The fate of U1 snRNP during anti-Fas induced apoptosis: specific cleavage of the U1 snRNA molecule

Abstract

During apoptosis, the U1-70K protein, a component of the spliceosomal U1 snRNP complex, is specifically cleaved by the enzyme caspase-3, converting it into a C-terminally truncated 40-kDa fragment. In this study, we show that the 40-kDa U1-70K fragment is still associated with the complete U1 snRNP complex, and that no obvious modifications occur with the U1 snRNP associated proteins U1A, U1C and Sm-B/B'. Furthermore, it is described for the first time that the U1 snRNA molecule, which is the backbone of the U1 snRNP complex, is modified during apoptosis by the specific removal of the first 5 ± 6 nucleotides including the 2,2,7-trimethylguanosine (TMG) cap. The observations that U1 snRNA cleavage is very specific (no such modifications were detected for the other U snRNAs tested) and that U1 snRNA cleavage is markedly inhibited in the presence of caspase inhibitors, indicate that an apoptotically activated ribonuclease is responsible for the specific modification of U1 snRNA during apoptosis. Cell Death and Differentiation (2000) 7, 70 ± 79.

The pathophysiology of photosensitivity in lupus erythematosus

“…The caspases activated during apoptosis cleave intracellular proteins into fragments that are bound by autoantibodies from some patients with LE (101,102). Further, proteins specifically phosphorylated during stress-induced apoptosis are tar-geted by antibodies from LE patient sera (103)(104)(105)(106). Granzyme B, a serine protease found principally in the cytotoxic granules of CTL and NK cells, can also cleave cellular autoantigens into unique fragments not detected in other forms of apoptosis.…”

Caspase-dependent cleavage of nucleic acids

“…During apoptosis, the U1-70K protein is specifically cleaved leading to the appearance of a 40 kDa fragment, 69,70 whereas the U1A, U1C and Sm-B/B' proteins are not detectably modified. 60,71 All proteins remain associated with the U1 snRNP complex in apoptotic cells. 71 The U1 snRNA molecule itself is a major target of autoimmunity in SLE-overlap syndromes, 72 and it has been shown that changes in the titer of anti-U1 snRNA autoantibodies may correlate with the severity of the disease.…”

“…60,71 All proteins remain associated with the U1 snRNP complex in apoptotic cells. 71 The U1 snRNA molecule itself is a major target of autoimmunity in SLE-overlap syndromes, 72 and it has been shown that changes in the titer of anti-U1 snRNA autoantibodies may correlate with the severity of the disease. 73 The most immunodominant epitopes of the autoantigenic U1 snRNA molecule are located in stemloop II (nts 49 ± 65 and 76 ± 90) and stemloop IV (nts 150 ± 153), 74 and were recently characterized in more detail using several recombinant human monoclonal antibodies (scFvs).…”

“…In all tested apoptotic mammalian cell lines (Jurkat, human T cell leukemia; HeLa, human cervix carcinoma; HL-60, human promyelocytic leukemia; 4B1, mouse cells overexpressing the human Fas-receptor), U1 snRNA was specifically cleaved. 71 Cleavage of U1 snRNA appeared to be a universal phenomenon, as this snRNA was cleaved in various cell types rendered apoptotic with a variety of stimuli (anti-Fas mAb 7C11, murine Fas-ligand, anisomycin, staurosporine, cycloheximide). Intriguingly, the nonautoantigenic U2, U3, U4, U5 and U6 snRNA molecules were not detectably modified.…”

“…The truncated U1 snRNA molecules migrate as a doublet in denaturing polyacrylamide gels, which is the result of cleavages occurring at the 5'-side of either c6 or c7 in the single-stranded 5'-end of the snRNA ( Figure 4A). 71 This might suggest that two flanking pseudouridines (c) are involved in the selection of cleavage sites. However, the U2 snRNA molecule is not cleaved despite the presence of a similar c-doublet in a single-stranded region near the 5'-end (nucleotides 7 and 8).…”