Mitogen-activated protein (MAP) kinase phosphorylation of MAP kinase kinase: determination of phosphorylation sites by mass spectrometry and site-directed mutagenesis.
Mitogen-activated protein kinase kinase (MKK) phosphorylates and activates mitogen-activated protein kinase (MAPK) in response to stimulation of various eukaryotic signaling pathways. Conversely, a recent report showed that MAPK phosphorylates MKK in vitro [Matsuda, S., Gotoh, Y., and Nishida, E. (1993) J. Biol. Chem. 268, 3277-3281]. To gain insight into the function of this feedback phosphorylation, we identified the major sites targeted for phosphorylation by MAPK and examined whether such a modification plays a role in regulating the basal and stimulated MKK activities. Two phosphopeptides generated by tryptic digestion of MAPK-phosphorylated MKK were identified by electrospray ionization mass spectrometry. Cyanogen bromide cleavage also yielded two phosphopeptides whose sequence overlapped with the tryptic phosphopeptides. Both sets of phosphopeptides contained candidate MAPK target sites at Thr292 and Thr386 that fit the consensus sequence ProXThr*Pro. Replacement of either Thr292 or Thr386 with alanine by site-directed mutagenesis reduced the phosphate incorporation respectively to 32 or 75% that of wild type MKK. Replacement of both threonine residues with alanine reduced phosphate incorporation to 2.5% that of wild type enzyme. Comparison of MAPK-phosphorylated vs. unphosphorylated MKK showed no significant differences in basal or Raf-1-stimulated MKK activity. We conclude that the phosphorylation of MKK at Thr292 and Thr386 does not interfere with catalysis in vitro.
Mansour
S J
SJ
Department of Chemistry, University of Colorado, Boulder 80309.
Resing
K A
KA
Candi
J M
JM
Hermann
A S
AS
Gloor
J W
JW
Herskind
K R
KR
Wartmann
M
M
Davis
R J
RJ
Ahn
N G
NG
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D016466:CHO Cells; D017871:Calcium-Calmodulin-Dependent Protein Kinases / Q000378:metabolism*; D006224:Cricetinae; D004789:Enzyme Activation; D006801:Humans; D013058:Mass Spectrometry; D020929:Mitogen-Activated Protein Kinase Kinases; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010449:Peptide Mapping; D010766:Phosphorylation; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism*; D017346:Protein Serine-Threonine Kinases / Q000235:genetics / Q000494:pharmacology; D011518:Proto-Oncogene Proteins / Q000235:genetics / Q000494:pharmacology; D019908:Proto-Oncogene Proteins c-raf; D011817:Rabbits; D051381:Rats; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D015398:Signal Transduction / Q000502:physiology
D011518:Proto-Oncogene Proteins; D011994:Recombinant Proteins; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; D019908:Proto-Oncogene Proteins c-raf; D017871:Calcium-Calmodulin-Dependent Protein Kinases; D020929:Mitogen-Activated Protein Kinase Kinases