PEP-19, neurogranin and neuromodulin are Small Neuronal IQ motif (SNIQs) calmodulin (CaM) binding proteins found in high abundance throughout the nervous system. Neurogranin knockout mice exhibit altered learning and memory, and have diverse defects in neuronal Ca2+ dynamics and multiple neuronal signaling pathways. This suggests that SNIQs play a fundamental role in CaM-dependent cell regulation, but there is a lack of experimental evidence to support a unifying mechanism that could explain these diverse effects. The only known activity of SNIQs is there ability to bind calmodulin in the presence or absence of Ca2+. We present the novel finding that SNIQs greatly accelerate the intrinsically slow kinetics of Ca2+ binding to the C-domain of CaM to achieve greatly enhanced rate of Ca2+ exchange by PEP-19, or a large decrease in Ca2+ affinity by neurogranin. We also show that SNIQs can affect Ca2+ binding to CaM even in the presence of another Ca2+-dependent CaM-binding protein. These observations demonstrate the potential for SNIQs to modulate the response of CaM to diverse Ca2+ signals it experiences in neurons, and they have broad implications for signal transduction in neuronal compartments that are rich in CaM and CaM-binding proteins. The experiments in this proposal will define the biochemical and structural basis for modulation of Ca2+ binding to CaM by SNIQs.
The Aims are: Identify Ca2+ modulatory domains in SNIQs and determine if they function when paired with other CaM binding motifs. Quantify binding and rate constants for dynamic interactions between Ca2+, CaM and SNIQs that define the ability of SNIQs to modulate the steady state and temporal activity of CaM. Characterize the relative ability of SNIQs to modulate the Ca2+ binding properties of CaM in the presence of CaM-dependent protein kinase II, calcineurin or cAMP phosphodiesterase. Determine the effect of SNIQs on CaM-dependent activity of these proteins. Define the structural basis for modulation of Ca2+ binding to CaM by SNIQs.
