During investigations of the regulation of
tyrosine hydroxylase (TH) by
protein phosphorylation, a novel
protein kinase activity has been discovered in rat
pheochromocytoma. Originally detected as a trace contaminant in preparations of highly purified TH, this novel
kinase activity phosphorylated TH at
serine 8 in the
proline-rich amino-terminal region of the
enzyme. This particular site is not phosphorylated by, nor is the amino acid sequence surrounding this site selective for, any of the classical (i.e. well characterized)
protein kinases. In this report, we describe the identification, characterization, and partial purification of this novel
protein kinase. By utilizing a synthetic
peptide corresponding to the amino-terminal region of TH, a selective assay for this
protein kinase was developed. The
kinase activity utilized
ATP and
magnesium, although
GTP could also be utilized as a
phosphate donor. The
kinase activity was found to co-purify with TH activity through
ammonium sulfate precipitation and
DEAE-cellulose chromatography and could be only partially resolved from TH by
heparin-agarose affinity chromatography. Substantial
kinase activity could be resolved from TH by
phosphocellulose chromatography. The novel
kinase migrates as a
protein with a molecular mass of approximately 45 kDa on gel permeation chromatography as well as
sucrose density gradient centrifugation. Studies of site specificity indicate that this Ser/Thr
kinase activity appears to be directed by an adjacent (carboxyl-terminal)
proline residue, exhibiting a minimal recognition sequence of -X-Ser/Thr-Pro-X-. In addition to TH, this
proline-directed protein kinase will also phosphorylate
synapsin I,
histone H1, and
glycogen synthase, suggesting that this
kinase may have multiple substrates in vivo. Additional findings indicate that the activity of
proline-directed protein kinase is increased transiently in PC12
pheochromocytoma cells following treatment with
nerve growth factor. Distinctions between this novel
kinase and other well characterized
protein kinases can be made on the basis of phosphorylation site specificity, chromatographic behavior, and physical characteristics.