Title: NMR solution structure of the C-terminal domain of the Fas apoptosis inhibitory molecule (FAIM)
Description: The Fas apoptosis inhibitory molecule (FAIM) is a recently discovered, soluble, cytosolic protein inhibitor of programmed cell death and is found in organisms throughout the animal kingdom. It is expressed in all tissue types, but shows brain-specific expression of an alternatively spliced long isoform that is upregulated during embryogenesis and persists in defined anatomical locations in the adult. The short isoform (FAIM-S) is known to be induced in B lymphocytes in response to stimulation of the surface-bound immunoglobulin receptor in the context of CD40 ligand exposure, thereby mediating resistance to signaling through the pro-apoptotic Fas receptor. Recently it was reported that expression of FAIM or other anti-apoptotic proteins can be used to arrest apoptosis of human keratinocytes exposed to Fas ligand, raising the prospect of using the biochemical properties of FAIM to improve the survival of skin allografts. There is evidence that isoforms of FAIM modulate signaling via Fas and the neurotrophin receptors TrkA and p75 NTR in neurons, but the precise mechanism of the activity or activities of FAIM remains unknown.

To obtain insights into the molecular structure and mechanism of FAIM, rational site-directed mutagenesis and buffer optimization are used in this work to obtain protein constructs stable at the concentrations necessary for nuclear magnetic resonance (NMR) spectroscopy. FAIM-S is shown via limited proteolysis, protein sequencing, and NMR spectroscopy to consist of two independently folding domains in contact with one another. The NMR solution structure of the C-terminal domain of FAIM is solved in isolation, validated using residual dipolar coupling data, and revealed to be a novel protein fold, a non-interleaved seven-stranded beta sandwich. Analysis of the structure and sequence reveals several residues that are strictly conserved, but non-structural, and are therefore likely to be involved in functionally significant interactions with the N-terminal domain or other binding partners. Chemical shift perturbation and site-directed spin labeling are used to elucidate contacts made between the N- and C-terminal domains.