Sema Domain

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. 

Class 3 semaphorins are the best functionally characterized group. The presence of a sema domain and dimerization are the minimal... 

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). 

Fig. 8. Structures of Sema3A-65K and Sema4D. (A) The structure of Sema3A-65K viewed from the top face of the molecule. The molecular surface (semi-transparent) is also indicated. The individual Sema3A-65K pseudo-repeats corresponding to the individual j3 propeller blades are colored (from -N to -C terminus) in red (1), orange (2), yellow (3), green (4), cyan (5), blue (6) and magenta (7). (B) The structure of Sema4D homodimer. The sema domain is in red, the PSI - in green, and the Ig - in blue. (See Color Insert.)...

Fig. 9. Non-covalent semaphorin dimerization mediated by the sema domains. (A) The Sema3A-65K dimer in the asymmetric unit of the crystals. (B) The interacting sema domains in the Sema4D dimer viewed in the same orientation as in (A). The expansive dimerization interface (total buried area of approximately 3000 A ) is generated by the approximation of four protruding loops from each monomer. These loops, located at the top face of the fi propeller are also implicated in interactions with the semaphorin receptors.

Takahashi, T., and Strittmatter, S. M. (2001). PlexinAl autoinhibition by the plexin sema domain. Neurcm 29, 429-439. 

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