Non-lysosomal degradation of misfolded human lysozymes with and without an asparagine-linked glycosylation site

UBR E3 ligases and the PDIA3 protease control degradation of unfolded antibody heavy chain by ERAD

Accumulation of unfolded antibody chains in the ER triggers ER stress that may lead to reduced productivity in therapeutic antibody manufacturing processes. We identified UBR4 and UBR5 as ubiquitin E3 ligases involved in HC ER-associated degradation. Knockdown of UBR4 and UBR5 resulted in intracellular accumulation, enhanced secretion, and reduced ubiquitination of HC. In concert with these E3 ligases, PDIA3 was shown to cleave ubiquitinated HC molecules to accelerate HC dislocation. Interestingly, UBR5, and to a lesser degree UBR4, were down-regulated as cellular demand for antibody expression increased in CHO cells during the production phase, or in plasma B cells. Reducing UBR4/UBR5 expression before the production phase increased antibody productivity in CHO cells, possibly by redirecting antibody molecules from degradation to secretion. Altogether we have characterized a novel proteolysis/proteasome-dependent pathway involved in degradation of unfolded antibody HC. Proteins characterized in this pathway may be novel targets for CHO cell engineering.

How early studies on secreted and membrane protein quality control gave rise to the ER associated degradation (ERAD) pathway: the early history of ERAD

All newly synthesized proteins are subject to quality control check-points, which prevent aberrant polypeptides from harming the cell. For proteins that ultimately reside in the cytoplasm, components that also reside in the cytoplasm were known for many years to mediate quality control. Early biochemical and genetic data indicated that misfolded proteins were selected by molecular chaperones and then targeted to the proteasome (in eukaryotes) or to proteasome-like particles (in bacteria) for degradation. What was less clear was how secreted and integral membrane proteins, which in eukaryotes enter the endoplasmic reticulum (ER), were subject to quality control decisions. In this review, we highlight early studies that ultimately led to the discovery that secreted and integral membrane proteins also utilize several components that constitute the cytoplasmic quality control machinery. This component of the cellular quality control pathway is known as ER associated degradation, or ERAD. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Regulation of N-linked core glycosylation: use of a site-directed mutagenesis approach to identify Asn-Xaa-Ser/Thr sequons that are poor oligosaccharide acceptors

N-linked glycosylation can profoundly affect protein expression and function. N-linked glycosylation usually occurs at the sequon Asn-Xaa-Ser/Thr, where Xaa is any amino acid residue except Pro. However, many Asn-Xaa-Ser/Thr sequons are glycosylated inefficiently or not at all for reasons that are poorly understood. We have used a site-directed mutagenesis approach to examine how the Xaa and hydroxy (Ser/Thr) amino acid residues in sequons influence core-glycosylation efficiency. We recently demonstrated that certain Xaa amino acids inhibit core glycosylation of the sequon, Asn37-Xaa-Ser, in rabies virus glycoprotein (RGP). Here we examine the impact of different Xaa residues on core-glycosylation efficiency when the Ser residue in this sequon is replaced with Thr. The core-glycosylation efficiencies of RGP variants with different Asn37-Xaa-Ser/Thr sequons were compared by using a cell-free translation/glycosylation system. Using this approach we confirm that four Asn-Xaa-Ser sequons are poor oligosaccharide acceptors: Asn-Trp-Ser, Asn-Asp-Ser, Asn-Glu-Ser and Asn-Leu-Ser. In contrast, Asn-Xaa-Thr sequons are efficiently glycosylated, even when Xaa=Trp, Asp, Glu or Leu. A comparison of the glycosylation status of Asn-Xaa-Ser and Asn-Xaa-Thr sequons in other glycoproteins confirms that sequons with Xaa=Trp, Asp, Glu or Leu are rarely glycosylated when Ser is the hydroxy amino acid residue, and that these sequons are unlikely to serve as glycosylation sites when introduced into proteins by site-directed mutagenesis.

