GGPS1 (human) — curation notes

UniProt: O95749 (GGPPS_HUMAN). Gene symbol GGPS1 (HGNC:4249). 300 aa, ~34.9 kDa.
Two isoforms (O95749-1 displayed; O95749-2 lacks residues 1..54, VSP_056578).

Identity / core function

GGPS1 encodes geranylgeranyl pyrophosphate synthase (GGPP synthase / GGPPSase), a
cytosolic trans-prenyltransferase of the FPP/GGPP synthase family (Pfam PF00348
polyprenyl_synt; PANTHER PTHR12001). It catalyzes sequential head-to-tail (1'-4)
condensations of isopentenyl diphosphate (IPP) onto an allylic diphosphate primer to
build the C20 isoprenoid geranylgeranyl diphosphate (GGPP).

UniProt lists three catalytic-activity reactions with distinct EC numbers, all supported
experimentally by the crystallographic study (PubMed:16698791):
- IPP + DMAPP → geranyl-PP (GPP) + PPi (EC 2.5.1.1)
- IPP + GPP → farnesyl-PP (FPP) + PPi (EC 2.5.1.10)
- IPP + FPP → geranylgeranyl-PP (GGPP) + PPi (EC 2.5.1.29) ← the physiologically defining step
[file:human/GGPS1/GGPS1-uniprot.txt "Reaction=isopentenyl diphosphate + (2E,6E)-farnesyl diphosphate = (2E,6E,10E)-geranylgeranyl diphosphate + diphosphate"]

The enzyme requires Mg2+ (binds 3 Mg2+ ions) [file:human/GGPS1/GGPS1-uniprot.txt "Binds 3 Mg(2+) ions"] and is
subject to product inhibition by GGPP [file:human/GGPS1/GGPS1-uniprot.txt "Subject to product inhibition by geranylgeranyl"].

Human recombinant enzyme makes GGPP from FPP + IPP:
PMID:10026212
PMID:9741684

Biological role

GGPP is the C20 isoprenoid lipid donor for protein geranylgeranylation. GGTase-I and
RabGGTase (GGTase-II) transfer GGPP onto C-terminal cysteines of small GTPases of the
Rho/Rac and Rab families, anchoring them to membranes. GGPP is also a precursor for
ubiquinone/dolichol side chains and other isoprenoids.
PMID:32403198

The enzyme sits in the mevalonate/isoprenoid branch downstream of FPP synthase; UniProt
records three UniPathway steps (geranyl-PP, farnesyl-PP, geranylgeranyl-PP biosynthesis).

Structure / oligomeric state

Human GGPPS is a homohexamer (trimer of homodimers); crystal structure at 2.7 Å with
Mg and geranylgeranyl phosphate bound (PubMed:16698791).
[file:human/GGPS1/GGPS1-uniprot.txt "Homohexamer; trimer of homodimers."]
The gel-filtration ~280 kDa oligomer was seen in the earliest characterization
PMID:10026212.
This hexameric self-association is the basis of the IPI "identical protein binding"
(GO:0042802) annotations (self-interaction O95749–O95749 in HuRI/interactome screens).

Localization

Cytosolic. HPA immunofluorescence → cytosol (IDA, GO_REF:0000052).
Disease paper (PubMed:32403198) immunohistochemistry:
- Dermal fibroblasts: uniform cytoplasmic + perinuclear + some organelles
PMID:32403198
- Skeletal muscle: prominent Z disc localization (co-labeled with desmin)
PMID:32403198

Note: cytoplasm/perinuclear/Z-disc are the annotated locations; the Z-disc/sarcomeric and
perinuclear pools likely reflect a tissue-specific, possibly non-catalytic
scaffolding/localization role suggested by the disease-mutation cluster (see below).

Disease

Biallelic (recessive) GGPS1 missense variants cause Muscular dystrophy, congenital
hearing loss, and ovarian insufficiency syndrome (MDHLO; MIM:619518)
(PubMed:32403198). Recurrent variants (F257C, Y259C, R261G, R261H; plus P15S) cluster in a
5-residue microdomain in the 11th α-helix on the outer surface of the hexamer, OUTSIDE the
catalytic barrel; this domain is conserved in animals but not plants.
PMID:32403198
Mutant enzymes retain 70–85% (bacterial) / ~50% (patient myoblast) activity
PMID:32403198.
The domain "faces outward, where it could potentially interact with as yet to be defined
binding partners involved in the localization or shuttling of the enzyme."

