A novel 30-kDa secretory protein, Acrp30 (adipocyte complement-related protein of 30 kDa), is exclusively produced in adipocytes and is induced over 100-fold during adipocyte differentiation. Acrp30 shares structural similarities with complement factor C1q and a hibernation-specific protein from Siberian chipmunks. It forms large homo-oligomers and undergoes post-translational modifications. Acrp30 is secreted into serum and is abundant, suggesting a role in energy homeostasis. Its secretion is enhanced by insulin, similar to adipsin, and its function is not yet known, though its structural resemblance to C1q is intriguing. Acrp30 is a secretory protein found in blood, detectable by Western blotting in mouse serum. It contains a potential N-glycosylation site but is not glycosylated. Post-translational modifications, such as hydroxylation of collagen domain proline residues, are observed. Insulin enhances Acrp30 secretion, indicating a regulated secretory pathway in adipocytes. Acrp30 forms homo-oligomeric structures, including trimers, hexamers, and larger species, and is likely involved in regulated exocytosis. The study confirms the existence of an insulin-regulated secretory pathway in adipocytes. Acrp30 and adipsin may be sorted into regulated secretory vesicles, which are exocytosed in response to insulin. Rab3 proteins, including Rab3A and Rab3D, are involved in this process. Acrp30 is a relatively abundant serum protein, accounting for up to 0.05% of total serum protein. Its function remains to be determined, but its structural and functional similarities to C1q and other proteins suggest a role in immune or metabolic processes. The study also highlights the importance of understanding the mechanisms of insulin signaling and regulated exocytosis in adipocytes.A novel 30-kDa secretory protein, Acrp30 (adipocyte complement-related protein of 30 kDa), is exclusively produced in adipocytes and is induced over 100-fold during adipocyte differentiation. Acrp30 shares structural similarities with complement factor C1q and a hibernation-specific protein from Siberian chipmunks. It forms large homo-oligomers and undergoes post-translational modifications. Acrp30 is secreted into serum and is abundant, suggesting a role in energy homeostasis. Its secretion is enhanced by insulin, similar to adipsin, and its function is not yet known, though its structural resemblance to C1q is intriguing. Acrp30 is a secretory protein found in blood, detectable by Western blotting in mouse serum. It contains a potential N-glycosylation site but is not glycosylated. Post-translational modifications, such as hydroxylation of collagen domain proline residues, are observed. Insulin enhances Acrp30 secretion, indicating a regulated secretory pathway in adipocytes. Acrp30 forms homo-oligomeric structures, including trimers, hexamers, and larger species, and is likely involved in regulated exocytosis. The study confirms the existence of an insulin-regulated secretory pathway in adipocytes. Acrp30 and adipsin may be sorted into regulated secretory vesicles, which are exocytosed in response to insulin. Rab3 proteins, including Rab3A and Rab3D, are involved in this process. Acrp30 is a relatively abundant serum protein, accounting for up to 0.05% of total serum protein. Its function remains to be determined, but its structural and functional similarities to C1q and other proteins suggest a role in immune or metabolic processes. The study also highlights the importance of understanding the mechanisms of insulin signaling and regulated exocytosis in adipocytes.