Modified J-chain

1. An IgM antibody comprising a modified J-chain, wherein the modified J-chain comprises one or more extraneous binding moieties introduced into a native human J-chain sequence of SEQ ID NO: 1 at one or more of the following locations:

(a) a C-terminus;

(b) an N-terminus; or

(c) in between cysteine residues 92 and 101.

2. The IgM antibody of claim 1 , wherein the extraneous binding moiety comprises a polypeptide introduced into the native human J-chain sequence of SEQ ID NO: 1 by direct or indirect fusion.

3. The IgM antibody of claim 2 , wherein the binding moiety is introduced by indirect fusion through a peptide linker.

4. The IgM antibody of claim 3 , wherein the indirect fusion is through a peptide linker at a C-terminus of the extraneous binding moiety.

5. The IgM antibody of claim 3 , wherein the indirect fusion is through a peptide linker at an N-terminus of the extraneous binding moiety.

6. The IgM antibody of claim 3 , wherein said peptide linker has a length that ranges from 1 to 20 amino acids.

7. The IgM antibody of claim 6 , wherein the length of the peptide linker is 15 amino acids.

8. The IgM antibody of claim 1 , wherein the extraneous binding moiety is introduced into the native human J-chain sequence of SEQ ID NO: 1 by chemical or chemo-enzymatic derivatization.

9. The IgM antibody of claim 8 , wherein the extraneous binding moiety is introduced into the native human J-chain sequence of SEQ ID NO: 1 by a chemical linker.

10. The IgM antibody of claim 9 , wherein the chemical linker is a cleavable or non-cleavable linker.

11. The IgM antibody of claim 10 , wherein the cleavable linker is a chemically labile linker or an enzyme-labile linker.

12. The IgM antibody of claim 10 , wherein the linker is selected from the group consisting of: N-succinimidyl-3-(2-pyridyldithio) propionate (SPDP), succinimidyl-4-(N-maleimidomethyl) cyclohexane-1-carboxylate (SMCC), N-succinimidyl-4-(2-pyridylthio) pentanoate (SPP), iminothiolane (IT), bifunctional derivatives of imidoesters, active esters, aldehydes, bis-azido compounds, bis-diazonium derivatives, diisocyanates, bis-active fluorine compounds, and any combination thereof.

13. The IgM antibody of claim 8 , wherein the modified J-chain is modified by insertion of an enzyme recognition site, and by post-translationally attaching the extraneous binding moiety at the enzyme recognition site through a peptide or non-peptide linker.

14. The IgM antibody of claim 1 , wherein at least one of the one or more extraneous binding moieties comprises a polypeptide capable of binding to a target, wherein the polypeptide is selected from the group consisting of: an antibody, an antigen-binding fragment of an antibody, an antibody-drug conjugate, an antibody-like molecule, an antigen-binding fragment of an antibody-like molecule, a ligand, a receptor, a virus-like particle, a protein toxin, an enzyme, and an alternative scaffold.

15. The IgM antibody of claim 14 , wherein the alternative scaffold is selected from the group consisting of: darpins, fibronectin domains, adnectins, and knottins.

16. The IgM antibody of claim 14 , wherein the antigen-binding fragment is selected from the group consisting of: F(ab′)2, F(ab)2, Fab′, Fab, Fv, and scFv.

17. The IgM antibody of claim 16 , wherein the antigen-binding fragment is an scFv.

18. The IgM antibody of claim 1 , which is monospecific.

19. The IgM antibody of claim 1 , which is bispecific.

20. The IgM antibody of claim 1 , which is multi-specific.

21. A composition comprising an IgM antibody comprising a modified J-chain, wherein the modified J-chain comprises one or more extraneous binding moieties introduced into a native human J-chain sequence of SEQ ID NO: 1 at one or more of the following locations:

(a) a C-terminus;

(b) an N-terminus; or

(c) in between cysteine residues 92 and 101.

22. The composition of claim 21 , which is a pharmaceutical composition, further comprising a pharmaceutically acceptable carrier.

23. The composition of claim 21 , which is a diagnostic composition.