Role of membrane environment and membrane-spanning protein regions in assembly and function of the Class II Major Histocompatibility complex

Class II Major Histocompatibility complex (MHC-II) is a polymorphic heterodimer that binds antigen-derived peptides and presents them on the surface of antigen presenting cells. This mechanism of antigen presentation leads to recognition by CD4 T-cells and T-cell activation, making it a critical element of adaptive immune response. For this reason, the structural determinants of MHC-II function have been of great interest for the past 30 years, resulting in a robust structural understanding of the extracellular regions of the complex. However, the membrane-localized regions have also been strongly implicated in protein-protein and protein-lipid interactions that facilitate Class II assembly, transport and function, and it is these regions that are the focus of this review. Here we describe studies that reveal the strong and selective interactions between the transmembrane domains of the MHC α, and invariant chains which, when altered, have broad reaching impacts on antigen presentation and Class II function. We also summarize work that clearly demonstrates the link between membrane lipid composition (particularly the presence of cholesterol) and MHC-II conformation, subsequent peptide binding, and downstream T-cell activation. We have integrated these studies into a comprehensive view of Class II transmembrane domain biology. [Abstract copyright: Copyright © 2018. Published by Elsevier Inc.

Thermodynamic competition between membrane protein oligomeric states

Self-assembly of protein monomers into distinct membrane protein oligomers provides a general mechanism for diversity in the molecular architectures, and resulting biological functions, of membrane proteins. We develop a general physical framework describing the thermodynamic competition between different oligomeric states of membrane proteins. Using the mechanosensitive channel of large conductance as a model system, we show how the dominant oligomeric states of membrane proteins emerge from the interplay of protein concentration in the cell membrane, protein-induced lipid bilayer deformations, and direct monomer-monomer interactions. Our results suggest general physical mechanisms and principles underlying regulation of protein function via control of membrane protein oligomeric state.Comment: 7 pages, 5 figure

Sites of Biosynthesis of Outer and Inner Membrane Proteins of Neurospora crassa Mitochondria

Outer and inner membranes of Neurospora crassa mitochondria were separated by the combined swelling, shrinking, sonication procedure. Membranes were characterized by electron microscopy and by marker enzyme activities. A red carotenoid pigment was found to be concentrated in the outer membrane. The inner mitochondrial membrane was resolved into about 20 protein bands on polyacrylamide gel electrophoresis, whereas the outer membrane shows essentially one single protein band. Only negligible incorporation of radioactive amino acids occurs into outer membrane when isolated mitochondria are synthesizing polypeptide chains. In agreement with this observation labeling of outer membrane protein is almost entirely blocked, when whole Neurospora cells are incubated with radioactive amino acids in the presence of cycloheximide, an inhibitor of cytoplasmic protein synthesis. Finally, the essential electrophoretic protein band from outer membrane does not become labeled when mitochondria are incubated with radioactive amino acids either in vitro or in vivo in the presence of cycloheximide. It is concluded that the vast majority, if not all, of the outer membrane protein is synthesized by the cytoplasmic system and that polypeptide chains formed by the mitochondrial ribosomes are integrated into the inner mitochondrial membrane

A saposin-lipoprotein nanoparticle system for membrane proteins.

A limiting factor in membrane protein research is the ability to solubilize and stabilize such proteins. Detergents are used most often for solubilizing membrane proteins, but they are associated with protein instability and poor compatibility with structural and biophysical studies. Here we present a saposin-lipoprotein nanoparticle system, Salipro, which allows for the reconstitution of membrane proteins in a lipid environment that is stabilized by a scaffold of saposin proteins. We demonstrate the applicability of the method on two purified membrane protein complexes as well as by the direct solubilization and nanoparticle incorporation of a viral membrane protein complex from the virus membrane. Our approach facilitated high-resolution structural studies of the bacterial peptide transporter PeptTSo2 by single-particle cryo-electron microscopy (cryo-EM) and allowed us to stabilize the HIV envelope glycoprotein in a functional state

Rampant exchange of the structure and function of extramembrane domains between membrane and water soluble proteins.

Of the membrane proteins of known structure, we found that a remarkable 67% of the water soluble domains are structurally similar to water soluble proteins of known structure. Moreover, 41% of known water soluble protein structures share a domain with an already known membrane protein structure. We also found that functional residues are frequently conserved between extramembrane domains of membrane and soluble proteins that share structural similarity. These results suggest membrane and soluble proteins readily exchange domains and their attendant functionalities. The exchanges between membrane and soluble proteins are particularly frequent in eukaryotes, indicating that this is an important mechanism for increasing functional complexity. The high level of structural overlap between the two classes of proteins provides an opportunity to employ the extensive information on soluble proteins to illuminate membrane protein structure and function, for which much less is known. To this end, we employed structure guided sequence alignment to elucidate the functions of membrane proteins in the human genome. Our results bridge the gap of fold space between membrane and water soluble proteins and provide a resource for the prediction of membrane protein function. A database of predicted structural and functional relationships for proteins in the human genome is provided at sbi.postech.ac.kr/emdmp

