GBPI, a novel gastrointestinal- and brain-specific PP1-inhibitory protein, is activated by PKC and inactivated by PKA.
The activities of PP1 (protein phosphatase 1), a principal cellular phosphatase that reverses serine/threonine protein phosphorylation, can be altered by inhibitors whose activities are themselves regulated by phosphorylation. We now describe a novel PKC (protein kinase C)-dependent PP1 inhibitor, namely GBPI (gut and brain phosphatase inhibitor). The shorter mRNA that encodes this protein, GBPI-1, is expressed in brain, stomach, small intestine, colon and kidney, whereas a longer GBPI-2 splice variant mRNA is found in testis. Human GBPI-1 mRNA encodes a 145-amino-acid, 16.5 kDa protein with pI 7.92. GBPI contains a consensus PP1-binding motif at residues 21-25 and consensus sites for phosphorylation by enzymes, including PKC, PKA (protein kinase A or cAMP-dependent protein kinase) and casein kinase II. Recombinant GBPI-1-fusion protein inhibits PP1 activity with IC50=3 nM after phosphorylation by PKC. Phospho-GBPI can even enhance PP2A activity by >50% at submicromolar concentrations. Non-phosphorylated GBPI-1 is inactive in both assays. Each of the mutations in amino acids located in potential PP1-binding sequences, K21E+K22E and W25A, decrease the ability of GBPI-1 to inhibit PP1. Mutations in the potential PKC phosphoacceptor site T58E also dramatically decrease the ability of GBPI-1 to inhibit PP1. Interestingly, when PKC-phosphorylated GBPI-1 is further phosphorylated by PKA, it no longer inhibits PP1. Thus, GBPI-1 is well positioned to integrate PKC and PKA modulation of PP1 to regulate differentially protein phosphorylation patterns in brain and gut. GBPI, its closest family member CPI (PKC-potentiated PP1 inhibitor) and two other family members, kinase-enhanced phosphatase inhibitor and phosphatase holoenzyme inhibitor, probably modulate integrated control of protein phosphorylation states in these and other tissues.
Liu
Qing-Rong
QR
Molecular Neurobiology Branch, National Institute on Drug Abuse-Intramural Research Program, NIH, Department of Health and Human Services, Box 5180, Baltimore, MD 21224, USA.
Zhang
Ping-Wu
PW
Lin
Zhicheng
Z
Li
Qi-Fu
QF
Woods
Amina S
AS
Troncoso
Juan
J
Uhl
George R
GR
D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites; D001921:Brain / Q000201:enzymology / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D004064:Digestive System / Q000201:enzymology / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010766:Phosphorylation; D011493:Protein Kinase C / Q000378:metabolism*; D054645:Protein Phosphatase 1 / Q000378:metabolism*; D011506:Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000302:isolation & purification / Q000378:metabolism; D014018:Tissue Distribution
C480492:PPP1R14D protein, human; C558665:PPP1R14D protein, rat; D011506:Proteins; D012333:RNA, Messenger; D011993:Recombinant Fusion Proteins; D017868:Cyclic AMP-Dependent Protein Kinases; D011493:Protein Kinase C; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1