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Introduction
Zinc transporter YiiP is an integral membrane protein found in the membrane of Esherichia coli. YiiP is a member of the cation diffusion facilitator family (LINK). Members of this family occur all throughout the biological realm. These diffusion facilitators export divalent transition metal ions from the cytoplasm to the extracellular space (CITE https://www.bnl.gov/isd/documents/71335.pdf). They work to regulate the amount of divalent metals inside of the cell, which is biologically relevant because while these metals are necessary for different biological functions, but in excess amounts can prove fatal to the cell.
Structure
YiiP is composed of dimers with each monomer consisting of 238 residues. YiiP has a "Y" shape conformation with two different functional conformations. A total of six helices comprise the transmembrane portion of each monomer (GREEN). Four of these helices are bundled together while the remaining two are oriented antiparallel to the bundle (GREEN). Movement of these helices play a role in the function of YiiP. An interlocked salt bridge connects the two domains with the Lys77 and the Asp207 from each domain - visualized here: (GREEN).This salt bridge acts as the hinge for the conformational changes that YiiP undergoes.Hydrophobic residues (GREEN) beneath the salt bridge further stabilize the two domains in the v-shaped void where the domains connect. YiiP has three zinc binding sites, two of which are known to play an active role in the function of YiiP. Site A (link) sits in extracellular space outside of the cell, while site C is situated inside of the cell to act as a sensor of intracellular zinc concentrations.
Function
YiiP functions to export zinc out of the cytoplasm of cells in order to regulate zinc levels within the cell. YiiP works via secondary transport and two conformations to sense zinc in the cell and transport zinc out of the cell into extracellular space using an electrochemical gradient of H+ ins antiported with zinc. The presence of cytoplasmic zinc (bound in the c-terminus domain) activates conformational changes that translocate the metal ions across the membrane and out of the cell.
Structural highlights
The Interlocking Salt Bridge
Active Sites
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