CD100 is a leukocyte semaphorin

CD100 was originally described as an activation molecule on the surface of human T lymphocytes. Its triggering through distinct epitopes leads to diff erent signals of costimulation with phorbol myristate acetate (PMA) or with CD3 and CD2. Interestingly, CD100 was shown to associate with different pa rtner molecules in T cells. First, CD100 can associate with CD45, a key mol ecule with protein tyrosine phosphatase activity involved in T-cell transdu ction: this association is physical and has functional consequences for bot h partners. Second, CD100 interacts in its cytoplasmic domain with a Ser/Th r kinase for which it represents a preferential substrate. Recently, CD100 was identified as a member of the semaphorin gene family. This family compr ises approximately 20 structurally related proteins. The first semaphorins were identified in the developing nervous system. Function has been shown f or only some of them and involves repulsion during growth cone guidance. Si nce CD100 was the first semaphorin identified in the immune system, this ra ises the possibility of the involvement of members of the semaphorin family in other physiological phenomena outside the nervous system.