The Proteasome Ubiquitin Receptor hRpn13 and Its Interacting Deubiquitinating Enzyme Uch37 Are Required for Proper Cell Cycle Progression.

Recently, we reported that bisbenzylidine piperidone RA190 adducts to Cys-88 of the proteasome ubiquitin receptor hRpn13, triggering accumulation of ubiquitinated proteins and endoplasmic reticulum stress-related apoptosis in various cancer cell lines. hRpn13 contains an N-terminal pleckstrin-like receptor for ubiquitin domain that binds ubiquitin and docks it into the proteasome as well as a C-terminal deubiquitinase adaptor (DEUBAD) domain that binds the deubiquitinating enzyme Uch37. Here we report that hRpn13 and Uch37 are required for proper cell cycle progression and that their protein knockdown leads to stalling at G0/G1 Moreover, serum-starved cells display reduced hRpn13 and Uch37 protein levels with hallmarks of G0/G1 stalling and recovery to their steady-state protein levels following release from nutrient deprivation. Interestingly, loss of hRpn13 correlates with a small but statistically significant reduction in Uch37 protein levels, suggesting that hRpn13 interaction may stabilize this deubiquitinating enzyme in human cells. We also find that RA190 treatment leads to a loss of S phase, suggesting a block of DNA replication, and G2 arrest by using fluorescence-activated cell sorting. Uch37 deprivation further indicated a reduction of DNA replication and G0/G1 stalling. Overall, this work implicates hRpn13 and Uch37 in cell cycle progression, providing a rationale for their function in cellular proliferation and for the apoptotic effect of the hRpn13-targeting molecule RA190.

Structure of the s5a:k48-linked diubiquitin complex and its interactions with rpn13.

Degradation by the proteasome typically requires substrate ubiquitination. Two ubiquitin receptors exist in the proteasome, S5a/Rpn10 and Rpn13. Whereas Rpn13 has only one ubiquitin-binding surface, S5a binds ubiquitin with two independent ubiquitin-interacting motifs (UIMs). Here, we use nuclear magnetic resonance (NMR) and analytical ultracentrifugation to define at atomic level resolution how S5a binds K48-linked diubiquitin, in which K48 of one ubiquitin subunit (the "proximal" one) is covalently bonded to G76 of the other (the "distal" subunit). We demonstrate that S5a's UIMs bind the two subunits simultaneously with a preference for UIM2 binding to the proximal subunit while UIM1 binds to the distal one. In addition, NMR experiments reveal that Rpn13 and S5a bind K48-linked diubiquitin simultaneously with subunit specificity, and a model structure of S5a and Rpn13 bound to K48-linked polyubiquitin is provided. Altogether, our data demonstrate that S5a is highly adaptive and cooperative toward binding ubiquitin chains.

Mycobacterium tuberculosis copper-regulated protein SocB is an intrinsically disordered protein that folds upon interaction with a synthetic phospholipid bilayer.

Multiple genes in Mycobacterium tuberculosis (Mtb) are regulated by copper including socAB (small orf induced by copper A and B), which is induced by copper and repressed by RicR (regulated in copper repressor). socA and socB encode hypothetical proteins of 61 and 54 amino acids, respectively. Here, we use biophysical and computational methods to evaluate the SocB structure. We find that SocB lacks evidence for secondary structure, with no thermal cooperative unfolding event, according to circular dichroism measurements. 2D NMR spectra similarly exhibit hallmarks of a disordered structural state, which is also supported by analyzing SocB diffusion. Altogether, these findings suggest that by itself SocB is intrinsically disordered. Interestingly, SocB interacts with a synthetic phospholipid bilayer and becomes helical, which suggests that it may be membrane-associated.

Proteasome subunit Rpn13 is a novel ubiquitin receptor.

