LRG1 is an abundant secreted glycoprotein rather than a membrane-spanning receptor or enzyme. The mature human plasma protein was sequenced as a single 312-residue polypeptide carrying one galactosamine- and four glucosamine-containing oligosaccharides PMID:3856868. Its repeated architecture was already evident from the sequence PMID:3856868. The early paper's final suggestion that LRG might be membrane-derived or membrane-associated was a sequence-based interpretation, not a membrane-localization experiment PMID:3856868. Current sequence annotation instead shows a cleaved signal peptide and no transmembrane segment.
LRG1 expression is not endothelial-specific. It is induced during granulocytic differentiation, and transcripts occur in neutrophils and progenitors PMID:12223515. In the retinal angiogenesis study, Lrg1 was described as a secreted glycoprotein and was largely vascular in mouse retina; low constitutive LRG1 was also observed in normal adult human retinal vessels [PMID:23868260 "When ranked according to fold change, a gene encoding a secreted glycoprotein of unknown function, namely leucine-rich alpha-2-glycoprotein-1 (Lrg1), emerged as the most significantly up-regulated."; PMID:23868260 "Consistent with the data obtained in the mouse, we observed low levels of constitutive LRG1 expression in normal adult human retinal vessels and weakly, but not exclusively, in vessels in other human tissues including breast, skin and intestine (Supplementary Fig 4b)."].
Proteomic detections in plasma, colostrum, urine-derived exosomes, saphenous-vein ECM fractions, and neutrophil granule pathways are localization/context evidence. They do not by themselves establish a molecular function. Several cached proteomics articles do not name LRG1 in the narrative, so their LRG1 calls likely depend on supplementary peptide tables or database records.
The most direct mechanistic paper is PMID:23868260. In primary endothelial cells, reciprocal immunoprecipitations placed LRG1 with TGFBR2, ALK1/ACVRL1, ALK5/TGFBR1, and endoglin PMID:23868260. One-to-one ectodomain experiments directly supported binding to ALK5, TGFBR2, and endoglin PMID:23868260. By contrast, ALK1 association was conditional on endoglin and enhanced by TGF-beta1, so the ACVRL1 IPI should not be paraphrased as an unconditional one-to-one interaction PMID:23868260.
LRG1 is a modulator rather than a standalone TGF-beta agonist in these endothelial assays. LRG1 alone did not activate SMAD1/5 or SMAD2/3, whereas LRG1 plus TGF-beta1 strongly enhanced pro-angiogenic SMAD1/5 signaling PMID:23868260. Knockdown of ALK1, TGFBR2, or endoglin abrogated LRG1-driven signaling or tube formation, supporting a receptor-complex mechanism rather than a generic growth-factor effect.
The angiogenic role spans cultured endothelial assays and mouse disease models. LRG1 was mitogenic only in the TGF-beta1 context and promoted angiogenesis PMID:23868260. Lrg1-null mice showed reduced pathological ocular angiogenesis, and antibody blockade attenuated angiogenesis [PMID:23868260 "Mice lacking Lrg1 develop a mild retinal vascular phenotype but exhibit a significant reduction in pathological ocular angiogenesis."; PMID:23868260 "LRG1 antibody blockade inhibits this switch and attenuates angiogenesis."]. The core conclusion is strongest for extracellular modulation of endothelial TGF-beta receptor-complex signaling and its pro-angiogenic output; anatomical disease contexts should remain explicit.
LRG1 does not impose the same TGF-beta branch in every cell. In the bleomycin mouse lung model, Lrg1 loss suppressed fibrosis and reduced SMAD2 phosphorylation [PMID:29279415 "In LRG KO mice, lung fibrosis was significantly suppressed, as indicated by attenuated Masson's trichrome staining and lower collagen content than those in WT mice."; PMID:29279415 "Moreover, in the lungs of LRG KO mice, phosphorylation of Smad2 was reduced and expression of α-SMA was decreased relative to those in WT mice."]. In cultured fibroblasts, LRG enhanced TGF-beta-induced SMAD2 and profibrotic targets, but endoglin was dispensable [PMID:29279415 "In vitro experiments indicated that LRG enhanced the TGF-β-induced phosphorylation of Smad2 and the expression of Serpine1 and Acta2, the downstream of Smad2, in fibroblasts."; PMID:29279415 "Although endoglin, an accessory TGF-β receptor, is essential for LRG to promote TGF-β signaling in endothelial cells during angiogenesis, we found that endoglin did not contribute to the ability of LRG to enhance Smad2 phosphorylation in fibroblasts."]. These data establish a mouse/fibroblast profibrotic context; they should not be generalized into a universal ALK1/SMAD1/5 mechanism or direct evidence for human idiopathic pulmonary fibrosis.
PMID:32887674 shows context-dependent effects rather than a uniformly beneficial or harmful wound role. In normal wounds, Lrg1 deficiency delayed closure through impaired inflammation, re-epithelialization, and angiogenesis PMID:32887674. The study links LRG1 to neutrophil adhesion PMID:32887674, keratinocyte migration PMID:32887674, and dermal endothelial proliferation/migration/tube formation PMID:32887674.
Diabetes reverses the net wound outcome: Lrg1 deletion protected mice from diabetes-induced delayed closure, associated with reduced NETosis PMID:32887674. This is a mouse global-knockout result supplemented by human neutrophil/cell-line experiments, not proof that every LRG1 source or every human diabetic wound behaves identically.
The two proteome-scale interactome papers underlying CYCS IPI rows do not expose the LRG1-CYCS pair in their cached narrative text; the pair is supplied by IntAct/GOA. Those screen records should not alone be interpreted as a physiological pathway. However, an independent biochemical study makes cytochrome-c binding biologically plausible: surface plasmon resonance found human LRG binding to horse and snake cytochrome c, but not yeast cytochrome c PMID:20442399. The authors proposed extracellular cytochrome c released from dead cells as an endogenous ligand PMID:20442399. Important boundary: the reported binding constants used nonhuman cytochrome-c proteins, and the proposed neutralization function was inferred rather than tested as an in-vivo human pathway.
The human LRG1 crystal structure is a horseshoe-like LRR solenoid with four N-glycosylation sites PMID:37121976. Biochemical and cell assays showed that removing glycans, especially at N325, increases high-affinity LPHN2 binding and promotes LPHN2-dependent angiogenic and neurotrophic responses PMID:37121976. This identifies a second receptor axis and a glycosylation-dependent molecular switch. The functional assays were mouse tissue explants and a diabetic mouse model; physiological LRG1 deglycosylation and the prevalence of this mechanism in humans remain unresolved.
The reviewed P02750 record defines a 347-residue precursor with a cleaved signal peptide at residues 1-35 and a mature chain spanning residues 36-347. The mature protein contains eight leucine-rich repeats and multiple experimentally annotated glycans; no alternative protein isoforms are reported. PDB 8H24 is an apo structure of the mature human protein, so it establishes the LRR-solenoid fold but not the architecture of an LRG1-receptor or LRG1-cytochrome-c complex. This is consistent with the primary structural report PMID:37121976, whose authors explicitly identify the missing complex structures PMID:37121976.
The PANTHER classification in the current UniProt record places LRG1 in a broad SALM/Kekkon-labeled family context. That automated family label was not used to transfer neuronal adhesion, synaptic, or Kekkon-specific functions to human LRG1: the reviewed sequence, secretory biology, direct receptor evidence, and LRG1 primary literature take precedence, and no SALM-derived molecular function was added.