Interactions (IPI, IntAct)

UniProt INTERACTION block lists: ATOX1 (O00244), self GGPS1 (O95749, NbExp=7), SDCBP
(O00560). These derive from high-throughput binary interactome screens:
- PMID:25416956 (Rolland et al., Cell 2014, HI-II-14): protein binding (with ATOX1 O00244) + identical protein binding (self)
- PMID:32296183 (Luck et al., Nature 2020, HuRI): protein binding (with SDCBP O00560) + identical protein binding (self)
- PMID:31515488 (Fragoza et al., Nat Commun 2019): identical protein binding (self)
Individual GGPS1 mentions are not in the article body text (data are in supplementary
interactome tables); these are systematic Y2H/binary screens. Self-interaction is
biologically consistent with the obligate homohexamer. The heterotypic hits (ATOX1, SDCBP)
are HT binary screen results with no dedicated follow-up establishing a specific cellular
function; treat "protein binding" as non-informative (over-annotation for core function).

Annotation summary (decisions)

• Catalytic MF GO:0004311 (GGPP synthase activity): ACCEPT for IDA (10026212, 9741684) and
  IMP (32403198). The redundant IBA and IEA(EC/RHEA) copies of the same correct core MF →
  ACCEPT (own correct core function, not over-annotation).
• EC-derived partial-reaction MFs GO:0004161 (DMAPP-transtransferase, EC 2.5.1.1) and
  GO:0004337 ((2E,6E)-FPP synthase, EC 2.5.1.10): these are the DMAPP→GPP and GPP→FPP
  chain-elongation steps the same active site performs en route to GGPP. Documented in
  UniProt catalytic-activity block. KEEP_AS_NON_CORE (real activities but the physiological
  product/role is GGPP, not GPP/FPP release).
• GO:0004659 prenyltransferase activity (IEA, InterPro): correct parent of the enzyme's
  activity but less informative than GO:0004311 → MARK_AS_OVER_ANNOTATED (general parent).
• BP GO:0033386 (GGPP biosynthetic process): ACCEPT (core). GO:0008299 isoprenoid
  biosynthetic process (IBA + IEA) and GO:0006720 isoprenoid metabolic process (IDA + IEA):
  correct but general parents → KEEP_AS_NON_CORE.
• UniPathway BP GO:0033384 (GPP biosynthetic process) and GO:0045337 (FPP biosynthetic
  process): the intermediate steps; KEEP_AS_NON_CORE (real but not the defining product).
• CC GO:0005829 cytosol (IDA + 3× Reactome TAS): ACCEPT. GO:0005737 cytoplasm (IEA + IMP):
  ACCEPT (correct, more general). GO:0048471 perinuclear region (IEA + IMP) and GO:0030018
  Z disc (IEA + IMP): supported by PubMed:32403198 IHC; KEEP_AS_NON_CORE (tissue/context
  specific, not the bulk cytosolic pool).
• Protein binding GO:0005515 (IPI, 2×): MARK_AS_OVER_ANNOTATED (bare, uninformative;
  HT binary screens; never REMOVE per policy).
• Identical protein binding GO:0042802 (IPI, 3×): ACCEPT/KEEP_AS_NON_CORE — self-interaction
  reflects the real homohexamer, but it is a structural property, not the core catalytic
  function → KEEP_AS_NON_CORE.

Cross-refs

• Reactome: R-HSA-9717830 (GGPS1 hexamer transfers IPPP to GPP), R-HSA-9717834 (…to DMAPP),
  R-HSA-1655832 (expression of GGPS1). PDB: 2Q80, 6R4V, 9CSL, etc.
• Drug target: nitrogen-containing bisphosphonates (zoledronate etc.) and DGBP inhibit
  GGPPS (DrugBank entries in UniProt).