The protein translocation channel binds proteasomes to the endoplasmic reticulum membrane

Abstract

Misfolded secretory proteins are transported across the endoplasmic reticulum (ER) membrane into the cytosol for degradation by proteasomes. A large fraction of proteasomes in a cell is associated with the ER membrane. We show here that binding of proteasomes to ER membranes is salt sensitive, ATP dependent, and mediated by the 19S regulatory particle. The base of the 19S particle, which contains six AAA-ATPases, binds to microsomal membranes with high affinity, whereas the 19S lid complex binds weakly. We demonstrate that ribosomes and proteasomes compete for binding to the ER membrane and have similar affinities for their receptor. Ribosomes bind to the protein conducting channel formed by the Sec61 complex in the ER membrane. We co-precipitated subunits of the Sec61 complex with ER-associated proteasome 19S particles, and found that proteoliposomes containing only the Sec61 complex retained proteasome binding activity. Collectively, our data suggest that the Sec61 channel is a principal proteasome receptor in the ER membrane.

Degradation of a Cytosolic Protein Requires Endoplasmic Reticulum-associated Degradation Machinery

“…Visualization of the ER reveals that this organelle is present throughout the cytosol and makes contact with virtually all organelles (61,68). Additionally, a significant portion of proteasomes in yeast may be localized to the ER/nuclear envelope membrane (69,70). Thus, the ER is properly positioned to fulfill a generalized role in cellular quality control, in contrast to the previous view of its role being strictly confined to ERAD.…”

CD4 and BST-2/Tetherin Proteins Retro-translocate from Endoplasmic Reticulum to Cytosol as Partially Folded and Multimeric Molecules

“…The Sec61 channel is only 15-25 Å wide and during synthesis can accommodate the transport of nonglycosylated unfolded polypeptides of 5-8 Å (63), suggesting that a pore of at least four times as wide is needed. It has been proposed but not proven completely that the Sec61 channel could form the retro-translocation pore by dynamically interacting with different molecular partners and thereby changing its permeability properties (64). Pores of larger size such as the ones formed by perforin (130 -300 Å wide) (65,66), which allow the movement of granzymes of 45-50 Å width (65), are large enough to compromise tight control of the transported material (67).…”

The Retrotranslocation Protein Derlin-1 Binds Peptide:N-Glycanase to the Endoplasmic Reticulum

“…Retrotrans- location of misfolded proteins may utilize the Sec61 translocon, which has been shown to bind the proteasome (Kalies et al, 2005). Alternatively, certain glycoproteins such as HC may utilize another pathway.…”

“…A retrotranslocation channel formed by Sec61 complex is also shown in this model that may be involved in the translocation of a subset of misfolded protein substrates. Recently, the Sec61 channel has been demonstrated to be a membrane receptor for the proteasome (Kalies et al, 2005), suggesting that the degradation of misfolded proteins does indeed occur in close vicinity to the ER. Whether the retrotranslocation channel formed by Derlin-1 also directly interacts with the proteasome remains to be determined.…”

“…Earlier work exploiting this system demonstrated a retrotranslocation channel consisting of Sec61 complex that catalyzed the US2-mediated dislocation of MHC class I HC (Wiertz et al, 1996;Ye et al, 2001). Very recently Sec61 channel has also been demonstrated to be a membrane receptor for the proteasome (Kalies et al, 2005). Recently, another mechanism for dislocation has been described, comprising of an ER membrane protein Derlin-1, which dislocates MHC class I HC in conjunction with p97, Ufd1, and Np14 (Lilley and Ploegh, 2004;Ye et al, 2004).…”