Cell signaling utilizes the frequency of calcium stimuli to produce diverse physiological outcomes. From fertilization of oocytes and cardiac facilitation, to muscle contraction and action potential firing, periodic fluctuations in calcium levels play a key role in determining cell fate. During my thesis work, I studied a protein signaling complex that responds to the frequency of calcium stimuli: calcium/calmodulin dependent protein kinase II (CaMKII). The CaMKII holoenzyme is a dodecameric kinase assembly that activates and autophosphorylates in a manner dependent on the frequency of calcium stimuli. Its best-studied role is in the post-synaptic neuron, where it responds to calcium pulse frequencies to initiate changes important for Long Term Potentiation.

My work investigated the activation mechanism of CaMKII, with the goal of understanding the enzyme's cooperative response to calcium-saturated calmodulin. This work elucidates the mechanism for CaMKII activation; by demonstrating that CaMKII is not a simple `coincidence detector' as previously postulated, by showing that CaMKII is cooperatively activated by calcium-saturated calmodulin, and by demonstrating that cooperative activation is mediated by the inter-subunit `capture' of regulatory segments. This work demonstrates that the cooperative response of CaMKII can by modulated by the length of the linker region where splice-form deletions and insertions occur. Similar modifications have been shown to shift the frequency-dependent activation of the enzyme, suggesting that our proposed mechanism enables the enzyme to tune its frequency response depending on developmental or tissue-specific expression. In addition, analysis of interactions exhibited by a peptide inhibitor of CaMKII revealed docking sites which other proteins, such as the NMDA-receptor, may utilize in order to localize and affect the frequency response of the CaMKII. Finally, I investigated the exchange of CaMKII subunits between holoenzyme complexes, and discuss the implications of such a process in prolonging the response to calcium stimuli beyond the lifespan of individual protein subunits.