Curation Notes for Variovorax paradoxus ppk (A0A0H2M4T4, VPARA_28600)

Overview

• UniProt Accession: A0A0H2M4T4 (A0A0H2M4T4_VARPD)
• Ordered Locus Name: VPARA_28600
• Gene Symbol: ppk
• Organism: Variovorax paradoxus (NCBITaxon:34073, strain TBEA6; for species-level genomic context in the sequenced strain S110 genome announcement, see PMID:21183664)
• Protein Function: Polyphosphate kinase 1 (PPK1, 710 aa, EC 2.7.4.1) that catalyzes the reversible transfer of the terminal $\gamma$-phosphate of ATP to long-chain inorganic polyphosphate (polyP).

Domain Architecture & Biochemical Mechanism (UniProtKB:A0A0H2M4T4)

1. Conserved Four-Domain PPK1 Architecture:
2. N-terminal domain (FT DOMAIN 18..119, Pfam:PF13089 PP_kinase_N / InterPro:IPR025198 PPK_N_dom, InterPro:IPR036832 PPK_N_dom_sf)
3. Middle domain (FT DOMAIN 128..310, Pfam:PF02503 PP_kinase / InterPro:IPR024953 PP_kinase_middle, InterPro:IPR036830 PP_kinase_middle_dom_sf)
4. C1 catalytic domain (FT DOMAIN 338..501, Pfam:PF17941 PP_kinase_C_1 / InterPro:IPR041108 PP_kinase_C_1)
5. C2 C-terminal domain (FT DOMAIN 511..674, Pfam:PF13090 PP_kinase_C / InterPro:IPR025200 PPK_C_dom2)
6. Matches full-length HAMAP:MF_00347 (Polyphosphate_kinase), NCBIfam:TIGR03705 (poly_P_kin), and InterPro:IPR003414 (PP_kinase).
7. Autophosphorylation Active Site (His441):
8. FT ACT_SITE 441 (Phosphohistidine intermediate) within the conserved C1 domain (UniProtKB:A0A0H2M4T4).
9. Catalyzes ATP + (phosphate)n = ADP + (phosphate)n+1 (EC 2.7.4.1), requiring $\text{Mg}^{2+}$ as a cofactor (FT BINDING 381, ligand="Mg(2+)").
10. Prospective Hypothesis on Intracellular Metal & REE Sequestration:
11. As a prospective hypothesis (not yet experimentally assayed in V. paradoxus), polyanionic polyphosphate chains synthesized by PPK1 (VPARA_28600) may form intracellular granules that chelate divalent transition metals ($\text{Cd}^{2+}, \text{Zn}^{2+}, \text{Co}^{2+}, \text{Ni}^{2+}, \text{Cu}^{2+}$) and trivalent lanthanides ($\text{Ln}^{3+}$), complementing the polyphosphate-mobilizing PF03976 PPK2 phosphotransferases VPARA_00520 (UniProtKB:A0A0H2MPR3, DR InterPro; IPR022300; PPK2-rel_1.) and VPARA_50220 (UniProtKB:A0A0H2LUF2, DR NCBIfam; TIGR03707; PPK2_PA0141; 1.) encoded in the same strain TBEA6 proteome (Proteomes:UP000035170).

Review of Existing GOA Annotations

• GO:0008976 (polyphosphate kinase activity, IEA via UniProtKB-EC / InterPro / ARBA) -> ACCEPT (core enzymatic activity, EC 2.7.4.1).
• GO:0006799 (polyphosphate biosynthetic process, IEA via UniProtKB-UniRule / InterPro) -> ACCEPT (core biological process).
• GO:0009358 (polyphosphate kinase complex, IEA via InterPro IPR003414) -> ACCEPT (canonical PPK1 oligomeric kinase complex).

Proposed New Annotations

• GO:0005524 (ATP binding) — enables, supported by FT BINDING 55 (ligand="ATP") in UniProtKB:A0A0H2M4T4.
• GO:0000287 (magnesium ion binding) — enables, supported by FT BINDING 381 (ligand="Mg(2+)") in UniProtKB:A0A0H2M4T4.
• GO:0005829 (cytosol) — located_in, supported by CC -!- FUNCTION in UniProtKB:A0A0H2M4T4.