Detection of protein concentrations using a pH-step titration method

A stimulus-response method based on the application of a pH step is proposed for the detection of protein immobilized in a membrane on top of an ion-sensitive field-effect transistor (ISFET). The ISFET response to a step-wise change in pH, applied at the interface between the membrane and the surrounding solution, depends on the concentration of protein immobilized in the membrane because proton-dissociation reactions of the protein cause a delayed diffusion of protons and hydroxyl ions through the membrane. Our theoretical description shows that the delay in ISFET response is linearly related to the concentration of protein immobilized in the membrane. Experiments performed with lysozyme as a model protein show the feasibility of this detection principle

Pengaruh Outer Membrane Protein Helicobacter Pylori Terhadap Perubahan Histopatologi Mukosa Lambung Dan S-IgA Pada Mus Musculus Outbred Balb/C

Helicobacter pylori (H. pylori) merupakan bakteri penyebab inflamasi mukosa lambung. Faktor virulensi bakteri berperan pada patogenesis penyakit infeksi oleh bakteri yang pada umumnya dapat merangsang sistem imun. Secara umum antigen yang merupakan faktor virulensi ini terdapat dalam Outer Membrane protein (OMP). Pemberian antigen secara per oral mampu menginduksi respon imun mukosal dengan cara membentuk Secretory IgA (S-IgA). Penelitian ini bertujuan untuk mengetahui pengaruh OMP H. pylori terhadap histopatologi mukosa lambung Mus musculus Outbred Balb/C dan peningkatan konsentrasi S-IgA pada Mus musculus Outbred Balb/C. Metode Penelitian yang digunakan adalah Post-test only kontrol group design. Bakteri H. pylori dikultur kemudian dilakukan isolasi OMP dengan menggunakan bahan n-Octyl-ß-D-Glucopyranoside (NOG) 0,5% melalui isolasi bertahap dan dilakukan SDS-PAGE. Setelah itu dilakukan coupling dengan CTB dan diberikan ke mencit secara intragastrik dengan dosis 100 μgml-1 setiap minggu. Pada akhir minggu ke-2, ke-4, ke-6 dan ke-8 dilakukan pemeriksaan histopatologis dan kadar S-IgA. Hasil penelitian menunjukkan terdapat perbedaan yang signifikan (p<0,05) pada perhitungan jumlah sel polimorfonuklear (PMN), jumlah sel Mononuklear (MN) dan kadar S-IgA antara kelompok kontrol dan perlakuan. Berdasarkan hasil penelitian dapat disimpulkan bahwa OMP H. pylori dapat menyebabkan kerusakan mukosa lambung menginduksi S-IgA Mus musculus Outbred Balb/C

Modeling membrane nanotube morphology: the role of heterogeneity in composition and material properties.

Membrane nanotubes are dynamic structures that may connect cells over long distances. Nanotubes are typically thin cylindrical tubes, but they may occasionally have a beaded architecture along the tube. In this paper, we study the role of membrane mechanics in governing the architecture of these tubes and show that the formation of bead-like structures along the nanotubes can result from local heterogeneities in the membrane either due to protein aggregation or due to membrane composition. We present numerical results that predict how membrane properties, protein density, and local tension compete to create a phase space that governs the morphology of a nanotube. We also find that there exists a discontinuity in the energy that impedes two beads from fusing. These results suggest that the membrane-protein interaction, membrane composition, and membrane tension closely govern the tube radius, number of beads, and the bead morphology

A new approach to immunoFET operation

A new method is presented for the detection of an immunological reaction taking place in a membrane, which covers the gate area of an ISFET. By stepwise changing the electrolyte concentration of the sample solution, a transient diffusion of ions through the membrane-protein layer occurs, resulting in a transient membrane potential, which is measured by the ISFET. The diffusion rate is determined by the immobile charge density in the amphoteric protein layer, which changes upon formation of antibody—antigen complexes. No membrane potential is induced at zero fixed charge density as occurs at a protein characteristic pH. Isoeletric points of embedded proteins can be determined by detecting the zero potential response.\ud \ud Up to now, the authors have studied the membrane adsorption of lysozyme, human serum albumin (HSA) and the immune reaction of HSA with the antibody anti-human serum albumin (αHSA). The influence of protein parameters on the amplitude of the transient can be described with an empirical equation. Assuming Langmuir behaviour, the protein concentration in the solution can well be correlated with the concentration in the membrane.\ud \ud This new detection method is unique concerning direct measurements of charge densities and isoelectric points of amphoteric macromolecules adsorbed in the membrane. The simple procedure of one incubation stage followed by one detection stage, without separate washing and labelling techniques, gives direct information about specific charge properties of the macromolecules to be studied