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Acrp30/adiponectin is an adipocyte-derived serum protein with important roles in regulation of lipid and glucose metabolism, but which of its isoforms are biologically active remains controversial. We addressed this issue by first characterizing the structure of each individual Acrp30 oligomer and the determinants responsible for multimer formation. Freeze etch electron microscopy showed the trimer to exhibit a ball-and- stick-like structure containing a large globular sphere, an extended collagen stalk, and a smaller sphere on the opposite end of the stalk. The hexamer consists of two adjacent trimeric globular domains and a single stalk composed of collagen domains from two trimers. Although not necessary for trimer formation or stability, two of the three monomers in an Acrp30 trimer are covalently linked by a disulfide bond between cysteine residues at position 22. In contrast, assembly of hexameric and higher …