Purified mammalian HSP-70 KDA activates phosphoprotein phosphatases in vitro.
We have examined protein phosphorylation in the presence of purified mammalian HSP-70 kDa using the phosphoproteins in the rabbit reticulocyte lysate system as a model. Purified HSP-70 added to the rabbit reticulocyte lysate decreased the general protein phosphorylation by 50-80% as measured by PAGE analysis of proteins labelled with gamma-(32P)-ATP. Reduction in protein phosphorylation was not due to the ATPase activity of HSP-70 as measured by thin layer chromatography. The reduction in protein phosphorylation was also not due to the reduced activities of the protein kinases. However, using (32P)-labelled phosphorylase-alpha as a substrate in the phosphatase assay system indicated increases in the activity of protein phosphatase 1(PP-1) and/or 2A (PP-2A) by 20-40% relative to control in the presence of increasing concentrations of HSP-70. Using a variety of activators and inhibitors of the two major protein phosphatases, PP-1 and PP-2A, we found that Mn2+ caused a similar pattern of dephosphorylation of proteins as measured by PAGE analysis. Both okadaic acid and microcystin, two protein phosphatase inhibitors, largely counteracted the HSP-70 effect as measured by gel electrophoresis or when (32P)-labelled phosphorylase-alpha was used as a substrate. We conclude that in this system HSP-70 activates specific protein phosphatases.
Stanford University School of Medicine, Department of Radiation Oncology, CBRL, CA 94305.
N F
NF
Mivechi
L D
LD
Trainor
G M
GM
Hahn
D000818:Animals; D002478:Cells, Cultured; D006224:Cricetinae; D004789:Enzyme Activation; D006360:Heat-Shock Proteins / Q000502:physiology*; D006801:Humans; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D010766:Phosphorylation; D054645:Protein Phosphatase 1; D013379:Substrate Specificity; D014407:Tumor Cells, Cultured
D006360:Heat-Shock Proteins; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1