatic activity.

L-arginine and superoxide dismutase prevent or reverse cerebral hypoperfusion after fluid-percussion traumatic brain injury

To determine whether treatment with L-arginine or superoxide dismutase (SOD) would prove effective in reducing cerebral hypoperfusion after traumatic brain injury (TBI), we measured cerebral blood flow (CBF) using laser Doppler flowmetry (LDF) in rats treated before or after moderate (2.2 atm) fluid-percussion (FP) TBI. Rats were anesthetized with isoflurane and prepared for midline FP TBI and then for LDF by thinning the calvaria using an air-cooled drill. Rats were then randomly assigned to receive sham injury, sham injury plus L-arginine (100 mg/kg, 5 min after sham TBI), TBI plus 0.9% NaCl, TBI plus L-arginine (100 mg/kg, 5 min post-TBI), TBI plus SOD (24,000 U/kg pre-TBI + 1600 units/kg/min for 15 min after TBI), or TBI plus SOD and L-arginine. A second group of rats received TBI plus saline, L-, or D-arginine (100 mg/kg, 5 min after-TBI). After treatment and TBI or sham injury, CBF was measured continuously using LDF for 2 h and CBF was expressed as a percent of the preinjury baseline for 2 h after TBI. Rats treated with saline or D-arginine exhibited significant reductions in CBF that persisted throughout the monitoring period. Rats treated with L-arginine alone or in combination with SOD exhibited no decreases in CBF after TBI. CBF in the SOD-treated group decreased significantly within 15 min after TBI but returned to baseline levels by 45 min after TBI. These studies indicate that L-arginine but not D-arginine administered after TBI prevents posttraumatic hypoperfusion and that pretreatment with SOD will restore CBF after a brief period of hypoperfusion.

Oligomeric rings of the Sec61p complex induced by ligands required for protein translocation

The heterotrimeric Sec61p complex is a major component of the protein-conducting channel of the endoplasmic reticulum (ER) membrane, associating with either ribosomes or the Sec62/63 complex to perform co- and posttranslational transport, respectively. We show by electron microscopy that purified mammalian and yeast Sec61p complexes in detergent form cylindrical oligomers with a diameter of approximately 85 A and a central pore of approximately 20 A. Each oligomer contains 3-4 heterotrimers. Similar ring structures are seen in reconstituted proteoliposomes and native membranes. Oligomer formation by the reconstituted Sec61p complex is stimulated by its association with ribosomes or the Sec62/63p complex. We propose that these cylindrical oligomers represent protein-conducting channels of the ER, formed by ligands specific for co- and posttranslational transport.

Glycosylation structure and enzyme activity of lecithin:cholesterol acyltransferase from human plasma, HepG2 cells, and baculoviral and Chinese hamster ovary cell expression systems

The glycosylation state of lecithin:cholesterol acyltransferase (LCAT) may be important in determining its enzymatic activity. We compared glycosylation structure, enzyme kinetics, and phosphatidylcholine (PC) acyl specificity of human LCAT from four sources: human plasma (pLCAT), media from HepG2 cells (HepG2 LCAT), media from SF21 cells infected with a recombinant baculovirus (bLCAT) and media from stably transfected Chinese hamster ovary (CHO) cells (CHO LCAT). bLCAT was underglycosylated (molecular weight approximately 50 kDa) and resistant to digestion by N-glycanase F, endoglycosidase F, and neuraminidase. CHO and HepG2 LCAT were overglycosylated (approximately 68 kDa and approximately 70-75 kDa) compared to pLCAT (approximately 65 kDa). CHO LCAT, like pLCAT, was sensitive to N-glycanase F and neuraminidase but not to endoglycosidase F. HepG2 LCAT demonstrated resistance to N-glycanase F and endoglycosidase F. Apparent Km values for all four enzymes were similar (1.4-9.2 microM cholesterol) for recombinant high density lipoproteins (rHDL) containing sn-1 16:0, sn-2 18:1 PC (POPC). Apparent Vmax values (nmol cholesteryl ester formed/h per micrograms) were 52.6 for pLCAT, 48.6 for CHO LCAT, 15.3 for bLCAT, and 8.3 for HepG2 LCAT. Changes in PC acyl specificity in the presence and absence of cholesterol were characterized by comparing the ratio of LCAT activity on rHDL containing sn-1 16:0, sn-2 20:4 PC (PAPC) or POPC (PAPC/POPC activity ratio). The ratios for pLCAT, bLCAT, CHO LCAT, and HepG2 LCAT activity were 0.63, 0.49, 0.56, and 0.51 with cholesterol and 0.34, 0.29, 0.36, and 0.99 without cholesterol, respectively. We conclude that LCAT source influences glycosylation structure, which affects the apparent Vmax for cholesteryl ester formation with only minor changes in apparent Km or acyl substrate specificity.

