Nepl19 (CG14523) is a neprilysin-like protein of the M13 metallopeptidase family in Drosophila melanogaster. Its gene is one of five tandemly duplicated Nepl genes (Nepl17-Nepl21) at cytological band 98F1, and it belongs to the insect-specific PANTHER subfamily PTHR11733:SF238. The 671-residue protein has an N-terminal signal peptide and no transmembrane anchor, so it is predicted to be secreted. It keeps the M13 N-terminal and C-terminal peptidase domains and the downstream ENIAD zinc-ligand motif. In the HExxH motif, however, both the first zinc-binding histidine and the catalytic glutamate are replaced by glutamine (QQLAH), so Nepl19 is predicted to be a non-catalytic pseudopeptidase. Its closest tandem paralogs keep an intact HExxH motif, which places the loss in the Nepl19 lineage. The gene is transcribed, but no experimental study of the protein exists. Any ligand, binding partner or physiological role is unknown. Binding and sequestering neuropeptides that active neprilysins would otherwise cleave has been proposed for secreted Nepl proteins in general, but this has not been tested for Nepl19.
Summary: Nepl19 descends from the PAINT node PTN000199753, whose ancestral M13 proteins are type II membrane ectoenzymes. Nepl19 itself has a cleavable signal peptide and no transmembrane segment, so it has diverged from that membrane-anchored ancestral state. It is predicted to be secreted.
Reason: The PAINT placement of a plasma-membrane location at PTN000199753 is sound for the ancestral membrane-anchored neprilysins (MME, ECE1, KEL, PHEX donors). Nepl19, however, belongs to the secreted class of Drosophila Nepl proteins. Its sequence has a SignalP signal peptide at residues 1-24 and no hydrophobic anchor, which is evidence that the inherited location has been lost in this lineage. The location supported for this protein is the extracellular region. No experiment has localized the protein, so the replacement remains a prediction and is not proposed as core.
Propagation Review
Root cause:PROPAGATION BAD
Failure modes:COMPARTMENT OR COMPLEX MISMATCH
Sources checked:
PANTHER:PTN000199753 · PTN000199753 SUPPORTS SOURCE BUT NOT TARGET
The donors at this node (including human MME, ECE1, KEL and PHEX, rodent orthologs and two fly M13 genes) are membrane-anchored ectoenzymes, so the plasma-membrane location fits the ancestor. Nepl19 is a descendant that lost the type II membrane anchor and has an N-terminal signal peptide instead.
The selected UniProt sequence has a SignalP-derived signal peptide at 1–24 and a mature chain at 25–671, with no annotated transmembrane segment. The mature C-terminal sequence ends in TSPQKCQLFAVNLD, lacking a hydrophobic membrane anchor.
two Nep and the majority (18) of Nepl proteins lack a predicted transmembrane domain and, instead, possess a predicted signal peptide suggesting that they are secreted
Summary: This electronic plasma-membrane location comes from an ARBA-predicted "single-pass type II membrane protein" subcellular-location line. The Nepl19 sequence contradicts that prediction, because it has an N-terminal signal peptide and no membrane anchor.
Reason: The UniProt subcellular-location statement behind this row rests only on an ARBA rule, which copies the type II membrane topology of active neprilysins. The Nepl19 sequence has a SignalP signal peptide and no transmembrane segment, and the Drosophila Nep/Nepl survey classifies most Nepl proteins as secreted. The row matches the IBA row and is modified to extracellular region in the same way.
The selected UniProt sequence has a SignalP-derived signal peptide at 1–24 and a mature chain at 25–671, with no annotated transmembrane segment. The mature C-terminal sequence ends in TSPQKCQLFAVNLD, lacking a hydrophobic membrane anchor.
two Nep and the majority (18) of Nepl proteins lack a predicted transmembrane domain and, instead, possess a predicted signal peptide suggesting that they are secreted
Summary: InterPro2GO assigns proteolysis to every protein that matches the M13 peptidase family and domain signatures. Nepl19 matches those signatures only because it keeps the M13 fold. It has lost the first zinc ligand and the catalytic glutamate of HExxH, and it carries a NOT peptidase activity annotation (IKR) that this review accepts.
Reason: Under GO, a gene product is involved in proteolysis only if it does some of the work, normally by hydrolysing the peptide bond. Here the term was inferred from a domain signature alone, and the catalytic residues that signature normally implies are missing (H584 and E585 of human neprilysin map to Q511 and Q512). Nepl19 has no known substrate, no known partner protease and no known regulatory activity that could support participation by another route. Keeping a positive proteolysis assertion next to the accepted NOT peptidase activity rows would contradict them. This is an electronic mapping that fails for a pseudoenzyme, so the row is removed. If a role in a neprilysin-regulated process is ever shown, for example by peptide sequestration, it would need its own experimental annotation.
The reference HEITH motif at 584–588 maps to QQLAH at Nepl19 511–515. Nepl19 retains the M13 fold and downstream ENIAD region but replaces the first zinc-binding histidine and catalytic glutamate with glutamines.
The 21 Nepl-genes encode proteins that exhibit significant similarity to neprilysins in their primary structure, but lack one or more of the motifs required for catalysis.
Summary: This NOT peptidase activity assertion is accepted. In the exact Q9VAS1 sequence, the first zinc-binding histidine and the catalytic glutamate of the HExxH motif are replaced by glutamines. Two independent alignment strategies place these substitutions consistently, and an active mouse neprilysin control keeps all of the sites.