Role of glycosylation and disulfide bond formation in the beta subunit in the folding and functional expression of Na,K-ATPase

Initial folding is a prerequisite for subunit assembly in oligomeric proteins. In this study, we have compared the role of co-translational modifications in the acquisition of an assembly-competent conformation of the beta subunit, the assembly of which is required for the structural and functional maturation of the catalytic Na,K-ATPase alpha subunit. Cysteine or asparagine residues implicated in disulfide bond formation or N-glycosylation, respectively, in the Xenopus beta1 subunit were eliminated by site-directed mutagenesis, and the assembly efficiency of the mutants and the functional expression of Na+,K+ pumps were studied after expression in Xenopus oocytes. Our results show that lack of each one of the two most C-terminal disulfide bonds indeed permits short term but completely abolishes long term assembly of the beta subunit. On the other hand, lack of the most N-terminal disulfide bonds allows the expression of a small number of functional Na+,K+ pumps at the cell surface. Elimination of all three but not of one or two glycosylation sites produces beta subunits that remain stably expressed in the endoplasmic reticulum, in association with binding protein but not as irreversible aggregates. The assembly efficiency of nonglycosylated beta subunits is decreased but a reduced number of functional Na+,K+ pumps is expressed at the cell surface. The lack of sugars does not influence the apparent K+ or ouabain affinity of the Na+,K+ pumps. Thus, these data show that disulfide bond formation and N-glycosylation may play important but qualitatively distinct roles in the initial folding of oligomeric protein subunits. Moreover, the results suggest that an endoplasmic reticulum degradation pathway exists, which is glycosylation-dependent.

N-Glycosylation affects endoplasmic reticulum degradation of a mutated derivative of carboxypeptidase yscY in yeast

The endoplasmic reticulum (ER) of eukaryotic cells contains a quality control system, that is required for the proteolytic removal of aberrantly folded proteins that accumulate in this organelle. We used genetic and biochemical methods to analyse the involvement of N-glycosylation in the degradation of a mutant derivative of carboxypeptidase yscY in the ER of the yeast Saccharomyces cerevisiae. Our results demonstrate that N-glycosylation of this protein is required for its degradation since an unglycosylated species is retained stably in the ER. Cells that were devoid of the ER-processing alpha 1,2-mannosidase showed reduced degradation of the glycosylated substrate protein. Disruption of CNE1, a gene encoding a putative yeast homologue for calnexin, did not exhibit any effects on the degradation of this substrate protein in vivo. Also, the alpha 1,2-mannosidase-dependent reduction in the degradation rate did not show any correlation with the function of the CNE1 gene product. Our results suggest that the ER of yeast contains a glycosylation-dependent quality control system, as has been shown for higher eukaryotic cells.

The amino acid at the X position of an Asn-X-Ser sequon is an important determinant of N-linked core-glycosylation efficiency

N-Linked glycosylation is a common form of protein processing that can profoundly affect protein expression, structure, and function. N-Linked glycosylation generally occurs at the sequon Asn-X-Ser/Thr, where X is any amino acid except Pro. To assess the impact of the X amino acid on core glycosylation, rabies virus glycoprotein variants were generated by site-directed mutagenesis with each of the 20 common amino acids substituted at the X position of an Asn-X-Ser sequon. The efficiency of core glycosylation at the sequon in each variant was quantified in a rabbit reticulocyte lysate cell-free translation system supplemented with canine pancreas microsomes. The presence of Pro at the X position completely blocked core glycosylation, whereas Trp, Asp, Chi, and Leu were associated with inefficient core glycosylation. The other variants were more efficiently glycosylated, and several were fully glycosylated. These findings demonstrate that the X amino acid is an important determinant of N-linked core-glycosylation efficiency.