Activation of protein kinase C by phorbol esters disrupts the tegument of Schistosoma mansoni

The tegument of the human parasite Schistosoma mansoni is critical for parasite survival within the mammalian host. The role of protein kinase C (PKC), a major effector molecule in the phosphoinositide pathway, in maintaining the structural organization of this syncytial layer was examined in adult worms. Phorbol 12-myristate, 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDB), phorbol esters that activate PKC, induced formation of surface vesicles as determined by light and scanning electron microscopy. Similar results were seen with sn-2-dioctanoyl-glycerol, a synthetic analogue of diacylglycerol. No effect was seen in parasites incubated with 4-alpha-phorbol ester or alpha isomers of PMA or PDB, compounds that do not activate PKC. Vesicle formation was reversible in parasites treated with sn-2-dioctanoyl-glycerol but not with phorbol esters. The tegument of male worms was more sensitive to the effect of phorbol esters than females. Transmission electron microscopy revealed vacuolization of the tegument. These data suggest that signal transduction pathways may have a critical role in the maintenance of the structural integrity of the tegument of parasitic helminths.

Mapping the lateral distribution of photosystem II and the cytochrome b6/f complex by direct immune labeling of the thylakoid membrane

By direct immunolabeling we have mapped the distribution of photosystem II (PS II) and cytochrome b6/f on the surfaces of photosynthetic membranes isolated from spinach. Photosynthetic membranes were attached to a support and gently disrupted to expose the occluded outer stacked surface, prior to labeling. Polyclonal antibodies against PS II intensely labeled the outer stacked surfaces while the outer nonstacked surface had minimal labeling. This confirms previous fractionation and immunolocalization studies which demonstrated that PS II is largely restricted to the stacked regions of the membrane. Inside-out membranes were also heavily labeled with PS II antibodies. Antibodies against cytochrome f were evenly distributed between the stacked and nonstacked outer surfaces and were found clumped together on the membrane outer surface. Previous fractionation and immunolocalization studies have indicated that cytochrome b6/f is located in both the stacked and nonstacked regions, but this is the first report to provide direct evidence that the complex may be clustered in the membrane. The clustering of antibodies to cytochrome b6/f supports the idea that this electron transport component exists as a multimeric complex within the membranes and that such complexes are found in both stacked and nonstacked regions of the photosynthetic membrane. No evidence was seen of any special differentiation of the marginal regions of the membrane, which link stacked and nonstacked regions.

Titration calorimetry as a binding assay for lipid-binding proteins

Titration calorimetry has been evaluated as a method for obtaining binding constants and thermodynamic parameters for the cytosolic fatty acid- and lipid-binding proteins. An important feature of this method was its ability to accurately determine binding constants in a non-perturbing manner. The equilibrium was not perturbed, since there was no requirement to separate bound and free ligand in order to obtain binding parameters. Also, the structure of the lipid-protein complex was not perturbed, since native ligands were used rather than non-native analogues. As illustrated for liver fatty acid-binding protein, the method distinguished affinity classes whose dissociation constants differed by an order of magnitude or less. It also distinguished endothermic from exothermic binding reactions, as illustrated for the binding of two closely related bile salts to ileal lipid-binding protein. The main limitations of the method were its relatively low sensitivity and the difficulty working with highly insoluble ligands, such as cholesterol or saturated long-chain fatty acids. However, the signal-to-noise ratio was improved by manipulating the buffer conditions, as illustrated for oleate binding to rat intestinal fatty acid binding protein. Binding parameters are reported for oleate interactions with several wild-type and mutant lipid-binding proteins from intestine. Where possible, the binding parameters obtained from calorimetry were compared with results obtained from fluorescence and Lipidex binding assays of comparable systems.

Ligand-protein electrostatic interactions govern the specificity of retinol- and fatty acid-binding proteins

Cellular retinol-binding protein II (CRBP-II) and intestinal fatty acid-binding protein (I-FABP) are both expressed in small intestinal enterocytes and exhibit 31% sequence identity. I-FABP binds a single molecule of long-chain fatty acid and forms an ion-pair electrostatic interaction between the cationic side chain of arginine-106 and the anionic fatty acid carboxyl group. In contrast, CRBP-II binds all-trans-retinol or -retinal and contains a glutamine residue in the corresponding position, residue 109. We have characterized and compared the interactions of fatty acids and retinoids with I-FABP, CRBP-II, and two reciprocal mutant proteins. The mutants were designated CRBP-II(Q109R), where glutamine-109 was replaced by arginine, and I-FABP(R106Q), where arginine-106 was replaced by glutamine. As monitored by titration calorimetry and carbon-13 NMR spectroscopy, the fatty acid-binding properties of CRBP-II(Q109R) were found to be essentially identical to those of wild-type I-FABP. Both proteins bound 1 molecule of fatty acid with identical affinities (Kd = 0.2 microM). The enthalpic contribution to the total free energy of binding was large for both proteins: 66% and 87%, respectively. In addition, the carboxyl groups of fatty acids bound to both proteins were solvent-inaccessible. There was little or no change in the ionization state of the bound fatty acid over a wide pH range, as monitored by the chemical shift of the fatty acid carboxyl 13C resonance. Furthermore, the binding of fatty acid to both proteins was accompanied by a selective perturbation of the guanidino 13C resonance of a single arginine residue.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of excitotoxin exposure on metabolic rate of primary hippocampal cultures: application of silicon microphysiometry to neurobiology

