, 2007a). Together, these studies suggest that TARPs bind to AMPARs in a complex and distributed fashion, with a special role for the first extracellular loop, likely through a direct interaction with the AMPAR ligand-binding core. Although the crystallization mTOR tumor of AMPAR structural domains, such
as the ligand-binding core (Armstrong et al., 1998), as well as the full AMPAR tetramer (Sobolevsky et al., 2009), represented quantum leaps in our understanding of iGluR structure and function, the exact nature of the interaction between AMPARs and TARPs awaits either the crystal structure of a TARP or the cocrystallization of an AMPAR-TARP complex. Aside from determining the structural basis for AMPAR-TARP interactions, persistent check details questions remain regarding TARP stoichiometry. How many TARP molecules are associated with single-AMPAR complexes in native systems?
Can the trafficking and gating effects of TARPs be tuned by differences in stoichiometry? The dose dependence of TARP gating effects, reflected in miniature excitatory postsynaptic current (mEPSC) decay, provided the first tantalizing hint that AMPAR-TARP interactions may exhibit variable stoichiometry (Milstein et al., 2007). Since then, TARP modulation of KA efficacy has been a valuable metric for TARP stoichiometry in both heterologous and native systems. Fusion proteins, in which GluA subunits are bound to various TARP family members through linker domains, provide AMPAR-TARP complexes with defined stoichiometry.
Using these constructs to calibrate KA efficacy in heterologous cells, AMPARs are estimated to associate with either two or four TARPs, suggesting a degree of cooperativity in TARP binding. TARP stoichiometry, suggested by KA efficacy, was subsequently found to differ among hippocampal cell types, suggesting that gating effects could be modulated by differential TARP expression (Shi et al., why 2009). However, biochemical data has shown that AMPARs are capable of associating with one, two, three, or four stargazin molecules depending on its expression level, contradicting the notion of cooperative binding. In addition, AMPARs in CGNs were estimated to associate with only one stargazin molecule, which is sufficient to modulate KA efficacy (Kim et al., 2010). These contrasting results may be attributed, in part, to cell-type-specific differences in TARP subtypes and expression level. Clearly, further quantitative work will be required to clarify the possible TARP subtype and cell-type-specific regulation of stoichiometry. More broadly, there remains the possibility that TARP stoichiometry is not fixed throughout the lifecycle of an AMPAR, but rather that it can be dynamically regulated. Evidence that AMPAR-TARP complexes can undergo acute, agonist-dependent dissociation (Tomita et al., 2004), and can modify paired-pulse ratio (PPR) in hippocampal neurons (Morimoto-Tomita et al.