A multidisciplinary team led by researchers in Israel and the US has used X-ray crystallography to build on their bioinformatics studies and so reveal a unique intertwined protein structure. the protein could be a novel target for more effective diabetes drugs with fewer side effects.

MitoNEET was previously identified as a putative site for diabetes drugs, but the determination of the protein's three-dimensional structure makes it a clearer target for small molecule drug designers, thanks to work by University of California at San Diego (UCSD) biophysical chemist Patricia Jennings and physicist Mark Paddock, and their colleagues.

"This is the first time that a protein like this has ever been found," says Paddock, "It is a brand new structure, a unique beast, which makes it an exciting target for structure-based drug design." The structure shows two protomers intertwined to form a unique dimeric structure, explain the researchers, this constitutes a new fold not only among the 650 known Fe-S proteins structures but also among all known proteins. The results build on work by Sandra Wiley, Anne Murphy and Jack Dixon in UCSD's School of Medicine, and in collaboration with Herbert Axelrod and Aina Cohen at the Stanford Synchrotron Radiation Laboratory and Rachel Nechushtai at the Hebrew University of Jerusalem, Israel.

MitoNEET reveals itself as a loosely bound iron-sulfur protein, a characteristic that may be related to its behaviour. The team found that when MitoNEET binds to the thiazolidinedione insulin-sensitizing drug Actos, the binding process stabilises the cluster, tightening the bindings.

Previously, researchers thought this drug had an entirely different mode of action involving another protein. However, the work of Jerry Colca, now at Metabolic Solutions Development Company in Kalamazoo, Michigan, hinted at a possible mechanism involving MitoNEET. The current work inspired by Colca's findings, suggests that the thiazolidinediones may protect cells from the damaging effects of free iron by keeping the iron-sulfur cluster bound to MitoNEET. The protein may also act as a monitor of oxidative stress within the cell.

The researchers explain how bioinformatic analysis revealed MitoNEET to be a member of a small family of proteins containing a protein domain described as a CDGSH-type zinc finger, although lacking a zinc atom, it contains instead a cluster of iron-sulfur atoms. The analysis revealed a conserved protein sequence: C-X-C-X₂-(S/T)-X₃-P-X-C-D-G-(S/A/T)-H. This, say the researchers, is "a defining feature of this unique family of proteins and is likely involved in iron binding."

"MitoNEET may be an example of an ever increasing group of proteins found to have more than one function," adds Paddock, "I think we are at the beginning of what is sure to be an interesting and biologically important puzzle."

The team is now using the new structural information to design sophisticated experiments for testing structure-based drug design with MitoNEET as the target, working with Colca's group.

Related links:

• Proc Natl Acad Sci, 2007, 104, 14342-14347
• Jennings Page 

Article by David Bradley