Increasing evidence implicates glutamate receptor over-stimulation in the neurotoxicity associated with a host of metabolic insults, including seizures and hypoxia-ischemia. To begin to understand more completely the role of energy metabolism in the mechanism of neuron death following excitatory amino acid exposure, we investigated the effects of kainic acid exposure on metabolic rate in cultured hippocampal cells using a recently developed silicon microphysiometer. The device gives a continual real-time measure of metabolism in relatively small numbers of cells, as assessed by efflux of protons generated at least in part by ATP hydrolysis and lactic acid production. In the first half of this report, we characterize the feasibility of using this device for measuring cellular metabolism in hippocampal cultures. Metabolic rate in both astrocytes and neurons was readily detectable, with a high signal-to-noise ratio. The rate was proportional to the number of cells and was sensitive to metabolic enhancement or depression. We then utilized this device to study metabolic responses to the excitotoxin kainic acid. We observed a receptor-mediated, dose-dependent increase in metabolic rate upon stimulation by kainic acid, with an EC50 of approximately 100 microM. Exposure to toxic levels of kainic acid for 10 min produced an initial elevation (for 2 hr) in metabolic rate and then a gradual decline in metabolism over the next 8 hr that preceded a measurable loss of cell viability. This study further delineates a time window for the onset of kainic acid-induced damage. The results clearly show the feasibility of using silicon microphysiometry for assessing metabolism of brain cultures and for exploring the relationship between metabolism and synaptic activation.

Localization of light-harvesting complex II to the occluded surfaces of photosynthetic membranes

The photosynthetic membranes of green plants are organized into stacked regions interconnected by nonstacked regions that have been shown to be biochemically and structurally distinct. Because the stacking process occludes the surfaces of appressed membranes, it has been impossible to conduct structural or biochemical studies of the outer surfaces of the photosynthetic membrane in regions of membrane stacking. Although stacking is mediated at this surface, it has not been possible to determine whether membrane components implicated in the stacking process, including a major light-harvesting complex (LHC-II), are in fact exposed at the membrane surface. We have been able to expose this surface for study in the electron microscope and directly label it with antibodies to determine protein exposure. The appearance of the newly exposed outer stacked surface highlights the extreme lateral heterogeneity of the photosynthetic membrane. The surface is smooth in contrast to the neighboring nonstacked surface that is covered with distinct particles. Although some investigators have suggested the existence of a cytochrome b6/f-rich boundary region between stacked and nonstacked membranes, our results provide no structural support for this concept. To explore the biochemical nature of the occluded membrane surface, we have used an mAb against the amino terminal region of the LHC-II. This mAb clearly labels the newly exposed outer stacked surface but does not label the inner surface or the outer nonstacked surface. These experimental results confirm the presence of the amino terminal region of this complex at the outer surface of the membrane in stacked regions, and also show that this complex is largely absent from nonstacked membranes.

Two-dimensional crystals of the photosystem II reaction center complex from higher plants

By detergent treatment of isolated photosynthetic membranes from maize chloroplasts, we have prepared two-dimensional crystals of the photosystem II complex. Two distinct crystal forms are produced by this treatment. Analysis of Fourier transforms of the crystals shows that each crystal type is formed from two inverted layers. Within the rectangular 17.8 x 26.7 nm unit cell of each layer is a tetrameric structure enclosing a two-fold symmetry axis, a result implying that the basic structural unit of photosystem II is dimeric. Tris-washing, which removes proteins associated with the oxygen-evolving apparatus from the inner surface of the photosynthetic membrane, causes a distinct change in the structure of these tetramers and reveals a dimeric core complex which may be directly associated with the photosystem II machinery.

