Figure 15–34 The stepwise activation of CaM-kinase II. (A) Each CaM-kinase II protein has two major domains: an amino-terminal kinase domain (green) and a carboxyl-terminal hub domain (blue), linked by a regulatory segment. Six CaM-kinase II proteins are assembled into a giant ring in which the hub domains interact tightly to produce a central structure that is surrounded by kinase domains. The complete enzyme contains two stacked rings, for a total of 12 kinase proteins, but only one ring is shown here for clarity. When the enzyme is inactive, the ring exists in a dynamic equilibrium between two states. The first (upper left) is a compact state, in which the kinase domains interact with the hub, so that the regulatory segment is buried in the kinase active site and thereby blocks catalytic activity. In the second inactive state (upper middle), a kinase domain has popped out and is linked to the central hub by its regulatory segment, which continues to inhibit the kinase but is now accessible to Ca2+/calmodulin. If present, Ca2+/calmodulin will bind the regulatory segment and prevent it from inhibiting the kinase, thereby locking the kinase in an active state (upper right). If the adjacent kinase subunit also pops out from the hub, it will also be activated by Ca2+/calmodulin, and the two kinases will then phosphorylate each other on their regulatory segments (lower right). This autophosphorylation further activates the enzyme. It also prolongs the activity of the enzyme in two ways. First, it traps the bound Ca2+/calmodulin so that it does not dissociate from the enzyme until cytosolic Ca2+ levels return to basal values for at least 10 seconds (not shown). Second, it converts the enzyme to a Ca2+-independent form, so that the kinase remains active even after the Ca2+/calmodulin dissociates from it (lower left). This activity continues until the action of a protein phosphatase overrides the autophosphorylation activity of CaM-kinase II. (B) This structural model of the enzyme is based on x-ray crystallographic analysis.
The remarkable dodecameric structure of the enzyme allows it to achieve a broad range of intermediate activity states in response to different Ca2+ oscillation frequencies: higher frequencies tend to cause more subunits in the enzyme to reach the phosphorylated active state (see Figure 15–35). The behavior of CaM-kinase II is also controlled by the length of the linker segment between the kinase and hub domains. The linker is longer in some isoforms of the enzyme; in these isoforms, the kinase domains tend to pop out of the ring more frequently, making it more sensitive to Ca2+. These and other mechanisms allow the cell to tailor the responsiveness of the enzyme to the needs of different types of neurons. (Adapted from L.H. Chao et al., Cell 146:732–745, 2011. PDB code: 3SOA.)
Another remarkable property of CaM-kinase II is that the enzyme can use its intrinsic memory mechanism to decode the frequency of Ca2+ oscillations. This property is thought to be especially important at a nerve cell synapse, where changes in intracellular Ca2+ levels in a postsynaptic cell as a result of neural activity can lead to long-term changes in the subsequent effectiveness of that synapse (discussed in Chapter 11). When CaM-kinase II is exposed to both a protein phosphatase and repetitive pulses of Ca2+/calmodulin at different frequencies that mimic those observed in stimulated cells, the enzyme’s activity increases steeply as a function of pulse frequency (Figure 15–35).