Adipogenin promotes the development of lipid droplets by binding a dodecameric seipin complex.

The microprotein adipogenin (Adig) is predominantly expressed in adipose tissues. Here, we found that Adig interacts with seipin to form a stable, rigid complex. We present the structure of the seipin-Adig complex at an overall resolution of ~3.0 angstroms. The structure revealed that mammalian seipin assembles into two distinct oligomeric forms: undecamers and dodecamers. Adig selectively bound to the dodecameric form and enhanced seipin assembly by bridging and stabilizing adjacent subunits. Functionally, this complex promoted lipid droplet development at both early and late stages. In transgenic mice, adipocyte-specific overexpression of Adig increased fat mass and enlarged lipid droplets, whereas Adig deletion disrupted triglyceride accumulation in brown adipose tissues. Thus, Adig can modulate lipid storage through its structural and functional interactions with seipin. Editor's summary: Fat storage in the body relies on specialized structures called lipid droplets (LDs). Li et al. identified the microprotein adipogenin as a regulator of adipocyte LD size (see the Perspective by Wu and Yang). Adipogenin interacts with the membrane protein seipin and stabilizes the assembly of seipin dodecamers by bridging adjacent subunits. Functionally, seipin-adipogenin complexes promote the formation of fewer but larger LDs. In mice, adipocyte-specific adipogenin overexpression results in increased fat mass and larger LDs, whereas adipogenin deletion reduces fat accumulation and LD size, particularly in brown adipose tissue. Thus, adipogenin represents a modulator of adipocyte lipid storage that acts through a structural and functional partnership with seipin. —Stella M. Hurtley INTRODUCTION: Adipogenin (Adig) is an 80–amino acid microprotein that is highly expressed in adipose tissues and steatotic liver. A previous genome-wide association study suggested that human ADIG is associated with blood leptin levels, highlighting its importance in energy metabolism. At the molecular level, Adig's function is largely unknown: No interacting proteins have been identified. At the cellular level, conflicting results have emerged regarding Adig's subcellular location and role in adipogenesis. In mice, the basis and extent of Adig's physiological effects remain elusive. Here, we systematically characterized Adig function at the molecular, cellular, and physiological levels. RATIONALE: Microproteins typically exert their functions by binding to larger proteins and regulating their activities. We pulled down Adig from adipocytes and identified its interacting proteins by mass spectrometry. Upon the identification of a seipin-Adig complex, we resolved its structure using cryo–electron microscopy (cryo-EM), enabling us to determine Adig's effect on seipin configuration at an atomic scale. Because seipin plays a vital role in lipid droplet (LD) formation and growth, we explored the function of the seipin-Adig complex in these processes. Moreover, we generated adipocyte-specific Adig overexpression and deletion mice to investigate Adig's effect on adipose tissue expansion and lipid metabolism in vivo. RESULTS: We found that Adig is a highly conserved protein with a single transmembrane (TM) segment that localizes to the endoplasmic reticulum (ER). Notably, Adig and seipin can form a complex and stabilize each other. Cryo-EM analysis revealed two distinct oligomers: an undecameric seipin-alone complex at ~3.2-Å overall resolution and a dodecameric seipin-Adig complex at ~3.0-Å overall resolution. In the seipin-Adig complex map, extra densities, corresponding to seipin and Adig TM domains, were observed. Multiple approaches, including high-resolution imaging, gel filtration, and molecular dynamics simulations, revealed that Adig could facilitate the assembly of dodecameric seipin complexes. Seipin complexes with varying Adig contents modulated LD formation and growth. The presence of the seipin-Adig complex altered triacylglycerol (TAG) flux in the ER, leading to the formation of fewer, but larger, LDs. Additionally, the ER-to-LD trafficking of select lipid-synthesizing enzymes was accelerated in Adig-expressing cells. In mice, Adig overexpression in adipocytes promoted LD enlargement and adipose tissue expansion, whereas Adig deletion decreased the amount of the seipin complexes in adipocytes and impaired TAG accumulation in brown adipose tissues. CONCLUSION: In this study, we demonstrate that Adig complexes with seipin, forming a previously unrecognized dodecameric seipin complex. Furthermore, Adig stabilizes and promotes the assembly of this complex, thereby supporting LD growth in cells. In mice, modulating the expression of seipin-Adig complexes in adipose tissues by Adig overexpression or deletion substantially affects LD formation and expansion as well as lipid absorption by adipose tissues. This study reveals Adig as a key cofactor that modulates seipin function and fat storage in adipose tissue. We conclude that the oligomerization and function of seipin complexes can be modulated by Adig expression. Seipin-Adig complex promotes the development of lipid droplets.: The seipin-Adig complex manifests as a distinctive oligomer of 12 subunits. Adig functions to maintain a pool of preassembled seipin-alone or seipin-Adig complexes, poised to facilitate the formation and expansion of LDs in adipose tissue. Mechanistically, the presence of seipin-Adig complexes enhances TAG flux into LDs, increases the ER-to-LD relocation of LD-targeting proteins, and may also promote lipid synthesis. ACSL and PLIN are LD targeting-proteins. ACSL, acyl-CoA synthetase long chain family member; PLIN, perilipin. [Figure created with BioRender.com] [ABSTRACT FROM AUTHOR]