| Type: | Domain | Name: | Sema domain |
| Description: | 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 plexins [], receptors for multiple classes of semaphorins, in hepatocyte growth factor receptor, and in viral proteins [].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 radially arranged around a central axis. Each blade contains a four-stranded (strands A to D) antiparallel beta sheet. The inner strand of each blade (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 repeat forming 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 additional secondary structure elements inserted in most of the blades. The Sema domainuses a 'loop and hook' system to close the circle between the first and the last 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 outer edge of blade 6 [, , ]. | Short Name: | Semap_dom |