Studies on asparagine-linked protein glycosylation in differentiating skeletal muscle cells

The embryonic development of skeletal muscle proceeds by the adherence and fusion of myoblast cells to form multinucleated myotubes. In the present study, enzymes in the dolichol pathway for asparagine-linked glycoprotein synthesis and oligosaccharide chain composition were characterized in myoblasts and myotubes derived from the C2 (mouse) muscle cell line. The N-acetylglucosaminyltransferase responsible for chain initiation and the mannosyl- and glucosyltransferases for Dol-P-Man and Dol-P-Glc synthesis were characterized with respect to substrate, cation, and detergent dependence. Time course studies in the absence and presence of exogenous Dol-P revealed that myoblasts had a two- to threefold higher capacity than myotubes for Dol-sugar synthesis. Pulse-chase experiments following the elongation of the Dol-oligosaccharide by intact cells showed myoblasts to label oligosaccharide intermediates approximately fourfold greater than myotubes; myotubes, however, were more efficient than myoblasts for converting the intermediates to the glucosylated Dol-tetradecasaccharide. Oligosaccharide chains isolated from sarcolemma glycopeptides were analyzed by Con A, WGA, and QAE chromatography. There were no differences between myoblast and myotube oligosaccharides with respect to the proportion of tri-tetraantennary complex, biantennary complex, and high mannose chains. Hybrid chains were not detected. The major high mannose chain contained nine mannose residues. Sialyltransferase activity was identical. The results suggest that higher levels of Dol-P and protein acceptor contribute to the greater degree of protein glycosylation in myoblast vs myotube muscle cells.

Supramolecular structure of the thylakoid membrane of Prochlorothrix hollandica: a chlorophyll b-containing prokaryote

Prochlorothrix hollandica is a newly described photosynthetic prokaryote, which contains chlorophylls a and b. In this paper we report the results of freeze fracture and freeze etch studies of the organization of the photosynthetic thylakoid membranes of Prochlorothrix. These membranes exhibit four distinct fracture faces in freeze fractured preparations, two of which are derived from membrane splitting in stacked regions of the thylakoid membrane, and two of which are derived from nonstacked regions. The existence of these four faces confirms that the thylakoid membranes of Prochlorothrix, like those of green plants, display true membrane stacking and have different internal composition in stacked and non-stacked regions, a phenomenon that has been given the name lateral heterogeneity. The general details of these fracture faces are similar to those of green plants, although the intramembrane particles of Prochlorothrix are generally smaller than those of green plants by as much as 30%. Freeze etched membrane surfaces have also been studied, and the results of these studies confirm freeze fracture observations. The outer surface of the thylakoid membrane displays both small (less than 8.0 nm) and large (greater than 10.0 nm) particles. The inner surface of the thylakoid membrane is covered with tetrameric particles, which are concentrated into stacked membrane regions, a situation that is similar to the inner surfaces of the thylakoid membranes of green plants. These tetramers have never before been reported in a prokaryote. The photosynthetic membranes of Prochlorothrix therefore represent a prokaryotic system that is remarkably similar, in structural terms, to the photosynthetic membranes found in chloroplasts of green plants.

A structural analysis of gap and tight junctions in the rat liver during a dietary treatment that induces oval cell proliferation

The authors have investigated early changes in liver cell gap and tight junctions that occur when rats are fed a carcinogenic diet. Animals were fed a choline-deficient diet that contained 0.1% ethionine (CDE) for periods up to 6 weeks. Short-term feeding of this diet results in the rapid proliferation of so-called "oval cells" within the liver, which is reversible upon returning the rats to a normal diet. Livers from animals fed the diet were removed at various times during feeding and during recovery from the diet and were analyzed by light and electron microscopy. The freeze-fracture technique was used to produce extended views of the internal structure of liver cell membranes at each stage under study. The characteristic junctional complex surrounding canalicular regions in normal liver disappears after only 2 weeks of the CDE regimen. Gap junctions were not found after 4 weeks of the diet, and tight junctions became increasingly disorganized. Tight junction elements were observed, however, between hepatocytes and oval cells, which indicated that these two cell types do interact directly. Changes occur in the structural complexity of tight junction elements between hepatocytes and between hepatocytes and oval cells. Recovery from the CDE diet results in a rapid increase in junctional complexity, and the large gap junction plaques characteristic of normal liver are visible within 2 weeks after cessation of the CDE regimen. These and other observations demonstrate that reversible alterations in hepatocyte gap and tight junctions occur as a result of administration of a diet that induces oval cell proliferation. The relationship of these changes to those that have been reported during other processes of cell proliferation are discussed.

Heliobacterium chlorum: cell organization and structure

The basic cellular organization of Heliobacterium chlorum is described using the freeze-etching technique. Internal cell membranes have not been observed in most cells, leading to the conclusion that the photosynthetic apparatus of these organisms must be localized in the cell membrane of the bacterium. The two fracture faces of the cell membrane are markedly different. The cytoplasmic (PF) face is covered with densely packed particles averaging 8 nm in diameter, while the exoplasmic (EF) face contains far fewer particles, averaging approximately 10 nm in diameter. Although a few differentiated regions were noted within these fracture faces, the overall appearance of the cell membrane was remarkably uniform. The Heliobacterium chlorum cell wall is a strikingly regular structure, composed of repeating subunits arranged in a rectangular pattern at a spacing of 11 nm in either direction. We have isolated cell wall fragments by brief sonication in distilled water, and visualized the cell wall structure by negative staining as well as deep-etching.

