Ubiquitin Ligases of the N-End Rule Pathway: Assessment of Mutations in UBR1 That Cause the Johanson-Blizzard Syndrome

Background: Johanson-Blizzard syndrome (JBS; OMIM 243800) is an autosomal recessive disorder that includes congenital exocrine pancreatic insufficiency, facial dysmorphism with the characteristic nasal wing hypoplasia, multiple malformations, and frequent mental retardation. Our previous work has shown that JBS is caused by mutations in human UBR1, which encodes one of the E3 ubiquitin ligases of the N-end rule pathway. The N-end rule relates the regulation of the in vivo half-life of a protein to the identity of its N-terminal residue. One class of degradation signals (degrons) recognized by UBR1 are destabilizing N-terminal residues of protein substrates. Methodology/Principal Findings: Most JBS-causing alterations of UBR1 are nonsense, frameshift or splice-site mutations that abolish UBR1 activity. We report here missense mutations of human UBR1 in patients with milder variants of JBS. These single-residue changes, including a previously reported missense mutation, involve positions in the RING-H2 and UBR domains of UBR1 that are conserved among eukaryotes. Taking advantage of this conservation, we constructed alleles of the yeast Saccharomyces cerevisiae UBR1 that were counterparts of missense JBS-UBR1 alleles. Among these yeast Ubr1 mutants, one of them (H160R) was inactive in yeast-based activity assays, the other one (Q1224E) had a detectable but weak activity, and the third one (V146L) exhibited a decreased but significant activity, in agreement with manifestations of JBS in the corresponding JBS patients. Conclusions/Significance: These results, made possible by modeling defects of a human ubiquitin ligase in its yeast counterpart, verified and confirmed the relevance of specific missense UBR1 alleles to JBS, and suggested that a residual activity of a missense allele is causally associated with milder variants of JBS

The mammalian Arg/N-end rule pathway and missense mutations in human <i>UBR1</i> that underlie specific cases of the Johanson-Blizzard syndrome (JBS).

<p>(A) The mammalian N-end rule pathway. N-terminal residues are indicated by single-letter abbreviations for amino acids. Yellow ovals denote the rest of a protein substrate. ‘Primary’, ‘secondary’ and ‘tertiary’ denote mechanistically distinct subsets of destabilizing N-terminal residues (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#s1" target="_blank">Introduction</a>). C* denotes oxidized Cys, either Cys-sulfinate or Cys-sulfonate. MetAPs, Met-aminopeptidases. (B) Single-residue mutations in the UBR1 proteins of JBS patients #1 and #2. The positions of mutant residues are indicated both for the original mutations in human UBR1 and for their mimics in <i>S. cerevisiae</i>. (C) Same as in B but the mutation in UBR1 of patient #3 (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#s2" target="_blank">Results</a>).</p

Clinical features in JBS patients.

a<p>Ref. 1;</p>b<p>Ref. 6 and unpublished data.</p

Functional activity of yeast Ubr1 mimics of missense JBS-UBR1 mutants.