Proteasomal receptors that recognize ubiquitin chains attached to substrates are key mediators of selective protein degradation in eukaryotes. Here we report the identification of a new ubiquitin receptor, Rpn13/ARM1, a known component of the proteasome. Rpn13 binds ubiquitin through a conserved amino-terminal region termed the pleckstrin-like receptor for ubiquitin (Pru) domain, which binds K48-linked diubiquitin with an affinity of approximately 90 nM. Like proteasomal ubiquitin receptor Rpn10/S5a, Rpn13 also binds ubiquitin-like (UBL) domains of UBL-ubiquitin-associated (UBA) proteins. In yeast, a synthetic phenotype results when specific mutations of the ubiquitin binding sites of Rpn10 and Rpn13 are combined, indicating functional linkage between these ubiquitin receptors. Because Rpn13 is also the proteasomal receptor for Uch37, a deubiquitinating enzyme, our findings suggest a coupling of chain recognition and disassembly at the proteasome.

Ubiquitin docking at the proteasome through a novel pleckstrin-homology domain interaction.

Targeted protein degradation is largely performed by the ubiquitin-proteasome pathway, in which substrate proteins are marked by covalently attached ubiquitin chains that mediate recognition by the proteasome. It is currently unclear how the proteasome recognizes its substrates, as the only established ubiquitin receptor intrinsic to the proteasome is Rpn10/S5a (ref. 1), which is not essential for ubiquitin-mediated protein degradation in budding yeast. In the accompanying manuscript we report that Rpn13 (refs 3-7), a component of the nine-subunit proteasome base, functions as a ubiquitin receptor, complementing its known role in docking de-ubiquitinating enzyme Uch37/UCHL5 (refs 4-6) to the proteasome. Here we merge crystallography and NMR data to describe the ubiquitin-binding mechanism of Rpn13. We determine the structure of Rpn13 alone and complexed with ubiquitin. The co-complex reveals a novel ubiquitin-binding mode in which loops rather than secondary structural elements are used to capture ubiquitin. Further support for the role of Rpn13 as a proteasomal ubiquitin receptor is demonstrated by its ability to bind ubiquitin and proteasome subunit Rpn2/S1 simultaneously. Finally, we provide a model structure of Rpn13 complexed to diubiquitin, which provides insights into how Rpn13 as a ubiquitin receptor is coupled to substrate deubiquitination by Uch37.

MTMDAT-HADDOCK: high-throughput, protein complex structure modeling based on limited proteolysis and mass spectrometry.

BACKGROUND: MTMDAT is a program designed to facilitate analysis of mass spectrometry data of proteins and biomolecular complexes that are probed structurally by limited proteolysis. This approach can provide information about stable fragments of multidomain proteins, yield tertiary and quaternary structure data, and help determine the origin of stability changes at the amino acid residue level. Here, we introduce a pipeline between MTMDAT and HADDOCK, that facilitates protein-protein complex structure probing in a high-throughput and highly automated fashion. RESULTS: A new feature of MTMDAT allows for the direct identification of residues that are involved in complex formation by comparing the mass spectra of bound and unbound proteins after proteolysis. If 3D structures of the unbound components are available, this data can be used to define restraints for data-driven docking to calculate a model of the complex. We describe here a new implementation of MTMDAT, which includes a pipeline to the data-driven docking program HADDOCK, thus streamlining the entire procedure. This addition, together with usability improvements in MTMDAT, enables high-throughput modeling of protein complexes from mass spectrometry data. The algorithm has been validated by using the protein-protein interaction between the ubiquitin-binding domain of proteasome component Rpn13 and ubiquitin. The resulting structural model, based on restraints extracted by MTMDAT from limited proteolysis and modeled by HADDOCK, was compared to the published NMR structure, which relied on twelve unambiguous intermolecular NOE interactions. The MTMDAT-HADDOCK structure was of similar quality to structures generated using only chemical shift perturbation data derived by NMR titration experiments. CONCLUSIONS: The new MTMDAT-HADDOCK pipeline enables direct high-throughput modeling of protein complexes from mass spectrometry data. MTMDAT-HADDOCK can be downloaded from http://www.ifm.liu.se/chemistry/molbiotech/maria_sunnerhagens_group/mtmdat/together with the manual and example files. The program is free for academic/non-commercial purposes.

