Growth Control

Phosphorylation of ribosomal protein S6 at several amino acids at its C-terminus end is well known to be dependent on insulin/IGF signaling in animals [125]. It is required for the activity of S6 leading to the translation of 5' TOP mRNAs (mRNAs that contain an oligopyrimidine tract at their 5' transcriptional start site) coding for proteins of the protein synthetic apparatus. In plants, multisite phosphorylation of specific sites in S6 in analogy to the animal system has been reported for maize using 32P labeling and MALDI-TOF-MS [126]. Here, following phosphoprotein enrichment and MS-based peptide analysis, we could identify an S6 phosphopeptide in A. thaliana belonging to the C-terminal end of the protein. Phosphorylation of A. thaliana S6 ribosomal protein was also reported by Chang et al. [69] who subjected enriched ribosomes to j elimination and MS. Furthermore, studies exist describing aspects of a signal transduction pathway in plants similar to the animal pathway leading from membrane perception of mitogens to S6 phosphorylation and growth control [126-131]. Thus it seems that this important pathway, including multisite phosphorylation on specific residues at the C-terminus of S6, is indeed conserved in plants.

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