Fig. 7.

Potential convergence in plexin and semaphorin evolution. (A) S/T-rich domains (red brackets), which are sites for O-linked glycosylation likely contributing to a glycocalyx surrounding cells, are present in choanoflagellate Sema-FN1 and also in mammalian Sema4D and Plexin-B1. These domains are likely acquired independently during evolution of semaphorin/plexin proteins. (B) Peptide cleavage (dotted red lines) of extracellular domain in the juxtamembrane area of the extracellular domain occurs in choanoflagellate Sema-FN1 by autoproteolysis of the SEA domain, and in Drosophila Plexin-B and human Plexin-B2 (and Plexin-B1, not shown) by proteolytic cleavage by pro-protein convertases. This cleavage may provide a “breaking point’ as a protective mechanism against excessive mechanical pulling forces. See fig. 1 for colors of domains.