Structure of the Rpn13-Rpn2 complex provides insights for Rpn13 and Uch37 as anticancer targets.

Proteasome-ubiquitin receptor hRpn13/Adrm1 binds and activates deubiquitinating enzyme Uch37/UCHL5 and is targeted by bis-benzylidine piperidone RA190, which restricts cancer growth in mice xenografts. Here, we solve the structure of hRpn13 with a segment of hRpn2 that serves as its proteasome docking site; a proline-rich C-terminal hRpn2 extension stretches across a narrow canyon of the ubiquitin-binding hRpn13 Pru domain blocking an RA190-binding surface. Biophysical analyses in combination with cell-based assays indicate that hRpn13 binds preferentially to hRpn2 and proteasomes over RA190. hRpn13 also exists outside of proteasomes where it may be RA190 sensitive. RA190 does not affect hRpn13 interaction with Uch37, but rather directly binds and inactivates Uch37. hRpn13 deletion from HCT116 cells abrogates RA190-induced accumulation of substrates at proteasomes. We propose that RA190 targets hRpn13 and Uch37 through parallel mechanisms and at proteasomes, RA190-inactivated Uch37 cannot disassemble hRpn13-bound ubiquitin chains.

Structures of Rpn1 T1:Rad23 and hRpn13:hPLIC2 Reveal Distinct Binding Mechanisms between Substrate Receptors and Shuttle Factors of the Proteasome.

Three receptors (Rpn1/S2/PSMD2, Rpn10/S5a, Rpn13/Adrm1) in the proteasome bind substrates by interacting with conjugated ubiquitin chains and/or shuttle factors (Rad23/HR23, Dsk2/PLIC/ubiquilin, Ddi1) that carry ubiquitinated substrates to proteasomes. We solved the structure of two such receptors with their preferred shuttle factor, namely hRpn13(Pru):hPLIC2(UBL) and scRpn1 T1:scRad23(UBL). We find that ubiquitin folds in Rad23 and Dsk2 are fine-tuned by residue substitutions to achieve high affinity for Rpn1 and Rpn13, respectively. A single substitution in hPLIC2 yields enhanced interactions with the Rpn13 ubiquitin contact surface and sterically blocks hRpn13 binding to its preferred ubiquitin chain type, K48-linked chains. Rpn1 T1 binds two ubiquitins in tandem and we find that Rad23 binds exclusively to the higher-affinity Helix28/Helix30 site. Rad23 contacts at Helix28/Helix30 are optimized compared to ubiquitin by multiple conservative amino acid substitutions. Thus, shuttle factors deliver substrates to proteasomes through fine-tuned ubiquitin-like surfaces.

Ubiquitin and its binding domains.

Post-translational modification by ubiquitin (ubiquitination, ubiquitylation, ubiquitinylation) is used as a robust signaling mechanism in a variety of processes that are essential for cell homeostasis. Its signaling specificity is conferred by the inherent dynamics of ubiquitin, the multivalency of ubiquitin chains, and its subcellular context, often defined by ubiquitin receptors and the substrate. Greater than 150 ubiquitin receptors have been found and their ubiquitin-binding domains (UBDs) are structurally diverse and include alpha-helical motifs, zinc fingers (ZnF), pleckstrin-homology (PH) domains, ubiquitin conjugating (Ubc)-related structures and src homology 3 (SH3) domains. New UBD structural motifs continue to be identified expanding the ubiquitin-signaling map to proteins and structural families not previously associated with ubiquitin trafficking. In this manuscript, we highlight several ubiquitin receptors from the multiple UBD folds with a focus on the structural characteristics of their interaction with ubiquitin.