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Semaphorins, proteins characterized by an extracellular sema domain, regulate axon guidance, immune function and angiogenesis. The crystal structure of SEMA4D (residues 1-657) shows the sema topology to be a seven-bladed beta-propeller, revealing an unexpected homology with integrins. The sema beta-propeller contains a distinctive 77-residue insertion between beta-strands C and D of blade 5. Blade 7 is followed by a domain common to plexins, semaphorins and integrins (PSI domain), which forms a compact cysteine knot abutting the side of the propeller, and an Ig-like domain. The top face of the beta-propeller presents prominent loops characteristic of semaphorins. In addition to limited contact between the Ig-like domains, the homodimer is stabilized through extensive interactions between the top faces in a sector of the beta-propeller used for heterodimerization in integrins. This face of the propeller also mediates ligand binding in integrins, and functional data for semaphorin-receptor interactions map to the equivalent surface.

Original publication

DOI

10.1038/nsb977

Type

Journal article

Journal

Nat Struct Biol

Publication Date

10/2003

Volume

10

Pages

843 - 848

Keywords

Amino Acid Sequence, Antigens, CD, Crystallography, X-Ray, Dimerization, Humans, Ligands, Membrane Glycoproteins, Molecular Sequence Data, Protein Binding, Protein Structure, Tertiary, Semaphorins