The effects of galactolipid depletion on the structure of a photosynthetic membrane

The galactolipids monogalactosyldiglyceride and digalactosyldiglyceride together comprise more than 77% of the photosynthetic membrane lipids of higher plant chloroplasts. We have isolated a lipase from the chloroplasts of runner beans (Phaseolus vulgaris) which is highly specific for these galactolipids. This galactolipase promotes the hydrolysis of monogalactosyldiglyceride and digalactosyldiglyceride, in the process liberating two free fatty acids into the membrane bilayer, leaving the residual galactosyl glyceride group to diffuse into the aqueous bulk phase. Isolated spinach photosynthetic membranes were treated with this enzyme preparation and changes in membrane composition were studied with thin layer chromatography (for lipids), gel electrophoresis (proteins), and freeze-etching (membrane structure). After 30 min of lipolysis, nearly 100% of the galactolipids had been converted into membrane-associated fatty acids and water-soluble galactosyl glycerides. SDS PAGE showed that two proteins, one of which is possibly associated with the reaction center of photosystem II, were removed by the treatment. Despite the minor nature of changes in membrane protein composition, freeze-fracture and freeze-etch studies showed that striking changes in membrane structure had taken place. The large freeze-fracture particle on the E fracture face had disappeared in stacked regions of the membrane system. In addition, a tetrameric particle visible at the inner surface of the membrane had apparently dissociated into individual monomeric particles. The fact that these two structures are so dramatically affected by the loss of galactolipids strongly suggests that these lipids play a crucial role in maintaining their structure. Both structures are believed to be different views of the same transmembrane unit: a membrane-spanning complex associated with photosystem II. Our results are consistent with two possible interpretations: the intramembrane particles may be lipidic in nature, and hence lipolysis causes their disappearance; or galactolipids are necessary for the organization of a complex photosystem II-associated structure which is composed of a number of different molecular species.

The Rhodopseudomonas viridis photosynthetic membrane: arrangement in situ

The organization of photosynthetic membranes in the cytoplasm of the photosynthetic bacterium Rh. viridis has been examined by several techniques for electron microscopy. Thin sections of membrane stacks show that the regular lattice of membrane subunits reported in other studies can be observed in thin section. Tilting of sections in the electron microscope shows that the regular lattices of several membranes overlap in a way that suggests they are in register with each other. This observation can be confirmed by freeze-fracture images in which a regular arrangement of membrane lattices can be observed, each perfectly aligned. Analysis of the spacings of membrane pairs shows that the photosynthetic membranes of Rh. viridis are very closely apposed. The mean diameter of two membranes is 160A, and the average space between two such membranes is only 42A. When a recently developed atomic level model of Rh. viridis reaction center is superimposed against these spacings, each reaction center extends from the surface of its respective membrane far enough to make contact with an apposing membrane. The limited free space between membranes and regular alignment of lattices has a number of implications for how this membrane is organized to carry out the process of energy transfer.

Crystallization of the light-harvesting chlorophyll a/b complex within thylakoid membranes

We have found that treatment of the photosynthetic membranes of green plants, or thylakoids, with the nonionic detergent Triton X-114 at a 10:1 ratio has three effects: (a) photosystem I and coupling factor are solubilized, so that the membranes retain only photosystem II (PS II) and its associated light-harvesting apparatus (LHC-II); (b) LHC-II is crystallized, and so is removed from its normal association with PS II; and (c) LHC-II crystallization causes a characteristic red shift in the 77 degrees K fluorescence from LHC-II. Treatment of thylakoids with the same detergent at a 20:1 ratio results in an equivalent loss of photosystem I and coupling factor, with LHC-II and PS II being retained by the membranes. However, no LHC-II crystals are formed, nor is there a shift in fluorescence. Thus, isolation of a membrane protein is not required for its crystallization, but the conditions of detergent treatment are critical. Membranes with crystallized LHC-II retain tetrameric particles on their surface but have no recognizable stromal fracture face. We have proposed a model to explain these results: LHC-II is normally found within the stromal half of the membrane bilayer and is reoriented during the crystallization process. This reorientation causes the specific fluorescence changes associated with crystallization. Tetrameric particles, which are not changed in any way by the crystallization process, do not consist of LHC-II complexes. PS II appears to be the only other major complex retained by these membranes, which suggests that the tetramers consist of PS II.

