Thursday, March 17, 2011

Transporter Literature

Publications regarding this specific divalent metal ion transporter are still in their infancy, which is part of what makes this protein so fascinating. There is so much about it that is still unknown. The following summaries describe much of what is known about the transporter.
CorA proteins are widely agreed to be the main Mg2+ transporters in both bacteria and archaea. Because it is a membrane spanning protein, its structure was difficult to determine. Nonetheless, the structure has been determined. It is a cone shape with the large portin of the cone opening to the cytoplasm.
It has also been determined by the crystal structure of the protein that there are two potential binding sites for regulation. Both of these sites are found in the N-terminal domain. It is assumed that ions are allowed to flow in both directions as is the case in all CorA proteins.
The protein in question was first found in Thermotoga maritima, which is a rod shaped bacterium found in geothermal vents in the ocean. CorA proteins are not only found in bacteria and archaea, but also in humans and other eukaryotes lending even more validity to the study of this system.

(Lunin VV, Dobrovetsky E, Khutoreskaya G, et al. (April 2006). "Crystal structure of the CorA Mg2+ transporter". Nature 440 (7085): 833–7)
It has been found that CorA actually has two known substrates, Mg2+ and Co2+. Both of these bind to an interhelical loop that is on the small part of the protein that is exposed to the periplasmic space. This loop is also very negative in character, leading to the belief that the protein increases concentration of substrate near the mouth of the channel before transport. In addition it has been determined that binding in this location happens in the magnesium’s hydrated state. This means that the protein has not yet gone through the energetically costly process of dehydration.
CorA also has the specific inhibitor Co(III) hexamine chloride (HexCo3+). It has been determined that the interhelical loop that is responsible for substrate binding is not the main point of binding for the HexCo3+. This was determined by discovering that the loop holds a relatively low interaction with both the substrate and the inhibitor would not bind tightly enough to properly inhibit activity.
(Jian Hu Mukesh Sharma Huajun Qin Fei Philip Gao Timothy A. Cross June 5, 2009 The Journal of Biological Chemistry, 284,15619-15628)

The channel is gated in the cross membrane region by large hydrophobic residues. The channel, being formed by a five helical member group houses these hydrophobic group about half way up the channel. On the cytoplasmic side of the funnel, there is a large ring of positively charged residues. This gives rise to the idea that CorA channels are mediated from the divalent ions leaving the cell. Inside the pore, there are also two spots for supposed Mg2+ binging that suppose that cellular concentrations of the ion regulate pore gating activity.
(Vladimir V. Lunin et. al Nature 440, 833-837 (6 April 2006) | doi:10.1038/nature04642; Received 27 October 2005; Accepted 9 February 2006)

Tuesday, March 1, 2011

The Transporter -Inside and Out-

Feel free to click on the pictures, they get a lot bigger and a lot cooler.

Rainbow showing different subunits.

Alpha Helices and Beta Sheets with channel view. 

Spectrum b-factors coloring.

Atomwise view.