The invention provides constant regions incorporating a cysteine mutation and linked to a μ tailpiece and antibodies or fusion proteins incorporating the same. The constant regions include at least CH2 and CH3 regions of an IgG heavy chain constant region including a cysteine mutation and μ tailpiece. Antibodies or fusion proteins incorporating the constant regions gains the ability to form multivalent complexes, e.g., pentameric or hexameric structures. Antibodies or fusion proteins incorporating the constant regions also retain IgG properties including specific binding to protein G, which facilitates purification and may exhibit pH-dependent FcRn binding, which is associated with a relatively long in vivo half-life. Depending on the isotype and subtype, the nature of the antigen and presence of an additional IgG hinge domain, such antibodies or fusion proteins may also have properties of specific binding to protein A, and effector functions such as ADCC, CDC and opsonization.
