AslB (AtsB) is a 411-residue radical S-adenosyl-L-methionine (SAM) protein of the anaerobic sulfatase-maturating enzyme (anSME) family, encoded next to aslA, a gene for a putative periplasmic serine-type sulfatase. It has a radical SAM core with the CxxxCxxC motif that binds the SAM-cleaving [4Fe-4S] cluster, followed by a C-terminal SPASM domain whose cysteines are predicted to hold two auxiliary [4Fe-4S] clusters; all of these cluster ligands, the SAM-contact residues and the catalytic aspartate of the structurally characterised Clostridium perfringens enzyme are conserved. Characterised anSMEs oxidise the active-site cysteine or serine of sulfatases to C-alpha-formylglycine, the residue sulfatases need for catalysis. For AslB this role has not been established. Deleting aslB, together with ydeM and yidF, does not prevent E. coli K-12 from maturing heterologous cysteine-type sulfatases, which it does only under aerobic conditions by an unidentified oxygen-dependent system. Overproduced AslB modestly raised the activity of two human sulfatases co-expressed in E. coli B, but formylglycine formation was not shown. Whether AslB matures its neighbour AslA, a serine-type substrate that was never tested, or acts on some other protein is unknown, and no reaction of the purified protein has been reported.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0003824 catalytic activity | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: Root-level catalytic activity from the radical SAM InterPro signature. AslB has the intact radical SAM and SPASM machinery of the anSME family (file:ECOLI/aslB/aslB-bioinformatics/RESULTS.md), but no reaction has been demonstrated for the E. coli protein. Reason: Per the RADICAL_SAM project's treatment rule for generic rows: where no reaction is known, GO:0003824 is already as precise as the evidence allows. It is not false, since the protein belongs to a catalytic superfamily and keeps every catalytic residue of the reference enzyme, but there is no specific descendant that evidence for AslB supports, and its in vivo sulfatase-maturation role was not confirmed (PMID:17303125). Supporting Evidence: file:ECOLI/aslB/aslB-bioinformatics/RESULTS.md Neither E. coli paralog shows any sign of degeneration of the catalytic machinery file:ECOLI/aslB/aslB-uniprot.txt Belongs to the radical SAM superfamily. Anaerobic sulfatase-maturating enzyme family. |
| GO:0016491 oxidoreductase activity | IEA GO_REF:0000002 | UNDECIDED | Summary: Oxidoreductase activity mapped from the sulfatase-maturase radical SAM signature (IPR023867). Characterised family members are oxidoreductases: they oxidise a sulfatase active-site cysteine or serine to formylglycine (PMID:16766528, PMID:14749327). AslB keeps the full catalytic machinery, so the same class of chemistry is plausible, but no oxidation by AslB, and no substrate, has been shown. Reason: A family-level prediction of reaction class. The substrate-defining function of the family (sulfatase maturation) was not confirmed for AslB in K-12 deletion mutants (PMID:17303125), and the one positive report measures only sulfatase activity after overexpression in E. coli B (PMID:28882567). Without an assay of AslB itself the reaction class cannot be confirmed or excluded, so this row is neither accepted nor removed. Supporting Evidence: PMID:14749327 AtsB oxidizes serine to FGly by a radical mechanism that is initiated through reductive cleavage of SAM PMID:17303125 these putative anSME are not involved in Cys-type sulfatase maturation |
| GO:0051536 iron-sulfur cluster binding | IEA GO_REF:0000002 | MODIFY | Summary: Generic iron-sulfur cluster binding from the radical SAM signature. The clusters predicted for AslB are specifically [4Fe-4S] clusters: one in the radical SAM core and two in the SPASM domain, with all eleven cysteine ligands of the reference enzyme conserved (file:ECOLI/aslB/aslB-bioinformatics/RESULTS.md). Reason: Correct but general. Per the RADICAL_SAM project's treatment rule, replace with the descendant GO:0051539 4 iron, 4 sulfur cluster binding, which the gene already carries. Proposed replacements: 4 iron, 4 sulfur cluster binding Supporting Evidence: file:ECOLI/aslB/aslB-uniprot.txt Binds 3 [4Fe-4S] clusters (By similarity). |
| GO:0051539 4 iron, 4 sulfur cluster binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: [4Fe-4S] cluster binding from the anSME-type SPASM domain signature. Supported by conservation of the radical SAM CxxxCxxC motif (CNLNCDYC, residues 21 to 28) and of all eight SPASM cysteines that ligate the two auxiliary clusters in the C. perfringens enzyme (file:ECOLI/aslB/aslB-bioinformatics/RESULTS.md). Cluster binding has not been measured on AslB itself. Reason: True and as precise as the evidence allows. Per the RADICAL_SAM project's treatment rule, GO:0051539 is core only where catalysis is established; no reaction of AslB is known, so the clusters cannot yet be called the cofactors of a demonstrated activity. Supporting Evidence: file:ECOLI/aslB/aslB-bioinformatics/RESULTS.md all 11 cysteine ligands of the three [4Fe-4S] file:ECOLI/aslB/aslB-uniprot.txt Binds 3 [4Fe-4S] clusters (By similarity). |
