The present invention provides a substantially purified growth differentiation factor (GDF) receptor, including a GDF-8 (myostatin) receptor, as well as functional peptide portions thereof. In addition, the invention provides a virtual representation of a GDF receptor or a functional peptide portion thereof. The present invention also provides a method of modulating an effect of myostatin on a cell by contacting the cell with an agent that affects myostatin signal transduction in the cell. In addition, the invention provides a method of ameliorating the severity of a pathologic condition, which is characterized, at least in part, by an abnormal amount, development or metabolic activity of muscle or adipose tissue in a subject, by modulating myostatin signal transduction in a muscle cell or an adipose tissue cell in the subject. The invention also provides a method of modulating the growth of muscle tissue or adipose tissue in a eukaryotic organism by administering an agent that affects myostatin signal transduction to the organism.