====== Guanine nucleotide exchange factor ======
[[Guanine]] [[nucleotide]] exchange factors (GEFs) are [[protein]]s or [[protein domain]]s that activate monomeric [[GTPase]]s by stimulating the release of [[guanosine diphosphate]] (GDP) to allow binding of [[guanosine triphosphate]] ([[GTP]]).

A variety of unrelated structural domains have been shown to exhibit guanine nucleotide exchange activity. Some GEFs can activate multiple GTPases while others are specific to a single GTPase. 
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Guanine nucleotide exchange factors (GEFs) play important roles in many cellular processes, including regulation of the structural plasticity of dendritic spines. A GEF protein, adenomatous polyposis coli-stimulated GEF 1 (Asef1, ARHGEF4) is highly expressed in the nervous system. However, the function of [[Asef1]] has not been investigated in neurons. 

Oh et al., present evidence showing that [[Asef1]] negatively regulates the synaptic localization of postsynaptic density protein 95 (PSD-95) in the excitatory synapse by inhibiting Staufen-mediated synaptic localization of PSD-95. Accordingly, Asef1 expression impairs synaptic transmission in hippocampal cultured neurons. In addition, neuronal activity facilitates the dissociation of Asef1 from Staufen in a phosphoinositide 3 kinase (PI3K)-dependent manner. Taken together, our data reveal Asef1 functions as a negative regulator of synaptic localization of PSD-95 and synaptic transmission
((Oh JY, Lim CS, Yoo KS, Park HJ, Park YS, Kim EG, Lee YS, Kaang BK, Kim HK.
Adenomatous polyposis coli-stimulated GEF 1 (Asef1) is a negative regulator of
excitatory synaptic function. J Neurochem. 2018 Aug 20. doi: 10.1111/jnc.14570.
[Epub ahead of print] PubMed PMID: 30125942.
)).