Protein insertion into the membrane of the endoplasmic reticulum: The architecture of the translocation site

In mammalian cells, most membrane proteins are inserted cotranslationally into the membrane of the endoplasmic reticulum (ER), and secretory proteins are translocated across this membrane (for review, see Rapoport 1992). These processes are initiated in the cytoplasm by binding of the signal sequence of the na-scent polypeptide chain to the signal recognition parti-cle (SRP) (Walter and Blobel 1981). The resulting ribosome nascent chain SRP complex is then targeted to the ER membrane by an interaction with the mem-brane-bound SRP receptor (docking protein) (Gilmore et al. 1982; Meyer et al. 1982). At the membrane, the signal sequence is released from SRP in a GTP-requiring step, and the nascent polypeptide is trans-ferred into the ER translocation site (Connolly an

Intramembrane-cleaving aspartic proteases and disease: presenilins, signal peptide peptidase and their homologs

Recent studies demonstrate that presenilins (PSs) and signal peptide peptidase (SPP) are members of a novel protease family of integral membrane proteins that may utilize a catalytic mechanism similar to classic aspartic proteases such as pepsin, renin and cathepsin D. The defining features of the PSs and SPP are their ability to cleave substrate polypeptides within a transmembrane region, the presence of two active site aspartate residues in adjacent membrane-spanning regions and a conserved PAL motif near their COOH-terminus. PSs appear to be the catalytic subunit of multiprotein complexes that possess γ-secretase activity. Because this activity generates the amyloid β peptide (Aβ) deposited in the brain of patients with Alzheimer's disease (AD), PSs are considered therapeutic targets in AD. In contrast to PSs that are not active unless part of a larger complex, SPP does not appear to require protein co-factors. Because of its requirement for hepatitis C virus maturation and a possible immune modulatory role, SPP is also considered a potential therapeutic target. Four additional PS/SPP homologs have been identified in humans; yet, their functions have not been elucidated. Herein, we will review the recent advances in our understanding of the PS/SPP family of proteases as well as discuss aspects of intramembrane cleavage that are not well understoo

The Protein-Conducting Channel in the Membrane of the Endoplasmic Reticulum Is Open Laterally toward the Lipid Bilayer

Lipids and proteins were found to contact a nascent type II membrane protein, as well as a nascent secretory protein, during their insertion into the membrane of the endoplasmic reticulum. This suggests that the protein-conducting channel is open laterally toward the lipid bilayer during an early stage of protein insertion. Contact to lipids was confined to the hydrophobic core region of the respective signal or signal anchor sequence. Thus, the nascent polypeptide is positioned in the translocation complex such that the signal or signal anchor sequence faces the lipid bilayer, whereas the hydrophilic, translocating portion is in proteinaceous environment

Signal Sequence Processing in Rough Microsomes

Secretory proteins are synthesized with a signal sequence that is usually cleaved from the nascent protein during the translocation of the polypeptide chain into the lumen of the endoplasmic reticulum. To determine the fate of a cleaved signal sequence, we used a synchronized in vitro translocation system. We found that the cleaved signal peptide of preprolactin is further processed close to its COOH terminus. The resulting fragment accumulated in the microsomal fraction and with time was released into the cytosol. Signal sequence cleavage and processing could be reproduced with reconstituted vesicles containing Sec61, signal recognition particle receptor, and signal peptidase complex

Site-specific Photocross-linking Reveals That SecGlp and TRAM Contact Different Regions of a Membrane-inserted Signal Sequence

A chemically charged amber suppressor tRNA was used to introduce the photoactivatable amino acid (Tmd)Phe at a selected position within the signal sequence of the secretory protein preprolactin. This allowed the interactions of the NH -terminal, the central, and the COOH-terminal regions of the signal sequence to be investigated during insertion into the membrane of the endoplasmic reticulum (ER). We found that different regions of the nascent chains were photocross-linked to different ER proteins. The TRAM protein (translocating chain-associating membrane protein) contacts the NHz-terminal region of the signal sequence while the mammalian Sec6lp contacts the hydrophobic core of the signal sequence and regions COOH-terminal of this. These results suggest that the ER translocation complex is composed of heterologous protein subunits which contadcti stinct regions of nascent polypeptides during their membrane insertion

The intramembrane protease Sppl2a is required for B cell and DC development and survival via cleavage of the invariant chain

The protease Sppl2a cleaves the N-terminal fragment of invariant chain (CD74) and is required for efficient B cell development and function.B cell development requires tight regulation to allow for the generation of a diverse repertoire while preventing the development of autoreactive cells. We report, using N-ethyl-N-nitrosourea (ENU)–induced mutagenesis, the identification of a mutant mouse (chompB) with a block in early B cell development. The blockade occurs after the transitional 1 (T1) stage and leads to a decrease in mature B cell subsets and deficits in T cell–dependent antibody responses. Additionally, chompB mice have decreases in myeloid dendritic cells (DCs). The mutation was mapped to the intramembrane protease signal peptide peptidase-like 2a (Sppl2a), a gene not previously implicated in immune cell development. Proteomic analysis identified the invariant chain (CD74) as a key substrate of Sppl2a and suggests that regulated intramembrane proteolysis of CD74 by Sppl2a contributes to B cell and DC survival. Moreover, these data suggest that modulation of Sppl2a may be a useful therapeutic strategy for treatment of B cell dependent autoimmune disorders

Análisis de la norma ISO 9001 en la postventa automotriz : propuesta de aplicación ISO-9001 en el sector de postventa de Veneranda S.A. sucursal Villa María

Fil: Martoglio, Bruno. Universidad Nacional de Villa María; Argentina

Intramembrane-cleaving aspartic proteases and disease: Presenilins, signal peptide peptidase and their homologs

ISSN:0964-6906ISSN:1460-208

Intramembrane proteolysis promotes trafficking of hepatitis C virus core protein to lipid droplets

Hepatitis C virus (HCV) is the major causative pathogen associated with liver cirrhosis and hepatocellular carcinoma. The virus has a positive-sense RNA genome encoding a single polyprotein with the virion components located in the N-terminal portion. During biosynthesis of the polyprotein, an internal signal sequence between the core protein and the envelope protein E1 targets the nascent polypeptide to the endoplasmic reticulum (ER) membrane for translocation of E1 into the ER. Following membrane insertion, the signal sequence is cleaved from E1 by signal peptidase. Here we provide evidence that after cleavage by signal peptidase, the signal peptide is further processed by the intramembrane-cleaving protease SPP that promotes the release of core protein from the ER membrane. Core protein is then free for subsequent trafficking to lipid droplets. This study represents an example of a potential role for intramembrane proteolysis in the maturation of a viral protein