Pseudo-oligomer formation between related soluble and membrane-bound proteins. Implications on the attachment of proteins to receptors and membranes and the evolution of transport proteins.

Several animal proteins exist in related soluble and membrane-bound forms. In two instances of cobalamin endocytosis the extracellular cobalamin-transport proteins and their cell membrane receptors have been shown to be related. Membrane proteins, transport systems and receptors are known to be aggregates or dimers of subunits. Many soluble proteins also form oligomers, a phenomenon whose physiological significance has been little discussed. It is suggested that the binding of soluble proteins to cell membranes is often due to formation of a pseudo-oligomer between the related soluble and membrane-bound proteins. The soluble protein may have evolved as the result of a duplication of the gene coding for the membrane protein or of activation of the gene for an enzyme liberating a soluble part of the membrane protein. The soluble transport proteins apparently evolved when the organism became multicellular and developed a milieu interieur . Pharmacological implications of the pseudooligomer theory and the evolution of protein and peptide hormones are discussed.