Until recently, all that was known about the function of the BIR was that its physical presence was necessary for IAP function. New research on the structure and fuction of the BIR domain has recently been published, and is the focus of this web site. Recently, scientists have been trying to discover the answer to this question. It was known that the presence of the BIR is necessary for IAP function, but scientists were unsure of what residues were critical for its function. Truncating 24 residues from the amino terminus of the protein resulted in total loss of caspase inhibition.
Caspases are a specific set of death inducers that some IAPs inhibit.
The authors unexpectedly determined that residues in the linker region between the BIR domains were very important for caspase inhibition. However, the linker region alone is not capable of inhibiting caspases, so the authors reasoned that this region is important for binding between the IAP and the caspase. This observation is evidence that the linker region is important for caspase binding. Structure of BIR2 of XIAP BIR2 of XIAP has a structure that is typical of a BIR domain. Four amino acids chelate the zinc atom and stabilize the overall fold of the core.