PHOSPHATASE RESISTANCE OF ERK2 BRAIN KINASE PK40(ERK2)

We have previously shown that a brain protein kinase, termed PK40, cat alyzes the multiple phosphorylation of the KSP-repeat site of neurofil aments (NFs) and also can transform tau proteins into the paired helic al filament-like state as found in Alzheimer's disease (AD) brains. Pr otein sequence analysis suggests that PK40 is a form of the extracellu lar signal-regulated kinase ERK2. A subpopulation of ERK2 species in s oluble brain fractions can be efficiently phosphorylated and activated in cell-free systems, simply by adding Mg2+-ATP. Two phosphoisoforms of PK40 are formed in this process, which have a reduced gel mobility, very much like the ERK2 form obtained in cell culture by stimulation with growth factors. One of these low-mobility forms cannot be inactiv ated with protein phosphatase 2A (PP2A) or with tyrosine phosphatases. The second form can be slowly inactivated by PP2A. In this case two S er/Thr phosphates are removed at different rates during inactivation: One phosphate is very quickly removed to result in the formation of a high-mobility 39-kDa ERK2 species without consequence for activity; th e other, slowly removed Ser/Thr phosphate controls the activity but ha s no effect on the gel mobility of ERK2. These results show that forms of ERK2 exist with properties different from the previously character ized ERK2 (p42(mapk)) from stimulated cell cultures. The active ERK2 f orms produced in the presence of MS(2+)-ATP alone could provide an exp lanation for the existence of constitutive ERK2-like NF phosphorylatio n in vivo, Excessive formation of an ERK2 species resistant to inactiv ation by PP2A might be relevant to the persistent pathological tau hyp erphosphorylation in AD.