Shank protein

spinqualitylabelsall models Structure of Shank Family Proteins, 3l4f Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

      Shank Family Proteins are scaffolding proteins found in the postsynaptic density (PSD) of excitatory synapses. The PSD, a structure within dendritic spines and within the postsynaptic membrane, contains a complex assembly of proteins which organize neurotransmitter receptors and regulatory elements within a cytoskeletal matrix.^[1] It coordinates communication of incoming signals to cytoplasmic targets and contributes to neuronal plasticity by readily changing its composition and structure in response to neural signals.^[2] Shank proteins are believed to function as master organizer of the PSD owing to their ability to recruit and form multimeric complexes with postsynaptic receptors, signaling molecules, and cytoskeletal proteins, like AMPA, Neuroligin and NMDA glutamate receptors.^[3] Within the PSD, there are over 300 individual shank molecules, representing 5% of the total protein molecules within the PSD.^[4] Shanks contain five domains for protein-protein interactions, including an ankyrin repeat domain, used to bind acting regulating proteins, an Src homology 3 (Sh3) domain, used to bind AMPA receptors, a PDZ domain, used to bind G protein coupled receptors, several proline-rich domains, and a C-terminal SAM domain, which is responsible for mediating Shank multimerization. (See Image)^[1] Functionally, Shank is involved in the maturation of dendritic spines and is able to induce spine formation in neurons.^[3]

Chromosome 22q13 Deletion Syndrome

     Chromosome 22q13 deletion syndrome (22q13DS) is a neurobehavioral syndrome marked by neonatal hyptonia, global developmental delay, and autism spectrum disorder (ASD) features.^[3] The Shank-3 gene is located within this region of chromosome 22. Studies have revealed that point mutations in Shank-3 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 Shank-3 protein results in reductions in AMPA receptor mediated transmission and spine remodeling.^[4] Shank-3 heterozygous mice, who are haploinsufficient for the Shank-3 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 Shank-3 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 Shank-3, validating Shank-3’s role in autism.^[6] Due to the marked reduction in AMPA mediated transmission in Shank-3 mutants, compounds that enhance AMPA transmission (AMPAkinses) serve as potential therapeutic approaches to treating some ASDs.^[4]

βPIX Structure

     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]

Shank Family Protein Structure

     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 of βPIX forms a number of with the Shank PDZ domain. Shank-3-Arg 679 forms the with βPIX, tightly H-Bonding Glutamate 643, forming 2 weak bonds with Phe 696, and Van der Waals interactions with ring of Phe 696. 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 Oligomerization

     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 of Shank-3 can () 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 Shank-3 as master scaffolding proteins and illustrate how slight mutations can disrupt an entire PSD and synaptic function.