Identification of a phosphorylated form of phospho enolpyruvate carboxykinase from the yeast Saccharomyces cerevisiae

Abstract

A phosphoprotein of 65 kDa, as determined by SDS-gel electrophoresis, has been isolated from yeast crude extracts. This phospho form copurifies with phospho enolpyruvate carboxykinase in the enzyme purification procedure worked out in our laboratory (Tortora, P., Hanozet, G.M. and Guerritore, A. (1985) Anal. Biochem. 144, 179–185). Moreover, both proteins bind strongly to 5′AMP-Sepharose 4B in the presence of Mn 2+, whereas a substantially lower binding occurs if Mn 2+ is replaced by Mg 2+. This binding pattern is consistent with the well-known Mn 2+-dependence of yeast phospho enolpyruvate carboxykinase. These data suggest that the 65-kDa protein might be a phosphorylation product of the native enzyme. Furthermore, although the phospho form is not immunoprecipitated by anti-phospho enolpyruvate carboxykinase antibodies, addition of Protein A-Sepharose CL-4B to crude extracts preincubated with the antibodies results in the binding to the resin of the phospho form, thus providing immunological evidence for its identification as a modified form of native enzyme. The same 65-kDa phosphoprotein is detectable in extracts from cells grown in the presence of [ 32P]P i, as well as in cell extracts incubated with [γ- 32P]ATP. Moreover, digestion of the phosphoprotein with BrCN or with Staphylococcus aureus V8 proteinase, yields two and three fragments, respectively, which appear parallel to digestion products of phospho enolpyruvate carboxykinase, again supporting the proposed identification. Finally, analysis of the phosphorylated amino acids in the 65-kDa protein shows that phosphoserine is the only labelled phosphoamino acid.