Flexible programs for the prediction of average amphipathicity of multiply aligned homologous proteins: application to integral membrane transport proteins.

Simple flexible programs (TREEMOMENT and PILEUPMOMENT) are described for depicting the average amphipathicity (hydrophobic moment) along multiply aligned sequences of a family of evolutionarily related proteins. The programs are applicable to any number of aligned sequences and can be set for any desired angle corresponding to a residue repeat unit in a protein secondary structural element such as 100 degrees per residue for an alpha-helix or 180 degrees per residue for a beta-strand. These programs can be used to identify amphipathic regions common to the members of a protein family. The use of these programs is exemplified by showing that some families of integral membrane transport proteins (i.e. permeases of the bacterial phosphotransferase system (PTS) and the anion exchangers of animals) exhibit strikingly amphipathic alpha-helical structures immediately preceding the first hydrophobic transmembrane segment of their membrane-embedded domain(s). Other families, such as the major facilitator superfamily of uniporters, symporters and antiporters, do not exhibit this structural feature. The amphipathic structures in PTS permeases have been implicated in membrane insertion during biogenesis.