Two-dimensional crystals of a membrane protein: arrangement of subunits within the crystal sheet

Two-dimensional crystals have been prepared from the photosynthetic reaction center of Rhodopseudomonas viridis. Filtered images of these crystals show individual subunits approximately 4.5 nm in diameter arranged at a center-to-center distance of 6.4 nm. Our previous studies suggested that each subunit within such a sheet corresponds to a single photosynthetic reaction center. Air-dried and freeze-etched shadowed preparations of the crystals yield images which are quite different from negatively stained material. Rotary-shadowed surfaces of the crystals show rows of wedge-shaped particles separated by 3 nm furrows. Two such wedge-shaped particles occupy the 12.1 X 12.9 nm area in which four negatively stained subunits are normally visualized. Close analysis of these shadowed pictures suggests that both the shadowed and negatively stained images can be accounted for by a single model of subunit arrangement within the crystal. Within each 12.1 X 12.9 nm unit cell, two subunits are placed near one surface of the sheet, and two others are near the other surface. All four subunits are visible in negative stain. When the surface is shadowed, only the two subunits which project above the surface of the sheet accumulate appreciable amounts of the heavy metal shadow. Because of their close position, one subunit shades the other, forming the wedge-shaped appearance characteristic of the crystal. The only arrangement consistent with both shadowed and negatively stained images is one in which the two raised subunits occupy positions at either end of a diagonal across the unit cell. The analysis of shadowed images indicates that the plane group of the crystals is P22(1)2(1).

Photosynthetic reaction centers in artificial membranes: estimating protein dimensions by freeze-fracture and freeze-etching

Because estimates of size and shape for membrane proteins are difficult to obtain directly, many workers have incorporated purified proteins into artificial lipid bilayers. Freeze-fracturing has then been used to provide a measure of the approximate size and shape of the membrane protein. We have formed reconstituted membranes containing the photosynthetic reaction center of Rhodopseudomonas viridis, a photosynthetic bacterium. The size and shape of this reaction center is accurately known from studies of negatively stained crystals of the protein to be approximately 4.5 X 6.0 nm. Freeze-fracture images of the reaction center in phosphatidyl choline liposomes show particles formed after reconstitution with an average diameter of 12.3 nm, much larger than the actual size of the protein. Deep-etched images of the surfaces of the liposomes, in which each individual reaction center complex can be seen clearly, show why the diameter of the protein is exaggerated in freeze-fracture. The individual reaction centers tend to cluster into large groups, allowing several individual reaction centers to be visualized as a single (much larger) particle in freeze-fracture. Freeze-fracturing, although a valuable tool in the analysis of membrane structure in natural and artificial membranes, must be used with caution in the estimation of molecular sizes and shapes.

Structure of a bacterial photosynthetic membrane: integrity of reaction centers following proteolysis and detergent solubilization

The photosynthetic membranes of the purple bacterium Rhodopseudomonas viridis are composed of a semi-crystalline lattice of subunits. Proteolysis of isolated membranes with trypsin or pronase results in the degradation of polypeptides associated with the photosynthetic reaction center. However, two low molecular weight peptides which may form the light-harvesting complex survive the enzymatic treatment. The proteolysis does not affect the major absorbance peak (830 nm) associated with the reaction center. However, treatment of proteolyzed membranes with detergents such as LDAO abolishes the 830 nm absorbance peak. The 830 nm peak is stable following LDAO solubilization of non-proteolyzed membranes. These results suggest that a combination of covalent and non-covalent interactions are important in maintaining the configuration of the reaction center, and are consistent with a model of membrane organization in which the light-harvesting components are buried in a lipid phase of the membrane and reaction center components form the large structures which electron microscope studies have shown to extend from either membrane surface.

Formation of dolichol-linked sugar intermediates during the postnatal development of skeletal muscle

The postnatal development of skeletal muscle is characterized by changes in membrane function associated with N-linked glycoproteins. In the present study, early reactions involved in the synthesis of the dolichol-linked core oligosaccharide were examined in neonatal and adult rabbit skeletal muscle sarcoplasmic reticulum membranes. The initial rate of N-acetylglucosamine incorporation in the presence of exogenous dolichol phosphate was similar between neonate and adult (3.5-4.1 pmol of GlcNAc/min/mg). The Km values for UDP-GlcNAc and exogenous dolichol phosphate were similar. Tunicamycin (0.04-0.08 micrograms/ml) inhibited N-acetylglucosamine incorporation by 50%. UDP-GlcNAc pyrophosphatase activity was greater in neonatal membranes than adult (840 versus 350 pmol of GlcNAc-1-P/min/mg), explaining, in part, the greater enhancement of neonatal GlcNAc incorporation by pyrophosphatase inhibitors. Nucleotide-sugar pyrophosphatase inhibitors (alpha, beta-methylene ATP and dimercaptopropanol) increased the capacity of neonatal activity 4-fold and adult enzyme 2-fold. Analysis of dolichol-linked products by mild acid hydrolysis however, revealed that neonate had higher capacity for N,N'-diacetylchitobiosyl(pyro)phosphoryldolichol synthesis than adult. Mannosyltransferase and glucosyltransferase were elevated 6- and 5-fold in neonate compared to adult membranes. Neonate exhibited 4-fold greater GDP-Man pyrophosphatase activity than adult (500 versus 125 pmol of Man-1-P/min/mg). The Km for GDP-Man increased in the presence of exogenous dolichol phosphate. Increasing concentrations of exogenous dolichol phosphate did not equalize neonate and adult mannosyltransferase activity, indicating that the decline in activity during development was not due to a decrease in a pool of dolichol phosphate accessible to mannosyltransferase. Glucosyltransferase for the synthesis of glucosylphosphoryldolichol was also elevated 5-fold in neonatal compared to adult sarcoplasmic reticulum (7 versus 1.4 pmol of Glc/min/mg). In a previous study, it was reported that glycoprotein sialyltransferase activity decreased by a factor of 6.5 during the postnatal maturation and that total membrane hexose content of sarcoplasmic reticulum decreased by a factor of 8. Together, these results suggest that the postnatal development of skeletal muscle is characterized by coordinated changes in the expression of enzymes involved in both the "early" and "late" reactions of N-linked oligosaccharide biosynthesis.

