Compositional homology of membrane-protein systems and membrane-associated proteins: comparison with milk fat globule membrane and "membrane"-derived xanthine oxidase.

Amino acid compositions of the milk fat globule membrane protein and plasma membrane protein from other sources, as well as the compositions of milk fat globule membrane-derived xanthine oxidase and selected plasma membranes-associated proteins were compared by statistical difference index. Additionally, the average hydrophobicity of xanthine oxidase and selected membrane proteins were compared. These comparisons indicate high orders of apparent compositional homology between the various plasma membranes and membrane-associated proteins. Because the biological functions of membrane proteins are widely diverse, it is speculated that their "relatedness" may reflect on evolutionary convergence to similar amino acid compositions, necessitated by their in situ environment--the lipoidal bilayer. However, compositional relatedness should not imply sequential homology.