Intersubunit capture of regulatory segments is a component of cooperative CaMKII activation

Abstract

The CaMKII complex can enter a state of Ca2+-independent activity in response to high frequency Ca2+ pulses. Using functional and structural analyses, it is now shown that activation is cooperative and the basis of this is the intersubunit capture of the regulatory subunit of one kinase module by its neighbor. The dodecameric holoenzyme of calcium–calmodulin-dependent protein kinase II (CaMKII) responds to high-frequency Ca2+ pulses to become Ca2+ independent. A simple coincidence-detector model for Ca2+-frequency dependency assumes noncooperative activation of kinase domains. We show that activation of CaMKII by Ca2+–calmodulin is cooperative, with a Hill coefficient of ∼3.0, implying sequential kinase-domain activation beyond dimeric units. We present data for a model in which cooperative activation includes the intersubunit 'capture' of regulatory segments. Such a capture interaction is seen in a crystal structure that shows extensive contacts between the regulatory segment of one kinase and the catalytic domain of another. These interactions are mimicked by a natural inhibitor of CaMKII. Our results show that a simple coincidence-detection model cannot be operative and point to the importance of kinetic dissection of the frequency-response mechanism in future experiments.