<p>(A) Relative enzymatic activity of βgal in extracts from <i>S. cerevisiae</i> JD55 (<i>ubr1Δ</i>) that expressed His-βgal or Tyr-βgal, and also carried an empty vector, or an otherwise identical plasmid expressing wild-type <i>S. cerevisiae</i> Ubr1, or (separately) its three missense mutants Ubr1^V146L, Ubr1^H160R, or Ubr1^Q1224E. The activity of βgal was measured in triplicates, with standard deviations shown. (B) Relative levels of induction of the peptide transporter Ptr2 were assayed by measuring the activity of a plasmid-borne <i>lacZ</i> (βgal-encoding) reporter that was expressed from the P<i>_PTR2</i> promoter in <i>ubr1Δ S. cerevisiae</i> that carried either an empty vector or otherwise identical plasmids that expressed either wild-type Ubr1 <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Hwang1" target="_blank">[27]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Xia1" target="_blank">[28]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Turner1" target="_blank">[52]</a> or its indicated mutants. Cells were grown to A₆₀₀ of ∼0.8 in SC(-Ura, -Leu) medium at 30°C, followed by measurements, in triplicate, of βgal activity in cell extracts, with standard deviations shown. (C) The lysine-requiring JD55 (<i>ubr1Δ</i>) <i>S. cerevisiae</i> strain was grown on plates containing 110 µM lysine (Lys) or 66 µM Lys-Ala dipeptide as the sole source of Lys in the medium <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Hwang1" target="_blank">[27]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Hwang3" target="_blank">[33]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Turner1" target="_blank">[52]</a>. JD52 (<i>ubr1Δ</i>) cells carried a vector plasmid or otherwise identical plasmids expressing wild-type Ubr1 or its missense mutants Ubr1^H160R, Ubr1^V146L and Ubr1^Q1224E. Cells were grown to A₆₀₀ of ∼1 in SC(-Leu) medium at 30°C, washed in sterile water, serially diluted 5-fold, spotted on SC(-Leu, -Lys) plates containing 110 µM Lys or 66 µM Lys-Ala, and incubated at 30°C for 3 days. (D) Cell extracts (equal total protein levels) from experiments described in panels A and B were subjected to SDS-PAGE, followed by immunoblotting with affinity-purified anti-Ubr1 antibody (upper panel) and anti-tubulin antibody (a loading control; lower panel). Asterisk indicates a protein that crossreacts with anti-Ubr1 antibody. (E) Extracts from human lymphocytes (equal amounts of total protein) were subjected to SDS-PAGE, followed by immunoblotting with antibody to human UBR1 (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#s4" target="_blank">Materials and Methods</a>). Lane 1, wild-type lymphocytes. Lane 2, same as lane 1 but from lymphocytes of patient #2 (see the main text and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone-0024925-g001" target="_blank">Figs. 1</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone-0024925-g002" target="_blank">2</a>). Lane 3, same as lane 1 but with lymphocytes from patient #3. Lane 4, same as lane 1, but with lymphocytes from a JBS patient with a homozygous nonsense mutation in <i>UBR1</i>, previously shown to have no detectable UBR1 (null UBR1 control) <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Varshavsky3" target="_blank">[17]</a>. Lane 5, same as a lane 1.</p

<i>S. cerevisiae</i> Ubr1 N-recognin.

<p>(A) A diagram of the 225 kDa <i>S. cerevisiae</i> Ubr1. The indicated evolutionarily conserved regions of Ubr1 are the UBR box, the BRR (basic residues-rich) domain, the Cys/His-rich RING-H2 domain, and the AI (<u>a</u>uto<u>in</u>hibitory) domain <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Tasaki1" target="_blank">[18]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Xie1" target="_blank">[30]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Hwang3" target="_blank">[33]</a>. Three missense mutations in patients #1-3 of the present work are indicated as well (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone-0024925-g001" target="_blank">Fig. 1B, C</a>). (B) Ribbon diagram of the <i>S. cerevisiae</i> UBR domain <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Choi1" target="_blank">[48]</a> in a complex with RLGES, the N-terminal region of the separase-produced fragment of Scc1, a subunit of cohesin <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Rao1" target="_blank">[75]</a>. The bound RLGES peptide is shown as a stick model, with carbon atoms colored yellow. Several residues are marked with a black sphere and numbered to facilitate the tracing of the polypeptide chain. The names of residues of the RLGES peptide are in red, with the letter ‘s’ (substrate) appended to their position numbers. Side-chains of residues in the UBR domain that are present near JBS mutations (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone-0024925-g001" target="_blank">Fig. 1B, C</a>) are shown in a stick form, with carbon atoms colored green. Three coordinated zinc ions of the UBR domain <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone.0024925-Choi1" target="_blank">[48]</a> are shown as red spheres. (C) Close-up view of the UBR region near the V146L mutation (patient #1; <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone-0024925-g001" target="_blank">Fig. 1B</a>). In panel B, this region of UBR is boxed and labeled as ‘C’. The residues of UBR that accommodate the position-2 Leu residue (‘Leu2s’) of the RLGES peptide substrate are shown and labeled. The van der Waals sphere of the mutant Leu residue, in the UBR1^V146L mutant, is shown as purple dots. (D) Close-up view of the UBR region near the H160R mutation (patient #2, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0024925#pone-0024925-g001" target="_blank">Fig. 1B</a>). In panel B, this region of UBR is boxed and labeled as ‘D’. The residues of UBR coordinating Zn3 atom are shown and labeled. The van der Waals sphere of the mutant Arg residue, in the UBR1^H160R mutant, is shown as purple dots. The views in (C) and (D) are oriented to maximize visibility of mutation-proximal residues.</p