Active Ca2+ channels, according to Hudmon et al. (page 537), get their own dedicated Ca2+ sensors that cause well-used channels to open with gusto.
Some Ca2+ channels, including voltage-gated L-type channels, let through more Ca2+ per opening when they are used frequently. This positive feedback, known as facilitation, allows fast-beating cardiac cells, for instance, to beat harder (as during exercise). The new findings reveal that local retention of a Ca2+/calmodulin-dependent kinase, CaMKII, is behind this ability.
CaMKII is activated by autophosphorylation in response to Ca2+/calmodulin. The authors find that CaMKII then tethers itself to the pore-forming α1C subunit of the L-type channel, which is abundant in heart muscle. Even upon dephosphorylation, CaMKII lingers at the channel.
From this position, the kinase can up-regulate channel activity when Ca2+ influx is frequent. The authors show that active CaMKII phosphorylates two regions of α1C that were previously found to regulate channel activity. Unlike NMDAR-bound CaMKII, which is constitutively active, α1C-bound CaMKII still depends on Ca2+/calmodulin. This difference might explain why NMDARs are up-regulated for the long term by a brief stimulus, whereas full activity of voltage-gated channels requires repeated activation.
Tethering to α1C is necessary for facilitation, even though the channel can be phosphorylated by free kinase. The close proximity may allow the kinase to outdo channel-defacilitating phosphatases. Only when Ca2+ influx is frequent, and CaMKII activity is repeatedly high, can CaMKII win the fight.