Activation-triggered subunit exchange between CaMKII holoenzymes facilitates the spread of kinase activity

An inactive CaMKII holoenzyme can be phosphorylated on T286 in the absence of Ca2+/CaM.

“Remaining to be demonstrated, if this mechanism is to propagate CaMKII activity, is that an inactive holoenzyme can be phosphorylated on T286 in the absence of Ca2+/CaM. This is what was demonstrated in the studies claiming to exhibit subunit exchange (406).”

Active CaMKII holoenzymes (T286D) can phosphorylate wild-type holoenzymes in the absence of Ca2+/CaM.

“It should be emphasized that the experiments of Stratton et al. (406) demonstrate that active holoenzymes (T286D) can phosphorylate wt holoenzymes in the absence of Ca2+/CaM.”

Recent biochemical evidence suggests that CaMKII may also satisfy the second requirement, i.e., for subunit exchange.

“Recent biochemical evidence suggests that CaMKII may also satisfy this second requirement, i.e., for subunit exchange (28, 406–408).”

More recent studies have found evidence that high kinase concentration enables subunit exchange and transphosphorylation between subunits within a holoenzyme can occur in the absence of Ca2+/CaM but on a much slower time course.

“However, more recent studies have found evidence that high kinase concentration enables subunit exchange and transphosphorylation between subunits within a holoenzyme can occur in the absence of Ca2+/CaM but on a much slower time course (406).”

Phosphorylation of inactive holoenzymes occurs when they are not bound to Ca2+/CaM, establishing a phosphorylation event for maintaining CaMKII activity in the face of protein turnover.

“Although it can be argued as to whether this occurs through interholoenzyme phosphorylation or through subunit exchange, this finding shows that phosphorylation of inactive holoenzymes occurs (not bound Ca2+/CaM), thus establishing a phosphorylation event for maintaining CaMKII activity in the face of protein turnover.”

Using TIRF microscopy to track single molecules of CaMKII labeled on their catalytic domains with fluorescent dyes, activation of two homogeneous populations of CaMKII holoenzymes each with a distinct fluorophore triggers substantial colocalization consistent with the exchange of subunits.

“With the use of total internal reflection fluorescence (TIRF) microscopy to track single molecules of CaMKII labeled on their catalytic domains with fluorescent dyes, it was found that activation of two homogeneous populations of CaMKII holoenzymes each with a distinct fluorophore triggers substantial colocalization consistent with the exchange of subunits (FIGURE 11) (406).”

Recent work from Kuriyan and collaborators nicely fulfills the second requirement, although there may be an alternative mechanism of interholoenzyme phosphorylation.

“The recent work from Kuriyan and collaborators (406) nicely fulfills this second requirement, although there may be an alternative mechanism of interholoenzyme phosphorylation (81).”