Genome-wide map of nuclear protein degradation shows NCoR1 turnover as a key to mitochondrial gene regulation.

Transcription factor activity and turnover are functionally linked, but the global patterns by which DNA-bound regulators are eliminated remain poorly understood. We established an assay to define the chromosomal location of DNA-associated proteins that are slated for degradation by the ubiquitin-proteasome system. The genome-wide map described here ties proteolysis in mammalian cells to active enhancers and to promoters of specific gene families. Nuclear-encoded mitochondrial genes in particular correlate with protein elimination, which positively affects their transcription. We show that the nuclear receptor corepressor NCoR1 is a key target of proteolysis and physically interacts with the transcription factor CREB. Proteasome inhibition stabilizes NCoR1 in a site-specific manner and restrains mitochondrial activity by repressing CREB-sensitive genes. In conclusion, this functional map of nuclear proteolysis links chromatin architecture with local protein stability and identifies proteolytic derepression as highly dynamic in regulating the transcription of genes involved in energy metabolism.

The mammalian tRNA ligase complex mediates splicing of XBP1 mRNA and controls antibody secretion in plasma cells.

The unfolded protein response (UPR) is a conserved stress-signaling pathway activated after accumulation of unfolded proteins within the endoplasmic reticulum (ER). Active UPR signaling leads to unconventional, enzymatic splicing of XBP1 mRNA enabling expression of the transcription factor XBP1s to control ER homeostasis. While IRE1 has been identified as the endoribonuclease required for cleavage of this mRNA, the corresponding ligase in mammalian cells has remained elusive. Here, we report that RTCB, the catalytic subunit of the tRNA ligase complex, and its co-factor archease mediate XBP1 mRNA splicing both in vitro and in vivo. Depletion of RTCB in plasma cells of Rtcb(fl/fl) Cd23-Cre mice prevents XBP1s expression, which normally is strongly induced during plasma cell development. RTCB-depleted plasma cells show reduced and disorganized ER structures as well as severe defects in antibody secretion. Targeting RTCB and/or archease thus represents a promising strategy for the treatment of a growing number of diseases associated with elevated expression of XBP1s.

Glycan modulation and sulfoengineering of anti-HIV-1 monoclonal antibody PG9 in plants.

Broadly neutralizing anti-HIV-1 monoclonal antibodies, such as PG9, and its derivative RSH hold great promise in AIDS therapy and prevention. An important feature related to the exceptional efficacy of PG9 and RSH is the presence of sulfated tyrosine residues in their antigen-binding regions. To maximize antibody functionalities, we have now produced glycan-optimized, fucose-free versions of PG9 and RSH in Nicotiana benthamiana. Both antibodies were efficiently sulfated in planta on coexpression of an engineered human tyrosylprotein sulfotransferase, resulting in antigen-binding and virus neutralization activities equivalent to PG9 synthesized by mammalian cells ((CHO)PG9). Based on the controlled production of both sulfated and nonsulfated variants in plants, we could unequivocally prove that tyrosine sulfation is critical for the potency of PG9 and RSH. Moreover, the fucose-free antibodies generated in N. benthamiana are capable of inducing antibody-dependent cellular cytotoxicity, an activity not observed for (CHO)PG9. Thus, tailoring of the antigen-binding site combined with glycan modulation and sulfoengineering yielded plant-produced anti-HIV-1 antibodies with effector functions superior to PG9 made in CHO cells.

The human anti-HIV antibodies 2F5, 2G12, and PG9 differ in their susceptibility to proteolytic degradation: down-regulation of endogenous serine and cysteine proteinase activities could improve antibody production in plant-based expression platforms.

The tobacco-related species Nicotiana benthamiana has recently emerged as a promising host for the manufacturing of protein therapeutics. However, the production of recombinant proteins in N. benthamiana is frequently hampered by undesired proteolysis. Here, we show that the expression of the human anti-HIV antibodies 2F5, 2G12, and PG9 in N. benthamiana leaves leads to the accumulation of discrete heavy chain-derived degradation products of 30-40 kDa. Incubation of purified 2F5 with N. benthamiana intercellular fluid resulted in rapid conversion into the 40-kDa fragment, whereas 2G12 proved largely resistant to degradation. Such a differential susceptibility to proteolytic attack was also observed when these two antibodies were exposed to various types of proteinases in vitro. While serine and cysteine proteinases are both capable of generating the 40-kDa 2F5 fragment, the 30-kDa polypeptide is most readily obtained by treatment with the latter class of enzymes. The principal cleavage sites reside within the antigen-binding domain, the VH -CH 1 linker segment and the hinge region of the antibodies. Collectively, these results indicate that down-regulation of endogenous serine and cysteine proteinase activities could be used to improve the performance of plant-based expression platforms destined for the production of biopharmaceuticals.