COMPARISON OF THE RAT NUCLEOLAR PROTEIN NOPP140 WITH ITS YEAST HOMOLOG SRP40 - DIFFERENTIAL PHOSPHORYLATION IN VERTEBRATES AND YEAST

Rat Nopp140, a nonribosomal protein of the nucleolus and coiled bodies , was characterized as one of the most highly phosphorylated proteins in the cell. Based on its subcellular location, its nuclear localizati on signal binding capacity, and its shuttling between the nucleolus an d the cytoplasm, Nopp140 was proposed to function as a chaperone in ri bosome biogenesis, This study shows that casein kinase II phosphorylat es Nopp140 to its unusual high degree and identifies the yeast SRP40 g ene product as immunologically and structurally related to rat Nopp140 . SRP40 encodes an acidic (pI = 3.9), serine-rich (49%) protein of 41 kDa whose carboxyl terminus exhibits 59% sequence identity to that of Nopp140 SRP40 localizes to the yeast nucleolus and is required at a sp ecific cellular concentration for optimal growth as indicated by the n egative effect on cell growth of both overexpression and deletion of i ts gene. Like Nopp140, SRP40 is phosphosylated by casein kinase II, bu t to a much lesser extent, While the parallels between these two prote ins suggest that SRP40 is the bona fide yeast Nopp140 homolog, their d isparities reflect the differences in nucleolar dynamics and regulatio n of ribosome biogenesis between yeast and vertebrates.