Neuroligin a type I membrane protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

, is a protein on the postsynaptic membrane that mediates synapse formation between neurons. Neuroligins mediate signaling across the synapse and affect the properties of neural networks by specifying synaptic functions. In humans, alterations in genes encoding neuroligins are implicated in autism and other cognitive diseases. It has been shown that neuroligin expression induces contact-mediated presynaptic differentiation in contacting axons

Axon

An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....

.

The extracellular domain of NLGN consists mostly of a region that is homologous to acetylcholinesterases

Acetylcholinesterase

"Acetylcholinesterase, also known as AChE or acetylcholine acetylhydrolase, is an enzyme that degrades the neurotransmitter acetylcholine, producing choline and an acetate group. It is mainly found at neuromuscular junctions and cholinergic nervous system, where its activity serves to terminate...

, but the amino acids

Amino acid

Amino acids are molecules containing an amine group, a carboxylic acid group and a side-chain that varies between different amino acids. The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen...

 important for catalysis in AChE are not conserved in NLGN, which lack esterase

Esterase

An esterase is a hydrolase enzyme that splits esters into an acid and an alcohol in a chemical reaction with water called hydrolysis.A wide range of different esterases exist that differ in their substrate specificity, their protein structure, and their biological function.- EC classification/list...

 activity. Furthermore, this AChE homologous region is crucial for the proper function of NLGN.

Neuroligins act as ligands for β-Neurexins

Neurexin

A neurexin is a presynaptic protein that helps to glue together neurons at the synapse. Neurexins are type I membrane proteins that can be classified into two types, α-NRXNs and β-NRXNs. The α-NRXNs are larger and have different amino-terminal extracellular sequences...

, which are located presynaptically. Neuroligin and β-neurexin "shake hands," resulting in the connection between the two neurons and the production of a synapse. Additionally, neuroligin has also been found to play a role in angiogenesis

Angiogenesis

Angiogenesis is the physiological process involving the growth of new blood vessels from pre-existing vessels. Though there has been some debate over terminology, vasculogenesis is the term used for spontaneous blood-vessel formation, and intussusception is the term for the formation of new blood...

.

Neuroligins also operate in honeybees and their functional roles in insects are likely similar to those in vertebrates.

Known neuroligin gene

Gene

A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s include NLGN1

NLGN1

Neuroligin-1 is a protein that in humans is encoded by the NLGN1 gene.-Interactions:NLGN1 has been shown to interact with NRXN1 and DLG4.-Further reading:...

, NLGN2

NLGN2

Neuroligin-2 is a protein that in humans is encoded by the NLGN2 gene.-Further reading:...

, NLGN3

NLGN3

Neuroligin-3 is a protein that in humans is encoded by the NLGN3 gene.-Further reading:...

, NLGN4X

NLGN4X

Neuroligin-4, X-linked is a protein that in humans is encoded by the NLGN4X gene.-Further reading:...

, and NLGN5.