Folding patterns of porin and bacteriorhodopsin.

Porin spans the outer membrane of Escherichia coli with most of the protein embedded within the membrane. It lacks pronounced hydrophobic domains and consists predominantly of beta-pleated sheet. These observations require the accommodation of polar and ionizable residues in an environment that has a low dielectric constant. Owing to a currently limited understanding of the constraints governing membrane protein structure, a minimal approach to structure prediction is proposed that identifies segments causing polypeptides to reverse their direction (turn identification). The application of this procedure avoids hydrophobicity parameters and yields a model of porin which is in good agreement with all experimental data available. The presence of polar and ionizable residues within membrane boundaries implies a dense (saturating) network of hydrogen bond donor and acceptor groups. Application to a paradigm of hydrophobic membrane proteins, bacteriorhodopsin, reveals a pattern consistent with its alpha-helical folding. The postulated structure includes significantly more polar residues in the membrane domain than have been assumed previously, suggesting that there are also hydrogen bonding networks in bacteriorhodopsin. Extensive networks permeating protein interior and surfaces would explain the extraordinary stability and the tight interactions between functional units in the formation of crystalline arrays of both proteins.