Evidence that the translocon may function as a hydropathy partitioning filter.

Developing a greater understanding of the function of the translocon-and the source of its selectivity for transmembrane helix insertion-are important steps toward deciphering the role of disease-causing mutations in membrane regions. To address these phenomena, we have prepared a library of helix-loop-helix ("hairpin") constructs derived from helices 3 and 4 of the first membrane domain of CFTR, in which position 232 was mutated individually to each of the 20 commonly-occurring amino acids. Using retention times on a reverse phase-HPLC C18 column to mimic the process of hairpin partitioning, we have quantitatively determined a hydropathy scale in the context of a bona fide membrane protein fragment that correlates to an in vivo hydropathy scale with r=-0.78-a value that rises to r=-0.92 when Asp and Glu are excluded due to protonation effects. Our results provide evidence that the translocon may act as a facilitator in the insertion selection process, effectively allowing the bilayer to "decide" through favorable non-polar solvation whether or not to allow a translocating helix to enter the membrane.