Phosphorylation in vitro and in vivo of Ribosomal Proteins from Saccharomyces cerevisiae

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Abstract
Crude ribosomes from Saccharomyces cerevisiae cultures were phosphorylated in vitro when incubated in the presence of [γ‐32P]ATP. Analysis of the ribosomal proteins with two‐dimensional electrophoresis revealed that of the 29 proteins identified in the small subunit, only protein S6 was phosphorylated. Of the 37 proteins identified in the large subunit, one was highly phosphorylated (L3) and two only slightly phosphorylated (L11 and L14). The protein kinase activity associated with the ribosomes was extracted with 1 MKC1 and was not dependent on adenosine 3′:5′‐monophosphate; it preferentially phosphorylated casein and phosvitin, but was less active on histones.Structural ribosomal proteins were also phosphorylated in vivo when the yeast cultures were incubated with [32P]orthophosphate; the radioactivity resistant to hydrolysis by hot perchloric acid was incorporated into the proteins of the two subunits. Radioactive phosphoserine was found by subjecting hydrolysates of ribosomal proteins to high‐voltage electrophoresis. After two‐dimensional electrophoresis, one poorly phosphorylated protein (S10) was identified in the small subunit. In the large subunit, one protein (L3) was highly labelled, and two proteins (L11 and L24) only slightly labelled.