Sequence-dependent scale for translocon-mediated insertion of interfacial helices in membranes

Biological membranes consist of a lipid bilayer studded with integral and peripheral membrane proteins. Most alpha-helical membrane proteins require protein-conducting insertases known as translocons to assist in their membrane insertion and folding. While the sequence-dependent propensities for a helix to either translocate through the translocon or insert into the membrane have been codified into numerical hydrophobicity scales, the corresponding propensity to partition into the membrane interface remains unrevealed. By engineering diagnostic glycosylation sites around test peptide sequences inserted into a host protein, we devised a system that can differentiate between water-soluble, surface-bound, and transmembrane (TM) states of the sequence based on its glycosylation pattern. Using this system, we determined the sequence-dependent propensities for transfer from the translocon to a TM, interfacial, or extramembrane space and compared these propensities with the corresponding probability distributions determined from the sequences and structures of experimentally determined proteins.