ACTIVATION OF CYCLIC-AMP-DEPENDENT PROTEIN-KINASE IN OKADAIC ACID-TREATED NEURONS POTENTIATES NEUROFILAMENT FRAGMENTATION AND STIMULATES PHOSPHORYLATION OF SER(2) IN THE LOW-MOLECULAR-MASS NEUROFILAMENT SUBUNIT

The activation of cyclic AMP-dependent protein kinase (PKA) in rat dor sal root ganglion (DRG) cultures increased phosphorylation of the low- molecular-mass neurofilament subunit (NFL) at a site previously identi fied as Ser(55) but had no effect on neurofilament integrity. When PKA was activated in DRG cultures treated with 20-250 nM okadaic acid, ne urofilament fragmentation was enhanced, and there was a corresponding increase in phosphorylation of NFL at a novel site. This site was also phosphorylated by PKA in vitro and was determined to be Ser(2) by mas s spectrometric analysis of the purified chymotryptic phosphopeptide. The PKA sites in NFL were dephosphorylated by the purified catalytic s ubunit of protein phosphatase-2A but not that of protein phosphatase-1 , and phosphoserine-2 was a better substrate than phosphoserine-55. Th e phosphorylation and dephosphorylation of Ser(2) and Ser(55) in NFL m ay therefore be involved in the modulation of neurofilament dynamics t hrough the antagonistic effects of PKA and protein phosphatase-2A.