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Abstract
Methylation has been widely studied in the context of histones and DNA methylation, but its predominance and function in non-histone proteins is poorly understood. In Saccharomyces cerevisiae, many substrates of lysine and arginine methyltransferases have been identified but the effect of methylation on their function and/or activity has not been elucidated.
This study investigated the methylation of proteins in the cell-cycle. Proteome-wide methylation analyses were performed by use of antibodies specific for lysine or arginine methylation. Gene expression analyses of methyltransferases were also investigated using Q-RT-PCR. The ribosomal lysine methyltransferase RKM1 showed G1/S specific cycling, whereas RKM2 and RKM3 remained relatively stable expression during the cell-cycle. The predominant arginine methyltransferase in yeast, HMT1 had stable gene expression apart from an early singular peak in S phase of the cell-cycle. Global visualization of lysine methylation led to the observation that the eukaryotic Elongation Factor 1A (eEF-1A) showed cell-cycle associated changes.
eEF-1A, and both methyltransferases responsible for methylating the protein Efm1p and See1p exhibited coordinate gene expression. However, gene expression levels of eEF-1A did not correlate with protein abundance levels probed using an anti-eEF-1A-antibody. Subsequent analysis of eEF-1A with tandem mass spectrometry revealed the protein levels as well as specific sites of methylation to be changing through the cell-cycle.
Furthermore, this study reports preliminary evidence of Rkm1p being capable of methylating eEF-1A. Since eEF-1A is involved in translation elongation, aminoacyl-tRNAs export from the nucleus, and interactions with the actin cytoskeleton, theories as to how methylation of eEF-1A could be implicated in intracellular transport of aa-tRNAs and/or cellular vesicles and organelles in the cell-cycle are discussed in detail.