1.C.80 The Cytotoxic Major Fimbrial Subunit (MrxA) Family

Banerjee et al. (2006) have purified a fimbrial shaft protein (MrxA) of Xenorhabdus nematophila. The soluble monomeric protein lysed larval hemocytes of Helicoverpa armigera. Osmotic protection of the cells with polyethylene glycol suggested that the 17-kDa MrxA subunit makes pores in the target cell membrane. The internal diameter of the pores was estimated to be >2.9nm. Electron microscopy confirmed the formation of pores by the fimbrial subunit. The MrxA protein oligomerizes in the presence of liposomes. Electrophysiological studies confirmed that MrxA forms large, voltage-gated, passive-diffusion channels in lipid bilayers. (Banerjee et al., 2006).

MrxA is a small protein of 179 aas with an N-terminal hydrophobic peak (residues 1-20) as well as a less hydrophobic central region (residues 100-120). In BLAST searches it retrieves hundreds of fimbrial/pilin/adhesin subunits including the P-pilus rod subunit, PapA, of E. coli, for which the X-ray structure is available (Verger et al., 2007).  The mrxA gene has been transferred into the genomes of tobacco and tomato via Agrobacterium-mediated transformation where it was stably expressed.  The transgenic plants appeared healthy and phenotypically normal but were toxic to the insects, showing 100 % insect mortality and reduced damage of the transgenic plants (Kumari et al. 2015). Thus this pilin subunit may be used for the control of H. armigera.

The transport reaction catalyzed by MrxA is:

Ions and small molecules (in)  Ions and small molecules (out)