![]() ![]() Yet, examples such as preprolactin, influenza virus hemagglutinin (HA) and human cytomegalovirus US11 show that cleavage can follow initial folding and/or glycosylation and sometimes requires longer nascent-chain lengths ( Daniels et al., 2003 Rutkowski et al., 2003 Rehm et al., 2001). The minimal requirement for cleavage is the emergence of the cleavage site in the ER lumen, which translates to ~70 synthesized residues of a nascent polypeptide chain ( Daniels et al., 2003 Kowarik et al., 2002 Hou et al., 2012), and its recognition by the signal-peptidase complex. Signal peptides are widely believed to be cleaved co-translationally ( Blobel and Dobberstein, 1975 Jackson and Blobel, 1977 Martoglio and Dobberstein, 1998), but cleavage may well occur anywhere from early co-translational to late post-translational, depending on the protein. In a second cleavage step signal peptides are cleared from the ER membrane by signal-peptide peptidase, an intramembrane rhomboid-like protease ( Weihofen et al., 2002). Signal-peptide cleavage is mediated by the signal-peptidase complex, which, like oligosaccharyl transferase, associates with the translocon ( Görlich et al., 1992 Gilmore, 1993). Cleavable signal peptides are variable in sequence but share characteristics of an N-terminal region with typically 0–2 basic residues, a membrane-spanning hydrophobic α-helix (H) region, and a C-terminal region that often contains a signal-peptide cleavage site ( von Heijne, 1983 von Heijne, 1984). Targeting of soluble and type-I transmembrane proteins to the ER is mediated via cleavable signal peptides, near-N-terminal hydrophobic stretches of 14–50 amino acids that are recognized by SRP ( von Heijne, 1985 Kurzchalia et al., 1986 Lütcke et al., 1992 Walter and Blobel, 1981 Blobel and Dobberstein, 1975 Hegde and Bernstein, 2006). Proteins destined for the secretory pathway are translated and translocated into the endoplasmic reticulum (ER), which provides a specialized environment for their folding, disulfide bond formation, and N-linked glycosylation. The change to early cleavage results in decreased viral fitness compared to wild-type HIV. We demonstrate that postponed cleavage of gp160 enhances functional folding of the molecule. A point mutation in the signal peptide breaks the alpha helix allowing co-translational cleavage. Conserved residues from the signal peptide and residues downstream of the canonical cleavage site form an extended alpha-helix in the ER membrane, which covers the cleavage site, thus preventing cleavage. We here detail the mechanism by which co-translational signal-peptide cleavage is prevented. Signal-peptide cleavage occurs only late after gp160 chain termination and is dependent on folding of the soluble subunit gp120 to a near-native conformation. ![]() Proper gp160 folding in the ER requires core glycosylation, disulfide-bond formation and proline isomerization. Like all other secretory proteins, the HIV-1 envelope glycoprotein gp160 is targeted to the endoplasmic reticulum (ER) by its signal peptide during synthesis. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |