Semaphorins are a class of secreted and membrane proteins that act as axonal growth cone guidance molecules. Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl An axon or nerve fiber is a long slender projectionof a nerve cell or Neuron, that conducts electrical impulses away from the neuron's Cell A growth cone is a dynamic Actin -supported extension of a developing Axon seeking its synaptic target They primarily act as short-range inhibitory signals and signal through multimeric receptor complexes. In Biochemistry, a receptor is a Protein molecule embedded in either the Plasma membrane or Cytoplasm of a cell to which a mobile signaling They are usually cues to deflect axons from inappropriate regions, especially important in neural system development. The nervous system is a Network of specialized cells that communicate information about an animal's surroundings and itself The major class of proteins that act as their receptors are called plexins. A plexin is a protein which acts as a receptor for Semaphorin.
Every Semaphorin is characterised by the expression of a specific region of about 500 amino acids called the Sema domain. The Sema domain is a Structural domain of Semaphorins which are a large family of secreted and transmembrane proteins some of which function as repellent
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Classes
There are 8 major classes of Semaphorins. The first 7 are ordered by number, from Class 1 to Class 7. The eighth group is Class V, where V stands for virus. A virus (from the Latin virus meaning Toxin or Poison) is a sub-microscopic infectious agent that is unable 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.
In humans, the genes are:
- SEMA3A, SEMA3B, SEMA3C, SEMA3D, SEMA3E, SEMA3F, SEMA3G
- SEMA4A, SEMA4B, SEMA4C ("SEMAF"), SEMA4D, SEMA4E, SEMA4F, SEMA4G
- SEMA5A, SEMA5B
- SEMA6A, SEMA6B, SEMA6C, SEMA6D,
- SEMA7A
Classes 1 and 6 are considered to be homologues of each other, since they are both membrane bound in vertebrates and invertebrates. The same applies to Classes 2 and 3, because they are both secreted proteins specific to phylum. A phylum ( Plural: phyla) is a Taxonomic rank between Kingdom and above Class.
Each Class of Semaphorin has many subgroups of different molecules that share similar characteristics. For example, Semaphorin Class 3 ranges from Sema-3A to Sema-3E. Each one of the Class 3 Semaphorins are expressed in different regions of the body during development and whilst some encourage the growth of axons, others inhibit it.
Semaphorin receptors
Different semaphorins use different types of receptors:
- Most Semaphorins use receptors in the group of proteins known as Plexins. A plexin is a protein which acts as a receptor for Semaphorin.
- Class 3 Semaphorins utilise a group of proteins known as Neuropilins as co-receptors with Plexins
- Class 7 Semaphorins are thought to use Integrins as their receptors. Neuropilin is a Protein receptor active in Neurons. Overview There are two forms of Neuropilins NRP-1 and NRP-2 Integrins are Cell surface receptors that interact with the Extracellular matrix (ECM and mediate various intracellular signals.
Functions
Semaphorins are very versatile. Their discovery was in regards to axon guidance in the limb buds of grasshoppers in 1992, but since then, it has been discovered that Semaphorins have a role in many processes. They not only guide axons in development, but also have major roles in immune function (Classes 4 and 7) and the development of bones. One of the most versatile semaphorin classes is the Sema-3 class, and specifically Sema-3A.
Sema-3A is responsible for repelling axons from the dorsal root ganglia, facial nerves, vagal nerves, olfactory-sensory, cortical nerves, hippocampal nerves, cerebellar, etc.
External links
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