Department of Biophysics, Bose Institute, Kolkata 700054
Proteins are incredible molecules. They perform a variety of functions at the molecular level like catalyzing chemical reactions, acting as molecular motors, transporting molecules, transducing signals or proving structural framework at the cellular level. Considerable proportion of proteins are embedded in membranes. Wallin and von Heijne (Protein Sci. 1998, 7, 1029-1038) estimated that about 20-30% of ORFs encode helix-bundle class of membrane proteins. Many transmembrane proteins act as gatekeepers, selectively allowing specific ions and other molecules to cross the hydrophobic membrane barrier. One such protein is the mycobacteriophage D29 belonging to transmembrane class of proteins of holin family (Wang et al. Annu Rev Microbiol. 2002, 54, 799–825) that forms permanent holes in the inner membrane of mycobacteria causing host cell death (Kamilla & Jain FEBS J. 2016, 283, 173-90).
The first transmembrane domain of D29 holin is unique in more than one way. It undergoes a helix to beta-hairpin transition in lipidic micelle and is sufficient to elicit bacterial host cell death. By inverting the sequence of this 28-residue TM1 domain, about the central Gly-Pro motif, Lella and Mahalakshmi (J. Phys. Chem. Lett. 2016, 7, 2298−2303) were able to re-engineer the membrane-breaking TM1 into a transmembrane nanopore ion channel.
There is considerable interest in engineering peptide-based synthetic membrane channels. For example, peptide-based channels such as cyclic D,L-a-peptides were shown to self assemble in bacterial membranes making the membrane permeable (Fernandez-Lopez et al.,Nature 2001,412, 452-455). Peptide rings, stacked in the membrane, have been shown to display channel-mediated ion-transport activity (Ghadiri et al.,Nature 1994, 369, 301-304). Recently there have been efforts to re-engineer the function of naturally occurring transmembrane proteins. For example, a re-engineered α-hemolysin, which normally allows single stranded DNA to pass through, could unzip double-stranded DNA in the channel (Liu et al.,J. Phys. Chem. Lett. 2011, 2,1372-1376). In this context, conversion of a membrane breaker peptide to a nanopore ion channel by Lella and Mahalakshmi is an important contribution. The mechanism behind the functional shift, upon sequence inversion, is not yet clear but a number of factors governed by electrostatics and hydrophobic effect may be important. The study opens up an exciting area of natural protein inspired designs with very useful functions and potential applications.