A1BG

A1BG is a secreted plasma glycoprotein of the immunoglobulin superfamily, composed of five immunoglobulin-like domains, that functions as a high-affinity inhibitor of cysteine-rich secretory proteins (CRISPs) and as a stabilizer of extracellular NAMPT to modulate NAD+ biosynthesis (PMID:3458201, PMID:15461460, PMID:39433128, PMID:40560034). A1BG forms a nanomolar-affinity 1:1 complex with CRISP-3 in plasma and inhibits CRISP-mediated sterol binding and export through a Mg²⁺-dependent interaction mapped to its third immunoglobulin-like domain (PMID:15461460, PMID:39433128). A1BG also directly binds and stabilizes NAMPT, increasing NAD⁺ production and enhancing PARP1-dependent DNA repair, thereby conferring cisplatin resistance in osteosarcoma (PMID:40560034). Cardiomyocyte-specific loss of A1BG in female mice causes dilated cardiomyopathy with intercalated disc disruption and metabolic dysregulation, revealing a sex-specific structural role in the heart (PMID:40270023).

1. 1986 High

   Determination of the complete primary structure of A1BG established it as a five-immunoglobulin-domain member of the immunoglobulin superfamily, providing the first molecular framework for understanding its architecture but leaving its function entirely unknown.

   Evidence Complete amino acid sequencing and computational homology analysis of human plasma A1BG

   PMID:3458201

   Open questions at the time

   • No binding partner or biological function identified
   • No information on tissue-specific expression or regulation
2. 2004 High

   Identification of CRISP-3 as a nanomolar-affinity 1:1 binding partner of A1BG in plasma provided the first functional context — suggesting A1BG sequesters CRISP proteins — but the mechanistic consequence of this interaction was unknown.

   Evidence Size-exclusion chromatography, co-immunoprecipitation, and SPR with Kd measurement using human plasma proteins

   PMID:15461460

   Open questions at the time

   • Functional consequence of CRISP-3 sequestration not demonstrated
   • Binding domain on A1BG not mapped
   • Whether A1BG interacts with other CRISP family members was untested
3. 2024 High

   Reconstitution experiments demonstrated that A1BG directly inhibits CRISP-mediated sterol binding and export via an interaction requiring Mg²⁺ coordination, mapped to A1BG's third Ig-like domain, converting A1BG from a passive binding partner to an active functional inhibitor of CAP superfamily proteins.

   Evidence Yeast coexpression sterol export assay, in vitro sterol-binding assay, domain mapping, and Mg²⁺-dependency studies with A1BG and CRISP2

   PMID:39433128

   Open questions at the time

   • Physiological relevance of CRISP sterol-export inhibition in mammalian systems not tested in vivo
   • Whether all five Ig-like domains contribute to other interactions is unknown
   • Structural basis of the Mg²⁺-dependent A1BG–CRISP interface not resolved
4. 2025 Medium

   Discovery that adipocyte-secreted A1BG binds and stabilizes NAMPT to boost NAD⁺ production and PARP1-mediated DNA repair revealed a second major functional axis — paracrine metabolic signaling — distinct from CRISP inhibition.

   Evidence Co-immunoprecipitation of A1BG–NAMPT, NAD⁺ measurement, PARP1 activity assays, A1BG knockdown/recombinant rescue, xenograft models, and patient-derived organoids in osteosarcoma

   PMID:40560034

   Open questions at the time

   • A1BG–NAMPT interaction demonstrated by co-IP in a single study; awaits independent validation and structural characterization
   • Whether A1BG–NAMPT interaction occurs outside the tumor microenvironment is untested
   • Relative contribution of A1BG to systemic NAD⁺ homeostasis is unknown
5. 2025 High

   Cardiomyocyte-specific A1BG knockout revealed an unexpected cell-autonomous, sex-specific requirement for A1BG in maintaining intercalated disc integrity and metabolic homeostasis in the heart, establishing a tissue-specific in vivo function beyond its plasma roles.

   Evidence Conditional A1bg knockout in murine cardiomyocytes with echocardiography, TEM of intercalated discs, RNA-seq, and MS-based interactome profiling

   PMID:40270023

   Open questions at the time

   • Molecular mechanism linking A1BG loss to intercalated disc disassembly not identified
   • Basis of the sex-specific phenotype (hormonal vs. chromosomal) is unresolved
   • Whether the cardiac function is mediated through CRISP or NAMPT interactions or novel partners is unknown

• A unifying model integrating A1BG's CRISP-inhibitory, NAMPT-stabilizing, and cardiac structural roles is lacking; it remains unknown whether these functions reflect a common molecular mechanism (e.g., protein stabilization via Ig-like domains) or independent activities in distinct compartments.
• No high-resolution structure of A1BG alone or in complex with any partner
• No comprehensive loss-of-function study in human genetics linking A1BG variants to disease
• Relative importance of secreted versus cell-surface/intracellular A1BG pools is uncharacterized