Isolated nucleic acid molecule encoding a fusion protein comprising an activin receptor

An isolated nucleic acid molecule comprising a first polynucleotide encoding at least one heterologous protein and a second polynucleotide selected from the group consisting of:

• (a) a polynucleotide comprising a polynucleotide sequence at least 95% identical to the polynucleotide sequence set forth in SEQ ID NO:
• 1;
• (b) a polynucleotide encoding a polypeptide comprising an amino acid sequence at least 95% identical to the amino acid sequence set forth in SEQ ID NO:
• 2, with or without a signal sequence, wherein the polypeptide is capable of binding myostatin, activin A, or GDF-11;
• and
• (c) a polynucleotide that hybridizes to (a) or (b) or the complete complement of (a) or (b) in the presence of 50% formamide, 6× sodium chloride/sodium citrate (SSC) at 42° C. and washing conditions of 60° C., 0.5×SSC, 0.1% sodium dodecyl sulfate (SDS), and
• wherein the protein encoded by the isolated nucleic acid molecule comprises at least one of the following characteristics:
• (1) the protein has activin A neutralizing activity IC50 (nM) vs. 20 nM activin that is less than the activin A neutralizing activity IC50 (nM) vs. 20 nM activin of a control protein comprising amino acids 1-124 of the sequence set forth in SEQ ID NO:5 linked to human IgG1 Fc;
• or (2) the protein has myostatin-neutralizing activity IC50 (nM) vs. 4 nM myostatin that is less than the myostatin-neutralizing activity IC50 (nM) vs. 4 nM myostatin of the control protein.