Ubiquitin dependent degradation of endoplasmic reticulum membrane-bound substrates - mechanisms and requirements
Nicole Zabel, Universität Stuttgart
Hrd1, als Abbausignal dienen. Zusätzlich werden nur die Ubiquitin-konjugierenden Enzyme Ubc1 und Ubc7 benötigt. Schlussendlich ist die AAA ATPase Cdc48 für die Membranextraktion aller abbaubaren Proteine verantwortlich. Zusammenfassend konnte in dieser Arbeit gezeigt werden, dass der zytosolische Teil von Membranproteinen des endoplasmatischen Retikulums eine große Rolle spielt, obwohl diese fehlgefaltete ER lumenale Domänen und intakte oder fehlgefaltete Membrandomänen tragen. Außerdem
... t die Art der zytosolischen Domäne den Weg des Abbaus dieser Proteine. Protein synthesis starts at ribosomes in the cytosol. 20 -40% of the proteome are secretory proteins    . These are equipped with motifs targeting them for import into the endoplasmic reticulum (ER). These hydrophobic stretches of amino acids are recognized and the respective proteins are able to enter the ER. There are various ways into the ER either by a signal recognition particle (SRP) dependent or independent mechanism, also called co-or posttranslational import, respectively. All import pathways merge at the translocon, which is embedded in the ER membrane. Secretory proteins enter the ER via the Sec61 translocon. During ER import, they are glycosylated. The core N-glycan is trimmed by glucosidase I (Gls1) and glucosidase II (Gls2). After removal of the glucose residues, several mannose moieties are cleaved off by α-1,2-mannosidase I (Mns1) and the α-1,2-specific exomannosidase (Mnl1 alias Htm1). This provides time for protein folding. In case the folding failed, the α-1,6-linked mannose moiety is exposed, further mannose residues are removed by α-1,2-mannose (Mnl2) and recognized by the lectin Yos9. The Hsp70/Hsp40 chaperone complex (Kar2, Scj1 and Jem1), as well as Yos9 bind to Hrd3, which is in complex with the ubiquitin ligase Hrd1 (alias Der3). Der1 also recognizes misfolded proteins and is recruited to the HRD ligase complex via Usa1. (Figure adapted form Berner et al. 2018) ER-and inner nuclear membrane-embedded proteins with a disordered membrane domain are members of the ERAD-M substrate family. Transmembrane helices are usually 12-35 amino acids long. They share unifying sequence features, vary widely in hydrophobicity and helical propensity 64 . Little is known about their recognition. But it seems that the ubiquitin ligases themselves are responsible for recognition of ERAD-M substrates. In a systematic analysis of the ubiquitin ligase Hrd1 (alias Der3), various hrd1 mutants were found with impaired ERAD-M substrate degradation 65 . Residues specific for the recognition of aberrant membrane domains are located in the transmembrane domain of Hrd1. It is hypothesized, that recognition is induced by the interaction of normally buried hydrophilic residues of the substrate membrane domain with hydrophilic residues of the ligase 41,65 . Sss1. This complex forms the alternative translocon. In case its binding partner Ssh1 is lacking, Sbh2 exposes polar residues. These residues are recognized by Doa10 and lead to fast degradation of Sbh2. Thus, orphan Sbh2 is a native ERAD-M substrate 66 . Previously, it was also found that a ubiquitin ligase, located in the inner nuclear membrane is responsible for ubiquitination of incorrectly localized ERAD-M substrates.