Hypothesis about the function of membrane-buried proline residues in transport proteins

In a survey of the bilayer-spanning regions of integral membrane proteins, membrane-buried proline residues were found in nearly all transport proteins examined, whereas membrane-buried regions of nontransport proteins were largely devoid of intramembranous proline residues. When amino acids from the complete sequences of representative sets of transport and nontransport membrane proteins were analyzed for the distribution of proline residues between aqueous vs. membranous domains, proline was shown to be selectively excluded from membranous domains of the nontransport proteins, in accord with expectation from energetic and structural considerations. In contrast, proline residues in transport proteins were evenly distributed between aqueous and membranous domains, consistent with the notion that functional membrane-buried proline residues are selectively included in transport proteins. As cis peptide bonds involving proline arise in proteins and have been implicated in protein dynamic processes, the cis-trans isomerization of an Xaa-Pro peptide bond (Xaa = unspecified amino acid) buried within the membrane--and the resulting redirection of the protein chain--is proposed to provide the reversible conformational change requisite for the regulation (opening/closing) of a transport channel. Parallel to this function, the relatively negative character of the carbonyl groups of Xaa-Pro peptide bonds may promote their participation as intramembranous liganding sites for positive species in proton/cation transport processes.