Structure-Function Studies of Scaffolding Proteins Involved in the Formation of Neuronal Connections: AIDA1 and CASKIN2
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Modular proteins serve assembly platforms and often actively regulate cellular signaling events. An intrinsic diversity of interaction modules, typical for scaffolding proteins, facilitates the organization of numerous protein partners into signaling cascades, contributing to the spatial precision, efficiency and fidelity of signal transduction. The role of complex molecular dynamics of postsynaptic density (PSD) proteins in synaptic plasticity is relatively new and yet to be fully understood. AIDA-1 is one of the most abundant members of the PSD protein family. Growing research evidence of multiple protein partnerships suggests that AIDA-1 functions as an essential PSD molecular scaffold, NMDA receptor functional mediator, and a synapse-to-nucleus messenger. The NMR structure of AIDA-1 carboxy-terminal phosphotyrosine binding domain (PTB), presented in this study, provided the structural basis for comparative analysis with the other PTB domain-containing proteins, Fe65 and X11/Mint1, that also participate in amyloid beta precursor protein (APP) processing and amyloid beta peptide (A) secretion. A combination of peptide arrays, mutagenesis and fluorescence based assays was employed to characterize the affinity and specificity of the AIDA-1 PTB domain and APP intracellular domain (AICD) interaction. Another modular protein of these studies is a pre-synaptic scaffolding protein, Caskin2. Presently, its function within the synapse is less clear compared to its more widely studied homolog, Caskin1. However, the structural differences between the two identified by our research suggest the possibility of distinct functional outcomes in the neuron. We demonstrated that Caskin2 Sterile Alpha Motif (SAM) assembles into an oligomeric architecture different from Caskin1, with the minimal repeating unit being a dimer, rather than a monomer. In invertebrates, Caskin has been functionally linked to LAR receptor tyrosine phosphatase functional pathways, implicated in axonogenesis and synaptogenesis. Using a combination of biophysical and biochemical methods, the partnership between Caskin2 and LAR Homo sapiens homologs was confirmed and characterized. These integrated structural and functional studies provide a platform for further elucidation of AIDA-1 and Caskin cellular functions.