2 ANGSTROM CRYSTAL-STRUCTURE OF AN EXTRACELLULAR FRAGMENT OF HUMAN CD40 LIGAND

Background: The CD40 ligand (CD40L) is a member of the tumor necrosis factor (TNF) family of proteins and is transiently expressed on the su rface of activated T cells. The binding of CD40L to CD40, which is exp ressed on the surface of B cells, provides a critical and unique pathw ay of cellular activation resulting in antibody isotype switching, reg ulation of apoptosis, and B cell proliferation and differentiation. Na turally occurring mutations of CD40L result in the clinical hyper-IgM syndrome, characterized by an inability to produce immunoglobulins of the IgG, IgA and IgE isotypes. Results: We have determined the crystal structure of a soluble extracellular fragment of human CD40L to 2 Ang strom resolution and with an R factor of 21.8%. Although the molecule forms a trimer similar to that found for other members of the TNF fami ly, such as TNF alpha and lymphotoxin-alpha, and exhibits a similar ov erall fold, there are considerable differences in several loops includ ing those predicted to be involved in CD40 binding. Conclusions: The s tructure suggests that most of the hyper-IgM syndrome mutations affect the folding and stability of the molecule rather than the CD40-bindin g site directly. Despite the bet that the hyper-IgM syndrome mutations are dispersed in the primary sequence, a large fraction of them are c lustered in space in the vicinity of a surface loop, close to the pred icted CD40-binding site.