Semaphorin/CD100 antigen <p>The Sema domain occurs in semaphorins, which are a large family of secreted and transmembrane proteins, some of which function as repellent signals during axon guidance. Sema domains also occur in a hepatocyte growth factor receptor, in SEX protein [<cite idref="PUB00000965"/>] and in viral proteins.</p><p>CD100 (also called SEMA4D) is associated with PTPase and serine kinase activity. CD100 increases PMA, CD3 and CD2 induced T cell proliferation, increases CD45 induced T cell adhesion, induces B cell homotypic adhesion and down-regulates B cell expression of CD23.</p><p>The Sema domain is characterised by a conserved set of cysteine residues,which form four disulphide bonds to stabilise the structure. The Sema domainfold is a variation of the beta propeller topology, with seven blades radiallyarranged around a central axis. Each blade contains a four-stranded (strands A to D) antiparallel beta sheet. The inner strand of eachblade (A) lines the channel at the centre of the propeller, with strands B andC of the same repeat radiating outward, and strand D of the next repeatforming the outer edge of the blade. The large size of the Sema domain is notdue to a single inserted domain but results from the presence of additionalsecondary structure elements inserted in most of the blades. The Sema domainuses a 'loop and hook' system to close the circle between the first and thelast blades. The blades are constructed sequentially with an N-terminal beta-strand closing the circle by providing the outermost strand (D) of the seventh(C-terminal) blade. The beta-propeller is further stabilised by an extensionof the N terminus, providing an additional, fifth beta-strand on the outeredge of blade 6 [<cite idref="PUB00015414"/>, <cite idref="PUB00015415"/>, <cite idref="PUB00015416"/>].</p><p>CD molecules are leucocyte antigens on cell surfaces. CD antigens nomenclature is updated at Protein Reviews On The Web (http://prow.nci.nih.gov/). </p>