AGBL1

Model summary

A soluble human metalloexopeptidase that trims glutamate residues from the C-termini of modified proteins. It uses an N-terminal armadillo-repeat scaffold to assemble with substrates and cofactors and a C-terminal zinc-dependent catalytic core to hydrolyze terminal glutamates. By editing polyglutamylation on cytosolic targets—including tubulin—it fine-tunes post-translational modification states that influence cytoskeletal behavior and other intracellular pathways. The enzyme operates primarily in the cytoplasm with regulated nucleolar residency, where assembly factors and modified substrates concentrate.

Predicted Gene Ontology terms

1. 01

   I begin with the InterPro architecture.

   The N-terminus (residues 1–266) carries two overlapping helical solenoid signatures: IPR011989 (Armadillo-like helical homologous_superfamily, residues 1–266) and IPR016024 (Armadillo-type fold homologous_superfamily, residues 8–267). Armadillo repeats form elongated, right-handed superhelices that create versatile protein–protein interaction scaffolds and often mediate subcellular targeting and assembly with regulatory partners. Midway through the sequence, a family-level envelope IPR050821 (Cytosolic carboxypeptidase family, residues 149–1073) spans from the Armadillo-rich N-terminus into the catalytic region, marking the protein as a cytosolic carboxypeptidase specialized for post-translational processing. Within the C-terminal half, IPR040626 (Cytosolic carboxypeptidase, N-terminal domain, residues 597–671) precedes the catalytic core and typically contributes to substrate positioning and metal coordination. The enzymatic heart is defined by IPR000834 (Peptidase M14, carboxypeptidase A domain, residues 732–1022), a zinc-dependent exopeptidase fold that removes residues from peptide/protein C-termini. Overlapping with this region, IPR033852 (Cytosolic aminopeptidase 1/4 domain, residues 754–1040) indicates a broader metallopeptidase scaffold that stabilizes the active site and supports exopeptidase chemistry. The ordered layout—N-terminal Armadillo scaffold for assembly and targeting, followed by a CCP-specific N-terminal module and a C-terminal M14 catalytic core—causally establishes a soluble, metal-dependent carboxypeptidase.

2. 02

   This architecture dictates molecular function.

   The M14 carboxypeptidase domain (IPR000834) and the CCP-specific modules (IPR040626, IPR050821) together create a metallocarboxypeptidase active site that hydrolyzes peptide bonds at the C-terminus of substrates. This directly supports GO:0004181 metallocarboxypeptidase activity. The Armadillo N-terminus explains how the enzyme assembles with substrates and cofactors and biases specificity toward polyglutamylated tails.

3. 03

   From function to process, the catalytic preference for acidic C-termini and the CCP family identity explain participation in terminal protein modifications.

   The enzyme trims glutamate residues appended to protein C-termini, thereby executing GO:0018410 C-terminal protein amino acid modification. Its demonstrated role in editing polyglutamylation of tubulin fits GO:0035609 protein side chain glutamylation, where carboxypeptidase activity counterbalances elongation by removing glutamate units to tune post-translational marks.

4. 04

   Cellular location follows from both targeting and substrate ecology.

   The absence of transmembrane segments and the presence of Armadillo scaffolds favor a soluble intracellular enzyme, aligning with GO:0005737 cytoplasm. The nucleolus (GO:0005730 nucleolus) is a plausible second site because nucleoli concentrate RNA processing and specific protein assemblies; CCP-family scaffolds and Armadillo repeats can mediate transient nucleolar residency where polyglutamylation-sensitive substrates or assembly factors reside. Thus, a cytoplasmic enzyme with regulated nucleolar enrichment is expected.

5. 05

   Mechanistically, the N-terminal Armadillo platform recruits substrates and adaptors, the CCP-specific N-terminal domain orients the polyglutamylated tail, and the M14 catalytic core—stabilized by the overlapping metallopeptidase scaffold—coordinates a catalytic metal to hydrolyze terminal glutamates.

   This causes precise trimming of polyglutamylated proteins such as tubulin, thereby tuning microtubule dynamics and other cytoskeleton-dependent pathways.

6. 06

   Given this model, likely interaction partners include polyglutamylated substrates and their assembly machinery.

   Tubulin heterodimers and microtubule-associated proteins are prime binding/catalytic clients. TTLL family polyglutamylases are functional counterparts whose products CCP edits; transient complexes with these enzymes would channel substrates. Cytosolic chaperones and nucleolar assembly factors may stabilize targeting. Together, these interactions position the enzyme as a soluble editing carboxypeptidase that sculpts post-translational polyglutamylation landscapes in the cytoplasm and nucleolus.