Team:UANL Mty-Mexico/Project/capture

Capture

Arsenic capture is achieved by two proteins in our system: ArsR and rhMT. E. coli's ArsR has been previously overexpressed to develop a biological device for arsenic biorremediation  (Kostal et al. 2004). rhMT is a recombinant human metallothionein with an increased capability of arsenic capture -the highest known to date- able of binding up to 6 arsenic molecules (Ngu et al. 2006), in contrast with ArsR which can only bind 1 arsenic molecule. Thus rhMT will be the main chelating component of our biorremediator device.

Oligomerization

Oligomerization has been proved to enhance or reduce some proteins' function (Ali and Imperiali 2005). ArsR is able to repress pArsR only as a homo-dimer (Wu and Rosen 1993). On the other hand, oligomerization of some metallothioneins results in an enhanced arsenic binding capability without compromising the protein's expression (Hong et al. 2000). We had chemically synthesized rhMT's CDS suitable for oligomerization (BBa:00000).

Hong et al. also observed that oligomerized metallothioneins form inclusion bodies.  To avoid such aggregation, solubility tags such as MBP and GST, are usually added (Hammartörm et al. 2002). We looked for the better suited solubility tag for our oligomer. DelProposto et al. (2008) reported a monomeric version of Mocr protein from T7 bacteriophage as a solubility tag, which together with MBP, yielded a better solubilization when compared to other common tags. Nevertheless MBP tends to form disulfide bonds that could interfere with metallothionein activity, given that its capture site is highly cystein rich. Thus Mocr will be used as the solubility tag for oligomerized rhMT. 

Transport

Arsenic is a toxic compound that is imported by passive diffusion through the membrane of E. coli. In order to increase the uptake of this metalloid, it is planned the expression of the aquaglycerol porin GlpF, which facilitates import of glycerol as well as arsenic and antimony (Meng et al. 2004). Combined with the rhMT construct it is expected to obtain a system with increased capability to chelate arsenic considering that metallothioneins “neutralize” the charge and toxic effect of the corresponding ligand (Sigel et al. 2009).

Figure 2. Genetic circuits for arsenic capture and transport.