Two-dimensional crystals formed from photosynthetic reaction centers

Photosynthetic reaction centers from the bacterium Rhodopseudomonas viridis were prepared after detergent solubilization of photosynthetic membranes. The purified reaction centers, in agreement with reports from other laboratories, contain four distinct polypeptides ranging in molecular weight from 28,000 to 41,000. When the detergent was gradually removed by dialysis under appropriate conditions, large two-dimensional sheets of reaction centers were formed, suitable for analysis by electron microscopy. The crystals were rectangular, and the dimensions of a single unit cell were 121 X 129 A. Each unit cell contained four distinct subunits, each with approximate dimensions of 45 X 60 A. The thickness of the sheet was 60 A. Preliminary studies of the sheets with negative staining indicated that the sheets show a high degree of order: as many as six orders are visible in transforms of the images. Because of the fact that in R. viridis the native membrane from which these reaction centers were purified also displays a crystal-like structure, comparative studies between a membrane and one of its components, each analyzed by Fourier techniques, are now possible.

Structure of a bacterial photosynthetic membrane. Isolation, polypeptide composition, and selective proteolysis

A procedure for the isolation of highly purified bacterial photosynthetic membranes from Rhodopseudomonas viridis is described. The purity of the final membrane fraction has been confirmed by electron microscopy. Seven major polypeptide bands are associated with the photosynthetic membranes, and all seven are resistant to solubilization in Triton X-100 detergent. Two pigmented bands with apparent molecular weights of 44K and 41K are thought to be cytochromes. The three polypeptides with apparent molecular weights of 38K, 32K, and 28K have been reported in reaction center preparations of other laboratories. Two low-molecular-weight (16K and 11K) bands bind bacteriochlorophyll b and may represent light-harvesting bacteriochlorophyll-protein complexes. The structures that were isolated seem to represent complete photosynthetic membranes, consisting of reaction center, electron transport, and light-harvesting components, all arranged in the regular lattice characteristic of viridis. Selective proteolysis of these membranes indicates that all membrane components are accessible to digestion by trypsin and pronase, except for the light-harvesting complexes.

Artifacts associated with quick-freezing and freeze-drying

We have studied the structures produced when nonbiological samples were subjected to quick-freezing and freeze-drying with a liquid helium cooled freeze-slamming device. Samples examined in this way included sodium chloride, sucrose, and Tris buffer. A variety of filamentlike and trabeculumlike structures were formed in these preparations. These structures may represent eutectic mixtures formed during the growth of small ice crystals during the freezing process, and exposed during the rapid sublimation of pure ice during the etching process. Samples of biological membranes (isolated chloroplast membranes) were prepared in various buffers by means of this technique. In distilled water, excellent replicas of membrane surfaces were obtained. In salt solutions, however, the membranes appeared to be embedded in a network of thin filaments appearing very much like a cytoskeletal lattice. It is concluded that extreme caution must be used when employing this preparation technique for studies of cell architecture, and that extensive washing of cell components in distilled water may be necessary to obtain faithful representations of cell structure.

Addition of lipid to the photosynthetic membrane: effects on membrane structure and energy transfer

We have carried out a series of experiments in which the lipid composition of the photosynthetic membrane has been altered by the addition of lipid from a defined source under experimental conditions. Liposomes prepared by sonication are mixed with purified photosynthetic membranes obtained from spinach chloroplasts and are taken through cycles of freezing and thawing. Several lines of evidence, including gel electrophoresis and freeze-fracture electron microscopy, indicate that an actual addition of lipid has taken place. Structural analysis by freeze-fracture shows that intramembrane particles are widely separated after the addition of large amounts of lipid, with one exception: large hexagonal lattices of particles appear in some regions of the membrane. These lattices are identical in appearance with lattices formed from a single purified component of the membrane known as chlorophyll-protein complex II. The suggestion that the presence of such lattices in lipid-enriched membranes reflects a profound rearrangement of photosynthetic structures has been confirmed by analysis of the fluorescence emission spectra of natural and lipid-enriched membranes. Specifically, lipid addition in each of the cases we have studied results in the apparent detachment of chlorophyll-protein complex II from photosynthetic reaction centers. It is concluded that specific arrangements of components in the photosynthetic membrane, necessary for the normal functioning of the membrane in the light reaction of photosynthesis, can be regulated to a large extent by the lipid content of the membrane.