| GO:0051604 protein maturation | ISS PMID:16766528 A new type of bacterial sulfatase reveals a novel maturation... | UNDECIDED | Summary: Sequence-similarity transfer of protein maturation from the C. perfringens Cys-type anSME, the founding member of the family (PMID:16766528). For E. coli AslB the evidence conflicts. In K-12, deleting aslB, ydeM and yidF did not impair maturation of heterologous Cys-type sulfatases, which occurred only aerobically (PMID:17303125). In E. coli BL21(DE3), overproduced AslB raised the activity of co-expressed human IDS 4.6-fold and of GALNS 1.5 to 2-fold (PMID:28882567), but formylglycine was not measured in that abstract. The natural candidate substrate, the adjacent Ser-type putative sulfatase AslA, has never been tested. Reason: The cited paper does not assay E. coli AslB, the only in vivo test in K-12 was negative for Cys-type substrates, and the positive report is an overexpression activity readout in a different strain. Ser-type maturation, the activity of the most similar characterised enzyme (Klebsiella AtsB, 44.7% identical), remains untested. The evidence neither establishes nor refutes a protein-maturation role, so the row is left undecided rather than removed. Supporting Evidence: PMID:17303125 knocking-out of aslB, ydeM and yidF does not impair Cys-type sulfatase maturation PMID:28882567 IDS activity was increased in 4.6-fold when co-expressed with AslB file:ECOLI/aslB/aslB-uniprot.txt it is not involved in Cys-type sulfatase maturation in vivo |
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Download this section (compressed HTML)Q: Does E. coli AslB convert Ser136 of its neighbour AslA to formylglycine under anaerobic conditions, and is aslA ever expressed in an active form?
Suggested experts: Olivier Berteau, Thomas Dierks, Squire J Booker
Q: Is the increased human sulfatase activity seen when AslB is overproduced in E. coli B due to formylglycine formation by AslB, or to an indirect effect on the endogenous oxygen-dependent maturation system?
Suggested experts: Olivier Berteau
Experiment: Express AslA (wild type and S136A) in wild-type and aslB-deleted E. coli K-12 under anaerobic, sulfate-limited conditions; detect formylglycine at position 136 by mass spectrometry and measure arylsulfatase activity. In vitro, assay reconstituted AslB with an AslA peptide spanning the (C/S)XPXR motif for SAM-dependent formylglycine and 5'-deoxyadenosine formation.
Hypothesis: AslB is a serine-type anaerobic sulfatase-maturating enzyme for AslA.
Type: genetic and in vitro enzymology
Experiment: Compare the proteome and metabolome of wild-type and aslB-deleted cells grown anaerobically, and search purified, reconstituted AslB for SAM cleavage stimulated by candidate peptides from proteins whose genes co-occur with aslB across enterobacteria.
Hypothesis: AslB acts on a non-sulfatase substrate, as other SPASM-domain enzymes do.
Type: comparative omics and enzymology
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The substrate and reaction of AslB are unknown: it is not established whether it converts the active-site serine of its neighbour AslA, or of any other protein, to formylglycine, or whether it acts on a different substrate altogether.
OPEN BIOLOGY MF_DARK
What is known: Established: AslB keeps all cluster-ligand cysteines, SAM-contact residues and the catalytic aspartate of the structurally characterised C. perfringens anSME, and is most similar to the Ser-type maturase AtsB. Deleting aslB, ydeM and yidF does not block maturation of heterologous Cys-type sulfatases in E. coli K-12. Ser-type maturation and the activity of purified AslB have not been tested.
Significance: E. coli K-12 carries two intact anSME-family enzymes (AslB and YdeM) but has no detectable sulfatase activity; whether AslB is a dormant sulfatase maturase or has been repurposed decides how the whole anSME-like SPASM group in enterobacteria should be annotated.
What would resolve it: Reconstitute purified AslB anaerobically and assay formylglycine formation on AslA peptides containing Ser136, measuring 5'-deoxyadenosine release; in parallel, test whether aslB deletion abolishes formylglycine in AslA expressed under anaerobic, sulfur-limited conditions.
Provenance (the field's own admissions):
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