Apoptosis consists of highly regulated pathways involving post-translational modifications and cleavage of proteins leading to sequential inactivation of the main cellular processes. Here, we focused on the apoptotic processing of one of the essential components of the mRNA splicing machinery, the U1-70K snRNP protein. We found that at an early stage of apoptosis, before the cleavage of the C-terminal part of the protein by caspase-3, the basal phosphorylation of the Ser140 residue located within the RNA recognition motif, increases very significantly. A caspase-dependent, PP1-mediated dephosphorylation of other serine residues takes place in a subset of U1-70K proteins. The U1-70K protein phosphorylated at Ser140 is clustered in heterogeneous ectopic RNP-derived structures, which are finally extruded in apoptotic bodies. The elaborate processing of the spliceosomal U1-70K protein we identified might play an important role in the regulated breakdown of the mRNA splicing machinery during early apoptosis. In addition, these specific changes in the phosphorylation/dephosphorylation balance and the subcellular localization of the U1-70K protein might explain why the region encompassing the Ser140 residue becomes a central autoantigen during the autoimmune disease systemic lupus erythematosus. Cell Death and Differentiation (2008) In higher eukaryotes, the process of gene expression includes transcription, pre-mRNA splicing/processing and translation in the cytoplasm. The macromolecular machinery involved in pre-mRNA splicing, the spliceosome, consists of five small nuclear ribonucleoparticles (snRNPs) called U1, U2 and U4-6 snRNP, and a large number of additional splicing factors. 1 Splicing factors are supposedly recruited from storage sites, that is interchromatin granules, in the interchromatin space to transcription sites at the periphery of condensed chromatin. The assembly of the spliceosome is initiated by recognition of the 5 0 splice site of pre-mRNA by U1 snRNP and mediated by serine/arginine-rich (SR) proteins. 2 Phosphorylation and dephosphorylation both have an important role in the recruitment of splicing factors and the subsequent regulation of spliceosome assembly and splicing catalysis. 3 The U1-snRNP-specific U1-70K protein is a highly phosphorylated protein, which contains two SR domains. Phosphorylation of both the SR protein ASF/SF2 and the first SR domain of the U1-70K protein are involved in vitro in their interaction. 4,5 In addition, thiophosphorylation of the U1-70K protein, making it resistant to dephosphorylation by phosphatases, results in complete inhibition of splicing, but not of spliceosome formation. 6 At least 11-13 natural variants of the U1-70K protein, partially due to phosphorylation, have been found by 2D analysis. 7,8 However, very little is known about specific phosphorylated residues in this protein or specific enzymes involved in the functional regulation of the U1-70K protein by phosphorylation/dephosphorylation.Apoptosis is a fast and orderly process consisting of a seq...

Autoantibodies against particular nuclear components, such as chromatin and snRNPs, are a characteristic feature of the autoimmune disease systemic lupus erythematosus. The last decade, evidence has suggested that apoptotic cells are the main source of autoantigens in this disease. Therefore, it has been proposed that protein modifications occurring during apoptosis lead to the formation of neo-epitopes, which can break the tolerance when apoptotic cells are not properly cleared. Indeed, many lupus autoantigens are prone to apoptosis-associated post-translational modifications and/or cleavage by caspases. In addition, lupus autoantigens are relocated from the nucleus to apoptotic blebs on the cell surface of early apoptotic cells. Therefore, to understand why certain nuclear proteins become autoantigens during apoptosis, it is important to know the apoptotic processing of these proteins. This review summarizes the current knowledge of apoptotic processing of lupus autoantigens and the possible effects on their encounter with the immune system in normal and autoimmune situations.