Evolutionary relationship of thymopoietin to immunoglobulins and cellular recognition molecules

The sequence of thymopoietin, a thymic hormone composed of 49 amino acids, was compared to sequences of immunoglobulins and related cellular recognition molecules. Maximum homology was found between an active site of thymopoietin and homologous regions of IgG, IgE, beta 2-microglobulin, HLA-B7 and the Thy-1 antigen predicted to mediate immunologic activities, including Fc receptor binding. These results suggest that thymopoietin may be derived from approximately one half of the ancestral domain common to immunoglobulins and related immune system molecules and provide further support for a general theory that predicts the location of active sites in molecules of the immune system.