WFIKKN1 and WFIKKN2 are large extracellular multidomain proteins consisting of a WAP, a follistatin, an immunoglobulin, two Kunitz-type protease inhibitor domains, and an NTR domain. Recent experiments have shown that WFIKKN2 protein binds mature GDF8/myostatin and myostatin propeptide and inhibits the biological activity of myostatin (Hill, J. J., Qiu, Y., Hewick, R. M., and Wolfman, N. M. (2003) Mol. Endocrinol. 17, 1144-1154). Here we show that the paralogue of this protein, WFIKKN1, also binds to both myostatin and myostatin propeptide and that both WFIKKN1 and WFIKKN2 bind GDF11, the growth and differentiation factor most closely related to myostatin, with high affinity. Structure-function studies on WFIKKN1 have revealed that the follistatin domain is primarily responsible for the binding of mature growth factor, whereas the NTR domain contributes most significantly to the interaction with myostatin propeptide. Analysis of the evolutionary histories of WFIKKN1/WFIKKN2 and GDF8/GDF11 proteins indicates that the functional association of an ancestral WFIKKN protein with an ancestor of GDF8/11 may date back to cephalochordates/urochordates. Although duplication of the corresponding genes gave rise to WFIKKN1/WFIKKN2 and GDF8/GDF11 in early vertebrates, the data presented here suggest that there is significant functional overlap of the paralogous proteins.