The 72-kDa Component of Signal Recognition Particle Is Cleaved during Apoptosis

Abstract

Proteins cleaved by apoptotic caspases are commonly recognized by autoantibodies found in the serum of patients with rheumatic disease. We report that the 72-kDa signal recognition particle (SRP) protein, a rare target of autoantibodies found in the serum of patients with dermatomyositis and systemic lupus erythematosus, is rapidly cleaved in Jurkat T cells treated with apoptotic (i.e. Fas ligation, treatment with ␥ or ultraviolet radiation, or co-culture with anisomycin or staurosporine) but not proliferative (CD3 cross-linking) stimuli. Cleavage of SRP 72 produces a 66-kDa amino-terminal fragment and a 6-kDa carboxyl-terminal fragment that is selectively phosphorylated on serine residues. Cleavage of SRP 72 is prevented by chemical and peptide caspase inhibitors, and by overexpression of bcl-2, an inhibitor of apoptotic cell death. Analysis of the carboxyl terminus of SRP 72 has identified a putative cleavage site (SELD/A) for group III caspases, and carboxyl-terminal serine residues that are highly conserved in phylogeny. Both serine phosphorylation and caspase cleavage of SRP 72 are observed in cells derived from human, dog, rat, and mouse. Canine SRP 72 is cleaved in vitro by recombinant caspase 3 but retains the ability to mediate transport of a signal peptide-containing protein into the endoplasmic reticulum lumen. The 72-kDa component of the SRP joins a growing list of autoantigens that undergo post-translational modifications during programmed cell death.Proteins modified by the proteases and kinases that are activated during apoptosis are often involved in both the execution phase of cell death and in the development of autoantibodies in patients with systemic lupus erythematosus and mixed connective tissue disease (reviewed in Ref. 1). For example, at least 17 proteins that are known to be cleaved by caspases during apoptosis are autoantigens, including the 70-kDa component of the U1-small nuclear ribonuclear protein complex (U1-70 kDa) (2), poly(A) ribose polymerase (3), DNAdependent protein kinase (DNA-PK) (4), hnRNP C1 and C2 (5), lamins A, B, and C (6), the nuclear mitotic apparatus protein (NuMA) (7, 8), topoisomerases 1 and 2 (8), the nucleolar protein UBF/NOR-90 (8), and ␣-fodrin (9, 10) (reviewed in Ref. 1). In addition, phosphorylated serine/arginine splicing factors have recently been shown to specifically associate with the U1-small nuclear RNP autoantigen complex during apoptosis (11, 12). These observations have led to the hypothesis that proteins modified during apoptosis can be presented to the immune system in a way that bypasses tolerance to self proteins. Although the molecular mechanisms by which this occurs are not known, the data suggests that patient-derived autoantisera may be useful in the identification of proteins that contribute to the execution phase of apoptosis.While screening a panel of human autoantisera for their ability to precipitate new phosphoproteins from apoptotic Jurkat cell lysates, we serendipitously identified several sera that precipitated phosphoproteins fr...

Caspases rule the intracellular trafficking cartel

“…Production of the 20-kDa moiety induces ER stress and Ca 2+ release, which promotes mitochondrial fission and release of cytochrome c, thus, amplifying apoptotic signal [158][159][160]. Nevertheless, because the primary function of that organelle is the synthesis of proteins to be exported to the Golgi, stopping the entry of proteins into the secretory pathway can be achieved by the inactivation of the ribonucleoprotein signal recognition particle components SRP54, SRP68, and SRP72 that capture nascent polypeptide chains emerging from the ribosome [29,161,162]. Another hypothesis that has been put forth is that protein synthesis is maintained but that changes occur to diminish translation of protein mRNAs to be targeted to the ER [26], hence, a lesser importance of the ER.…”

The regulation of autoreactive B cells during innate immune responses

“…These self-antigens can be modified by phosphorylation and/or alternate protein cleavage, revealing novel epitopes that were not available during the induction of B and T cell tolerance [108][109][110][111]. During cell death, self-antigens are distributed in large and small blebs on the cell surface and act as targets for autoreactive B cells in a membrane-bound, multimeric form [112][113][114].…”

Life and death decisions: regulation of apoptosis by proteolysis of signaling molecules

“…The observation that many autoantigens (e.g., La, 84 U1-70 kD, 85 and the 72 kD component of the signal recognition particle, SRP 72 86 ) are cleaved by caspases during cell death led our lab to use human autoimmune sera to identify proteins that are phosphorylated during apoptosis. 87 These experiments led to the discovery of a novel kinase pathway involved in the phosphorylation of an important family of RNA splicing factors, termed serine arginine (SR) proteins, during apoptosis.…”