A tyrosine-specific protein kinase from Ehrlich ascites tumor cells

Area of Science:

• Biochemistry
• Molecular Biology
• Enzymology

Background:

• The sodium-potassium pump (Na+,K+-ATPase) plays a critical role in cellular ion homeostasis.
• Regulation of Na+,K+-ATPase activity through phosphorylation is a key area of investigation.
• Protein tyrosine kinases are implicated in various cellular signaling pathways.

Purpose of the Study:

• To purify and characterize a protein tyrosine kinase that phosphorylates Na+,K+-ATPase.
• To investigate the substrate specificity of this kinase compared to other known tyrosine kinases.
• To identify potential regulatory mechanisms of Na+,K+-ATPase.

Main Methods:

• Purification of protein tyrosine kinase from Ehrlich ascites tumor cell membranes.
• Enzymatic assays using purified Na+,K+-ATPase (alpha and beta subunits), poly(Glu80Tyr20), and casein as substrates.
• Comparison with viral transforming gene products (Rous sarcoma virus, Fujinami sarcoma virus) and EGF receptor kinase.
• Molecular weight determination using gel filtration and SDS-PAGE.

Main Results:

• A protein tyrosine kinase was purified 500-fold, requiring Mn2+, Mg2+, or Fe2+ and inhibited by Cu2+ or Zn2+.
• The purified kinase phosphorylated the beta subunit of Na+,K+-ATPase approximately five times more efficiently than the alpha subunit.
• Unlike viral kinases, this enzyme showed poor phosphorylation of casein but was highly active on poly(Glu80Tyr20).
• The kinase has an estimated molecular weight of 60 kDa, with a potential autophosphorylated 60 kDa phosphopeptide identified.

Conclusions:

• A novel protein tyrosine kinase specifically phosphorylates the beta subunit of Na+,K+-ATPase.
• This kinase exhibits distinct substrate specificity compared to viral oncogene products and EGF receptor kinase.
• The findings suggest a potential role for this kinase in regulating Na+,K+-ATPase function in tumor cells.