Post-translational modifications regulating the activity of Sam68

Src-associated in mitosis 68 kDa (Sam68) is an RNA binding protein identified 10 years ago as a mitosis specific target for Src tyrosine kinase. It belongs to the signal transduction and activation of RNA metabolism (STAR) protein family. These proteins are thought to link signaling pathways with some aspect of RNA metabolism. Although the function of Sam68 is unknown it has been reported to play a role in several biological processes such as: viral replication, cell signaling, alternative pre-mRNA splicing, cell cycle regulation, and has been suggested to be a tumour suppressor and a transcriptional regulator.; It has been demonstrated that the activity of Sam68 can be regulated by post-translational modifications such as tyrosine or serine/threonine phosphorylation. Tyrosine phosphorylation was shown to inhibit Sam68 binding to synthetic poly(U) RNA, and serine/threonine phosphorylation was shown to influence the ability of Sam68 to alter splice site selection. Since Sam68 is predominantly a nuclear protein there are several post-translational modifications common to nuclear proteins that have not yet been described for Sam68. For example, acetylation, sumoylation and ubquitination are three post-translational modifications described for numerous nuclear proteins.; Here I report that Sam68 can be acetylated, and that acetylation positively regulates its binding to poly(U) RNA. As well, Sam68 is shown to be sumoylated. Conjugation with SUMO altered Sam68 subnuclear localization and its ability to act as a repressor of gene expression. Within B lymphocytes, I show Sam68 tyrosine phosphorylation to be induced by activation of the cells upon ligation of the B cell receptor. Src tyrosine kinase activity inhibited Sam68 SUMO modification and induced its mono- and poly-ubiquitination. Thus, Sam68 activity, localization and protein levels appear to be intricately regulated by acetylation, sumoylation and ubiquitination.; Post-translational modifications can diversify the function of a protein, and inappropriate modifications can result in disease or oncogenesis. Thus, characterizing post-translational modifications of molecules such as Sam68, and understanding the functional consequence of these changes on protein function, will be key towards deciphering the elaborate code dictating cellular biochemistry. Such information could reveal new targets for directed therapy of diseases or cancer resulting from deregulated signaling pathways.