The Sema domain, a protein interacting module, of Plexin A
Plexins serve as receptors of semaphorins and may be the ancestor of semaphorins. Members of the Plexin A subfamily are receptors for Sema1s, Sema3s, and Sema6s, and they mediate diverse biological functions including axon guidance, cardiovascular development, and immune function. Guanylyl cyclase Gyc76C and Off-track kinase (OTK), a putative receptor tyrosine kinase, modulate Sema1a-Plexin A mediated axon repulsion. Sema3s do not interact directly with plexin A receptors, but instead bind Neuropilin-1 or Neuropilin-2 toactivate neuropilin-plexin A holoreceptor complexes. In contrast to Sema3s, Sema6s do not require neuropilins for plexin A binding. In the complex, plexin As serve as signal-transducing subunits. An increasing number of molecules that interact with the intracellular region of Plexin A have been identified; among them are IgCAMs (in axon guidance events) and Trem2-DAP12 (in immune responses). The Sema domain is located at the N-terminus and contains four disulfide bonds formed by eight conserved cysteine residues. It serves as a ligand-recognition and -binding module.
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