A possible role of ER-60 protease in the degradation of misfolded proteins in the endoplasmic reticulum

Wild-type human lysozyme (hLZM) is secreted when expressed in mouse L cells, whereas misfolded mutant hLZMs are retained and eventually degraded in a pre-Golgi compartment (Omura, F., Otsu, M., Yoshimori, T., Tashiro, Y., and Kikuchi, M. (1992) Eur. J. Biochem. 210, 591-599). These misfolded mutant hLZMs are associated with protein disulfide isomerase (Otsu, M., Omura, F., Yoshimori, T., and Kikuchi, M. (1994) J. Biol. Chem. 269, 6874-6877). From the observation that this degradation is sensitive to cysteine protease inhibitors, such as N-acetyl-leucyl-leucyl-norleucinal and N-acetyl-leucyl-leucyl-methioninal, but not to the serine protease inhibitors, 1-chloro-3-tosylamido-7-amino-2-heptanone and (p-amidinophenyl)methanesulfonyl fluoride, it was suggested that some cysteine proteases are likely responsible for the degradation of abnormal proteins in the endoplasmic reticulum (ER). ER-60 protease (ER-60), an ER resident protein with cysteine protease activity (Urade, R., Nasu, M., Moriyama, T., Wada, K., and Kito, M. (1992) J. Biol. Chem. 267, 15152-15159), was found to associate with misfolded hLZMs, but not with the wild-type protein, in mouse L cells. Furthermore, denatured hLZM is degraded by ER-60 in vitro, whereas native hLZM is not. These results suggest that ER-60 could be a component of the proteolytic machinery for the degradation of misfolded mutant hLZMs in the ER.

The hydroxy amino acid in an Asn-X-Ser/Thr sequon can influence N-linked core glycosylation efficiency and the level of expression of a cell surface glycoprotein

N-Linked glycosylation usually occurs at the sequon, Asn-X-Ser/Thr. In this sequon, the side chain of the hydroxy amino acid (Ser or Thr) may play a direct catalytic role in the enzymatic transfer of core oligosaccharides to the Asn residue. Using recombinant variants of rabies virus glycoprotein (RGP), we examined the influence of the hydroxy amino acid on core glycosylation efficiency. A variant of RGP containing a single Asn-X-Ser sequon at Asn37 was modified by site-directed mutagenesis to change the sequon to either Asn-X-Cys or Asn-X-Thr. The impact of these changes on core glycosylation efficiency was assessed by expressing the variants in a cell-free transcription/translation/glycosylation system and in transfected tissue culture cells. Substitution of Cys at position 39 blocks glycosylation, whereas substitution of Thr dramatically increases core glycosylation efficiency of Asn37 in both membrane-anchored and secreted forms of RGP. The substitution of Thr for Ser also dramatically enhances the level of expression and cell surface delivery of RGP when the sequon at Asn37 is the only sequon in the protein. Novel forms of membrane-anchored and secreted RGP which are fully glycosylated at all three sequons were also generated by substitution of Thr at position 39.

Indication of possible post-translational formation of disulphide bonds in the beta-sheet domain of human lysozyme

Lysozyme has two distinct folding domains, and in most molecules the alpha-helical domain folds more quickly than the beta-sheet domain in vitro [Radford, Dobson and Evans (1992) Nature (London) 358, 302-307]. In order to investigate the relationship between the formation of disulphide bonds and protein folding in vivo, we carried out cysteine scanning mutagenesis to shift positions of the disulphide bonds in both the alpha-helical and beta-sheet domains of human lysozyme. Of the constructed mutants (nine in the beta-sheet domain and 13 in the alpha-helical domain), the mutant L79CC81A, in which Leu-79 and Cys-81 in the beta-sheet domain were replaced by Cys and Ala respectively, was secreted by yeast. The rest of the mutants were retained in the insoluble fraction of the cell, probably because of a failure of folding. The distance between the two alpha-carbons at positions 79 and 95 in the wild-type protein is too far to form a disulphide bond, but analysis of the primary structure revealed that the major part of L79CC81A was secreted with a non-native disulphide bond Cys79-Cys95 and two free cysteine residues at positions 65 and 77 in the beta-sheet domain. These results suggest that the beta-sheet domain of human lysozyme can tolerate the shift of locations of disulphide bonds, and the non-native folding of mutated polypeptide chains in in vivo folding. The free residues Cys-65 and Cys-77 formed a disulphide bond in vitro by air oxidation, yielding two isomers. On the basis of our previous results and present study it is suggested that the formation of Cys6-Cys128 is the first step of the in vivo correct folding of human lysozyme, and disulphide bonds in the beta-sheet domain are post-translationally formed in vivo.