2.B.23 The Homotetrameric Transmembrane Zn²⁺/Co²⁺:Proton Synthetic Antiporter, Rocker (Rocker) Family 

The computational design of functional integral membrane proteins from first principles represents a challenge. Joh et al. 2014 designed a membrane-spanning, four-helical bundle that transports first-row transition metal ions, Zn²⁺ and Co²⁺, but not Ca²⁺, across membranes. The conduction path was designed to contain two di-metal binding sites that bind with negative cooperativity. X-ray crystallography and solid-state and solution nuclear magnetic resonance indicated that the overall helical bundle is formed from two tightly interacting pairs of helices, which form individual domains that interact weakly along a more dynamic interface. Vesicle flux experiments showed that as Zn²⁺ diffuses down its concentration gradient, protons are antiported. Thus, a membrane protein with predefined structural and dynamic properties was artificially synthesized (Joh et al. 2014).  The homotetrameric bundle has two 2His4Glu di-Zn²⁺–binding sites with each site formed by the coalescence of an EXXH motif near the N termini of two helices and a Glu near the C termini of the other two helices. The resulting 25 residue-long antiparallel homotetrameric bundle shares no structural similarities with YiiP (TC# 2.A.4.7.1), the only natural Zn²⁺ transporter with a high-resolution structure. The crystal structures have PDB acc#s of 4P6L and 4P6J.  The sequence of the 26 aa peptide is:

YXKEIAHALFSALFALSELYIAVRYX

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