Reason: The IKR rests on loss of key residues, and the exact-sequence alignments confirm that loss. The re-review subfamily alignment shows that Nepl19's closest tandem paralogs (Nepl17, Nepl18, Nepl20, Nepl21) keep HExxH, so the loss belongs to Nepl19 and is not an artefact of family-level transfer. Mutagenesis of neprilysin metal ligands is known to abolish catalysis. No experimental peptidase assay exists for Nepl19, but the residue evidence is strong and consistent with the published classification of Nepl proteins as motif-deficient.
The reference HEITH motif at 584–588 maps to QQLAH at Nepl19 511–515. Nepl19 retains the M13 fold and downstream ENIAD region but replaces the first zinc-binding histidine and catalytic glutamate with glutamines.
The 21 Nepl-genes encode proteins that exhibit significant similarity to neprilysins in their primary structure, but lack one or more of the motifs required for catalysis.
IKRNOT PMID:34189422 Identification and bioinformatic analysis of neprilysin and ...
ACCEPT
Summary: This NOT peptidase activity assertion is accepted. FlyBase made it from the Drosophila Nep/Nepl motif survey, which classifies Nepl19 among the Nepl genes that lack catalytic motifs. The survey's conclusion matches the exact-sequence finding that HExxH has become QQLAH.
Reason: The cited survey checked every fly M13 protein for the HExxH, ENIAD, CxxW and NAY/FY motifs and placed Nepl19 in the motif-deficient Nepl class. The independent alignment reproduces the missing zinc ligand and catalytic glutamate. This is the same claim as the GO_REF:0000047 IKR row, and both are accepted.
The 21 Nepl-genes encode proteins that exhibit significant similarity to neprilysins in their primary structure, but lack one or more of the motifs required for catalysis.
The reference HEITH motif at 584–588 maps to QQLAH at Nepl19 511–515. Nepl19 retains the M13 fold and downstream ENIAD region but replaces the first zinc-binding histidine and catalytic glutamate with glutamines.
Q: Does secreted Nepl19 bind neuropeptides that are cleaved by the active neprilysins (Nep2, Nep4) and thereby sequester or protect them? In which tissue would this happen?
Q: Did the HExxH loss in Nepl19, which is absent from its intact 98F1 tandem paralogs, follow a shift to a binding-only role, and is the motif loss conserved in Nepl19 orthologs of other Drosophila species?
Suggested Experiments
Experiment: Express epitope-tagged Nepl19 in S2 cells or in vivo and confirm that it is secreted and not retained at the membrane, for example by western blot of conditioned medium against cell lysate.
Experiment: Assay recombinant Nepl19 for hydrolysis of standard neprilysin peptide substrates, with Nep2 as a positive control and Nepl19 in which Q511 and Q512 are reverted to H and E. This would test whether the motif substitutions alone account for the predicted loss of activity.
Experiment: Make a Nepl19-specific loss-of-function allele, which is difficult because of the 98F1 tandem paralogs, and profile its phenotypes in tissues where Nepl19 is expressed, including the spermatheca.
Knowledge Gaps
What is not known — curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: It is not known whether Nepl19 binds any peptide or protein ligand, and its molecular activity is therefore undetermined. The protein is predicted to be non-catalytic, and no binding partner, ligand or activity has been identified.
OPENBIOLOGYWHOLLY_DARK
What is known: Nepl19 is a transcribed M13-family protein that is predicted to be secreted. It keeps the M13 fold and the ENIAD motif, but the HExxH zinc ligand and catalytic glutamate are replaced by glutamine, so it is predicted to lack peptidase activity.
What would resolve it: Express the secreted protein, test it for binding to neuropeptides that are known neprilysin substrates, and study loss-of-function phenotypes.
Provenance (the field's own admissions):
"One function of the secreted Nepl proteins may be to bind and sequester peptide targets of the catalytically active neprilysins." — PMID:34189422
External Prediction Reviews
These computational predictions are reviewed separately from the GOA annotation set used for this review. The assessments below are from this project and do not constitute official GO annotations or endorsement by GO/UniProt. They are not included in the existing annotation review above.
Review rationale: Conventional metalloendopeptidase activity is refuted by specific disruption of the M13 catalytic apparatus. Exact-sequence alignments map human H584/E585 to Nepl19 Q511/Q512, replacing a zinc ligand and catalytic glutamate; the downstream zinc-binding/catalytic region remains aligned. Two MAFFT strategies agree, and the active mouse control preserves all five sites. The published Nepl classification and existing NOT peptidase annotations agree with this independently reproduced sequence finding. The closest tandem paralogs in the 98F1 cluster keep HExxH, so the loss is specific to Nepl19 and family-level transfer of the catalytic term does not apply. The gene already carries accepted NOT peptidase activity annotations, which the predicted child term contradicts. This is pseudoenzyme overannotation, without a claim that the protein lacks a noncatalytic role.
Supporting Evidence:
file:DROME/Nepl19/Nepl19-bioinformatics/RESULTS.md: "The reference HEITH motif at 584–588 maps to QQLAH at Nepl19 511–515. Nepl19 retains the M13 fold and downstream ENIAD region but replaces the first zinc-binding histidine and catalytic glutamate with glutamines."
PMID:34189422: "The 21 Nepl-genes encode proteins that exhibit significant similarity to neprilysins in their primary structure, but lack one or more of the motifs required for catalysis."