Stably tethered structures of defined compositions with multiple functions or binding specificities

A stably tethered structure comprising:

• a) two copies of a first fusion protein comprising a dimerization and docking domain (DDD) selected from the group consisting of residues 1-44 of human protein kinase A RIIα and RIIβ fused to a first protein or peptide, wherein the first protein or peptide is a first monoclonal IgG antibody or antigen-binding fragment thereof;
• and
• b) one copy of a second fusion protein comprising the amino acid sequence selected from the group consisting of the anchoring domain (AD) of A-kinase anchoring protein (AKAP) fused to a second protein or peptide, wherein the second protein or peptide is a second monoclonal IgG antibody or antigen-binding fragment thereof;
• wherein the first and second antibody or antigen-binding fragment thereof bind to a human antigen selected from the group consisting of CD20, CD22, CD74, CEACAM5 (carcinoembryonic antigen), EGP-1 (epithelial glycoprotein-1), HLA-DR, IGF-1R (insulin-like growth factor type I receptor), and AFP (α-fetoprotein), wherein the two copies of the DDD bind to the AD to form a stably tethered structure and wherein the antigen-binding fragment is selected from the group consisting of a Fab antibody fragment, a Fab' antibody fragment, an scFv antibody fragment, a diabody and a single domain antibody (DAB).