Semaphorins structure

Both plexins and semaphorins are structurally characterized by the presence of an extracellular sema domain which contains a seven-blade 3-propeller. The... 

The Sema domain consisting of about 500 amino acids is characterized by highly conserved cysteine residues that form intramolecular disulfide bonds. Crystal structures have revealed that the Sema domain folds in the manner of the (3 propeller topology which is also found in integrins or the low-density lipoprotein (LDL) receptors. Sema domains are found in semaphorins, plexins and in the receptor tyrosine kinases Met and Ron. 

Two crystal structures of semaphorin family members were recently reported including the structure of an observed in vivo proteolytic form of semaphorin-3A (Sema3A-65K, including the complete sema domain and part of the PSI domain) (Antipenko et at., 2003) and of the nearly full-length semaphorin-3D including the sema, PSI, and Ig domains (Love et al., 2003). 

Like most of the other known propeller structures, semaphorin uses a Velcro system to close the circle between the first and the last blades (Fulop and Jones, 1999 Paoli, 2001). The Serna s Velcro (Fig. 8A) is stronger than usually observed. Indeed, the N-terminus of the molecule, in addition to contributing the fourth antiparallel j3 strand of the 7th blade (purple), wraps further around and contributes a fifth, this time parallel strand to the 6th blade (blue). 

Love, G. A., Harlos, K., Mavaddat, N., Davis, S. J., Stuart, D. L, Jones, E. Y, and Esnouf, R. M. (2003). The ligand-binding face of the semaphorins revealed by the high-resolution crystal structure of SEMA4D. Nature Struct. Biol. 10, 843-848. 

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