Methods and compositions for producing secreted trimeric receptor analogs and biologically active fusion proteins
Methods and compositions for producing secreted soluble receptors and biologically active polypeptides in trimeric forms are disclosed. The process involves fusing the DNA template encoding a soluble receptor with a ligand binding domain or biologically active polypeptide to a DNA sequence encoding a C-propeptide of collagen, which is capable of self-assembly into a covalently linked trimer. The resulting fusion proteins are secreted as trimeric soluble receptor analogs, which can be used for more efficient neutralization of the biological activities of their naturally occurring trimeric ligands.
1. An isolated nucleic acid comprising consecutive nucleotides having a nucleotide sequence encoding a fused protein subunit comprising a signal peptide joined by in-frame fusion to a non-collagenous polypeptide comprising a ligand binding domain, which in turn is joined by in-frame fusion to a C-terminal portion of collagen which is capable of self-trimerizing said fused protein subunit to form a disulfide bond-linked trimeric fusion protein containing three ligand binding domains, wherein said trimeric fusion protein has a greater binding affinity to a ligand than a monomeric ligand binding domain.
2. The nucleic acid of claim 1 , wherein the C-terminal portion of collagen encoded by the nucleic acid is selected from the group consisting of pro.alpha.1(I), pro.alpha 2(I), pro.alpha.1(II), pro.alpha.1(III), pro.alpha.1(V), pro.alpha.2(V), pro.alpha.1(XI), pro.alpha.2(XI) and pro.alpha.3(XI).
3. The nucleic acid of claim 1 , wherein the signal peptide and the non-collagenous polypeptide are selected from two different secreted proteins.
4. The nucleic acid of claim 1 , wherein the signal peptide and the non-collagenous polypeptide are both from an identical native secreted protein.
5. The nucleic acid of claim 1 , wherein the nucleic acid encodes the C-terminal portion of collagen having only a C-propeptide without any linked glycine-repeat triple helical regions of collagen.
6. The nucleic acid of claim 1 , wherein the nucleic acid encodes the C-terminal portion of collagen having a glycine-repeat triple helical region of collagen linked to a C-propeptide.
7. The nucleic acid of claim 6 , wherein the nucleic acid encodes the C-terminal portion of collagen having an amino acid sequence shown as SEQ ID NO:2.
8. The nucleic acid of claim 1 , wherein the nucleic acid encodes the C-terminal portion of collagen having a mutated or deleted BMP-1 protease recognition sequence.
9. The nucleic acid of claim 8 , wherein the C-terminal portion of collagen encoded by the nucleic acid is selected from the group consisting of SEQ ID NO:4 and SEQ ID NO: 18.