Shank protein

     Chromosome 22q13 deletion syndrome (22q13DS) is a neurobehavioral syndrome marked by neonatal hyptonia, global developmental delay, and autism spectrum disorder (ASD) features.^[3] The SHANK3 gene is located within this region of chromosome 22. Studies have revealed that point mutations in SHANK3 can produce the entirety of neurodevelopmental symptoms associated with 22q13DS, accounting for 1% of all autism cases.^[5] At the molecular level, disruption of the full length Shank3 protein results in reductions in AMPA receptor mediated transmission and spine remodeling.^[4] Shank3 heterozygous mice, who are haploinsufficient for the Shank3 gene emitted fewer ultrasonic vocalizations during interactions with estrus female mice, a behavior reminiscent of that seen in Autism patients. Further, knockout mice of Shank have a decreased spine number, a diminished PSD size, decreased levels of proteins GKAP and Homer, and reduced synaptic transmission. Interestingly, overexpression of SHANK3 may also result in an ASD, supporting the hypothesis that Autism is caused by improper Excitatory/Inhibitory neuronal ratios in the brain.^[4] Measurements of broad miRNA expression levels in Autism patients uncovered dysregulated miRNAs for genes involved in ASDs like MeCP2, the cause of Rett Syndrome, NRXN-1, a gene implicated in ASDs, and Shank3, validating Shank3’s role in autism.^[6] Due to the marked reduction in AMPA mediated transmission in Shank3 mutants, compounds that enhance AMPA transmission (AMPAkinses) serve as potential therapeutic approaches to treating some ASDs.^[4]

     βPIX belongs to a group of guanine nucleotide exchange factors used by Rho GTPase family members, like Rac1 and Cdc42, which are known to regulate the actin cytoskeleton of synapses.^[7] PIX has an N-terminal Src homology 3 (SH3) domain which associates with PAK, a coiled-coil (CC) domain, which is critical for multimerization, and a C-terminal PDZ binding domain which interacts with the PDZ domain of Shank.^[7] The interaction of Shank with βPIX promotes the synaptic localization of βPIX and βPIX associated p21 Associated Kinase (PAK). Since PAK is known to regulate actin cytoskeletons and dendritic spines are actin-rich structures, it is believed that Shank recruits βPIX and associated proteins to spines to regulate the PSD.^[1]

     The contains 90 amino acids and folds into a compact consisting of a six-stranded β-sandwich flanked by two alpha helices.^[7] βPIX possess a via within its CC domain, a , and a at the C-terminus. Interestingly, only 1 Shank molecule is bound to the CC domain trimer of βPIX in an . The (BALL AND STICK AND SPHERE COMBO BURIED MODE) of βPIX forms a number of **hydrogen bonding and hydrophobic interactions** (FIGURE 2A) with the Shank PDZ domain. Shank3-Arg 679 forms the **most critical interaction** with βPIX, tightly binding Glutamate -3. Abolishing this interaction through mutagenesis completely eliminates the assembly. Upon binding of βPIX, the PDZ domain undergoes a significant . Lys 682 undergoes a nearly to make room for the βPIX PDZ binding domain.^[7]

     Shank proteins are positioned between scaffolding proteins that are bound to either neurotransmitter receptors or the actin cytoskeleton. This puts Shank proteins in a perfect position to nucleate the underlying structure of the PSD.^[2] The SAM domain of () to form large sheets composed of helical fibers stacked side by side. The proposed sheet structure with radially projecting protein interaction domains, is ideal architecture for a protein that must contact both membrane and cytoplasmic components at a synaptic surface.^[2] Models of this sort validate the importance of Shank3 as master scaffolding proteins and illustrate how slight mutations can disrupt an entire PSD and synaptic function.