Myostatin is a transforming growth factor-beta (TGF-beta) family member that plays roles as a negative regulator of muscle mass development. Follistatin is a myostatin-binding protein that can inhibit myostatin activity to promote muscle growth [1]. This is an important way for increasing growth especially in demo horses. Here, we aimed to prediction inhibition activity of equine myostatin protein with human follistatin, through bioinformatics tools. The Swiss-model server was applied to predict the third (three-dimensional) structure of horse myostatin protein and studies with SAVES 6.0 online server. After that, the interactions of myostatin with human follistatin were evaluated using variety of tools such as Verify3D, ERRAT and ClusPro2.0 online software [2, 3]. The results showed that the Verify3D for this protein was at least 88% of the amino acid residues have an average score of 3D-1D> = 0.2, which is acceptable for our protein. strategy evaluates proteins using a three-dimensional structure. This score varies from -1 (poor score) to +1 (good score). ERRAT is an online server that confirms the structure of a protein on the assumption of nuclear fusion between different types of atoms, with total quality index in this study was 83.52 which is acceptable. According to the Ramachandarn plot, for horse myostatin 88.7% of the amino acids in this structure were in the desired region, which is acceptable for predicting the third structure. The docking result showed that the N-terminal of follistatin was in contact with TGF-beta region in myostation that related to myostatin activity according to crystallography structure, derived from the docking of human follistatin with mouse myostatin. Finally, our research indicated that the binding energy in our predicted model was -903 which is close to the result of binding energy of the docking human follistatin with mouse myostatin which is reported –1323.3. Key word: Follistatin, Equus caballus, Myostatin, Bioinformatics, Molecular Docking References [1] Tobin, J. F., & Celeste, A. J. (2005). Myostatin, a negative regulator of muscle mass: implications for muscle degenerative diseases. Current Opinion in Pharmacology, 5(3), 328-332. [2] Kozakov, D. et al. (2017). The ClusPro web server for protein–protein docking. Nature Protocols, 12(2), 255-278. [3] Javadmanesh, A., Mohammadi, E., Mousavi, Z., Azghandi, M., & Tanhaiean, A. (2021). Antibacterial effects assessment on some livestock pathogens, thermal stability and proposing a probable reason for different levels of activity of thanatin. Scientific Reports, 11(1), 1-10.

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