Semaphorin

Semaphorins are a class of secreted and membrane proteins that were originally identified as axonal growth cone guidance molecules. They primarily act as short-range inhibitory signals and signal through multimeric receptor complexes. Semaphorins are usually cues to deflect axons from inappropriate regions, especially important in neural system development. The major class of proteins that act as their receptors are called plexins, with neuropilins as their co-receptors in many cases. The main receptors for semaphorins are plexins, which have established roles in regulating Rho-family GTPases. Recent work shows that plexins can also influence R-Ras, which, in turn, can regulate integrins. Such regulation is probably a common feature of semaphorin signalling and contributes substantially to our understanding of semaphorin biology.

Every semaphorin is characterised by the expression of a specific region of about 500 amino acids called the sema domain.

Semaphorins were named after the English word Semaphore, which originated from Greek, meaning sign-bearer.

The Semaphorins are grouped into eight major classes based on structure and phylogenetic tree analyses. The first seven are ordered by number, from class 1 to class 7. The eighth group is class V, where V stands for virus. Classes 1 and 2 are found in invertebrates only, whilst classes 3, 4, 6, and 7 are found in vertebrates only. Class 5 is found in both vertebrates and invertebrates, and class V is specific to viruses.

Classes 1 and 6 are considered to be homologues of each other; they are each membrane bound in invertebrates and vertebrates, respectively. The same applies to classes 2 and 3; they are both secreted proteins specific to their respective taxa.

Each class of Semaphorin has many subgroups of different molecules that share similar characteristics. For example, Class 3 Semaphorins range from SEMA3A to SEMA3G.

In humans, the genes are:

Different semaphorins use different types of receptors:

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