Metal removal by metallothionein and an acid phosphatase PhoN, surface-displayed on the cells of the extremophile, Deinococcus radiodurans

Highlights: • S-layer proteins (Hpi, SlpA) of D. radiodurans used as membrane localization vehicles. • Expression of Hpi- metallothionein fusion protein enhanced cadmium binding to cells. • Hpi protein per se bound uranium both in vivo and in vitro, but not cadmium. • Recombinant Hpi-PhoN and SLH-PhoN proteins efficiently precipitated uranium ex vivo. • Cd/U bioremediation by chimeric S-layer protein fusions with SmtA/PhoN established. The utility of surface layer proteins (Hpi and SlpA) of the radiation resistant bacterium, Deinococcus radiodurans, was investigated for surface display and bioremediation of cadmium and uranium. The smtA gene, from Synechococcus elongatus (encoding the metal binding metallothionein protein), was cloned and over-expressed in D. radiodurans, either as such or as a chimeric gene fused with hpi ORF (Hpi-SmtA), or fused to the nucleotide sequence encoding the SLH domain of the SlpA protein (SLH-SmtA). The expressed fusion proteins localized to the deinococcal cell surface, while the SmtA protein localized to the cytoplasm. Recombinant cells surface-displaying the SLH-SmtA or Hpi-SmtA fusion proteins respectively removed 1.5–3 times more cadmium than those expressing only cytosolic SmtA. The deinococcal Hpi protein layer per se also contributed to U binding, by conferring substantial negative charge to deinococcal cell surface. The ORF of an acid phosphatase, PhoN was fused with the hpi or SLH domain DNA sequence and purified. Isolated Hpi-PhoN and SLH-PhoN, immobilized on deinococcal peptidoglycan showed efficient uranium precipitation (446 and 160 mg U/g biomass used respectively). The study demonstrates effective exploitation of the deinococcal S layer protein components for (a) cell surface-based sequestration of cadmium, and (b) cell-free preparations for uranium remediation.