A chloroplast membrane lacking photosystem I. Changes in unstacked membrane regions

The structure and polypeptide composition of the photosynthetic membrane of a mutant of maize has been investigated. The thylakoid membranes of the mutant plants are deficient in Photosystem I activity, although Photosystem II is at near normal levels. SDS polyacrylamide gel electrophoresis of thylakoid membranes from the mutant shows them to be deficient in two polypeptide bands which have been associated with Photosystem I. Freeze-fracture studies of the membrane show that the absence of these polypeptides is associated with a measurable reduction in particle diameter on the unstacked protoplasmic fracture face. This fracture face is derived from the splitting of membranes in unstacked regions of the thylakoid membrane system. It is suggested that in membranes stacked by salts in vitro, Photosystem I activity may be confined to this region.

A chloroplast membrane lacking photosystem II. Thylakoid stacking in the absence of the photosystem II particle

The polypeptide composition and membrane structure of a variegated mutant of tobacco have been investigated. The pale green mutant leaf regions contain chloroplasts in which the amount of membrane stacking has been reduced (although not totally eliminated). The mutant membranes are almost totally deficient in Photosystem II when compared to wild-type chloroplast membranes, but still show near-normal levels of Photosystem I activity. The pattern of membrane polypeptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows several differences between mutant and wild-type membranes, although the major chlorophyll-protein complexes described in many other plant species are present in both mutant and wild-type samples. Freeze-fracture analysis of the internal structure of these photosynthetic membranes shows that the Photosystem II-deficient membranes lack the characteristic large particle associated with the E fracture face of the thylakoid. These membranes also lack a tetramer-like particle visible on the inner (ES) surface of the membrane. The other characteristics of the photosynthetic membrane, including the small particles observed on the P fracture faces in both stacked and unstacked regions, and the characteristic changes in the background matrix of the E fracture face which accompany thylakoid stacking, are unaltered in the mutant. From these and other observations we conclude that the large (EF and ES) particle represents an amalgam of many components comprising the Photosystem II reaction complex, that the absence of one or more of its components may prevent the structure from assembling, and that in its absence, Photosystem II activity cannot be observed.

Chloroplast membrane biogenesis in Chlamydomonas: correlation between the formation of membrane components and membrane structure

The y-1 mutant of Chlamydomonas reinhardi, when allowed to green in the presence of chloramphenicol (CAP), an inhibitor of protein synthesis on 70s ribosomes, form photosynthetic membranes which contain somewhat less chlorophyll than those of cells greened in the absence of the drug. Photosystem I and II activities are drastically reduced in the CAP-greened cells, and specific alterations in the polypeptide composition of the thylakoid membranes are also observed. We have examined the internal structure of the thylakoid membranes from cells greened in the presence and absence of CAP, and have found that the large particles observed on the exoplasmic fracture face (EF) are substantially reduced in size and number in the CAP-greened cells. This structural defect seems related to the absence of significant photo-system activities in the CAP-greened cells, despite the presence of most major membrane polypeptides. We suggest that CAP treatment results in a failure of the cell to organize functional reaction complexes, and is structurally reflected in the absence of large (EF) particles in such membranes. This defect can be repaired by allowing the affected cells to re-green in the absence of the drug, and the large particles reappear, paralleling an increase in photosynthetic activity.

Organization of the photosynthetic membrane in maize mesophyll and bundle sheath chloroplasts

The freeze-fracturing technique has been used to investigate membrane architecture in the mesophyll and bundle sheath chloroplasts of Zea mays. The structural organization of mesophyll chloroplasts is virtually identical to that of other species of higher plants which have been investigated with this technique. Characteristic distributions of particles of various sizes are seen on each fracture face after membrane splitting during the fracturing process, and these distributions indicate the differentiation of the membrane system into sacked (grana) and unstacked (stroma) regions, typical of grana-containing chloroplasts. Bundle sheath chloroplasts contain very few grana, and the thylakoids of these plastids are therefore largely unstacked. Analysis of artificially unstacked mesophyll chloroplasts indicates that this difference is not merely related to the presence or absence of adhesion between adjacent thylakoids, but reflects a substantial difference in membrane substructure between mesophyll and bundle sheath photosynthetic membranes. Bundle sheath thylakoids contain virtually the same number of small (P fracture face) particles as mesophyll thylakoids, but contain only 40% as many of the larger (E fracture face) tetrameric particles. These differences, together with biochemical data indicating the comparative deficiency of bundle sheath chloroplasts in Photosystem II activity, suggest that the E face particles are related to the presence or absence of Photosystem II activity.