Most integral membrane proteins are built according to a helical bundle-design with one or more apolar a-helices spanning the lipid bilayer. The membrane assembly of this class of proteins is guided by the apolar segments and their polar flanking regions. In particular, membrane proteins from a variety of sources have been shown to follow the positive inside-rule: positively charged residues abound in the non-translocated parts of the protein, but are scarce in the translocated parts. Recent work in a number of labs, including our own, has shown that addition or removal of positively charged amino acids from critical locations in the chain can have dramatic consequences for the topology of a membrane protein.
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