J. Biol. Chem., Vol. 268, Issue 4, 2451-2457, 02, 1993
The activity-controlling phosphorylation site is not the same in the four acidic ribosomal proteins from Saccharomyces cerevisiae
T Naranda, M Remacha and JP Ballesta
Centro de Biologia Molecular Severo Ochoa (Consejo Superior de Investigaciones Cientificas-Universidad Autonoma de Madrid), Spain.
By using site-directed mutagenesis and chemical analysis of
phosphopeptides, a unique phosphorylation site has been shown at serine 73
in the amino acid sequence of the Saccharomyces cerevisiae acidic ribosomal
protein YP1 beta (L44'). The mutation in this position prevents in vitro
phosphorylation by protein kinases that modify the wild-type polypeptide.
The unphosphorylatable mutated protein is unable to bind to the ribosomes
and to rescue the growth deficiency of yeast strains in which the
corresponding original gene is inactivated by gene disruption. Sequencing
of tryptic phosphopeptides has shown that acidic proteins YP1 alpha and YP2
alpha (L44) are also phosphorylated at positions near the carboxyl end.
These results contrast with the data indicating that in the highly
homologous protein YP2 beta, phosphorylation takes place at serine 19,
close to the amino terminus. The results show that phosphorylation is
definitely required for the biological activity of these ribosomal
proteins. However, the differences in the phosphorylation sites suggest
that the effect of this modification is not the same in all of them,
confirming the heterologous role of these peculiar ribosomal components. In
fact, the different context of the modification sites in the four
polypeptides suggests the existence of more than one protein kinase
specific for this set of proteins.
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