Zinc-finger antiviral protein (ZAP) is a host antiviral factor that specifically inhibits the replication of Moloney murine leukemia virus (MLV) and Sindbis virus (SIN) by preventing accumulation of the viral mRNA in the cytoplasm. In previous studies, we demonstrated that ZAP directly binds to its specific target mRNAs. In this article, we provide evidence indicating that ZAP recruits the RNA processing exosome to degrade the target RNA. ZAP comigrated with the exosome in sucrose or glycerol velocity gradient centrifugation. Immunoprecipitation of ZAP coprecipitated the exosome components. In vitro pull-down assays indicated that ZAP directly interacted with the exosome component hRrp46p and that the binding region of ZAP was mapped to amino acids 224 -254. Depletion of the exosome component hRrp41p or hRrp46p with small interfering RNA significantly reduced ZAP's destabilizing activity. These findings suggest that ZAP is a trans-acting factor that modulates mRNA stability.
RNA degradationT he degradation of mRNA is an important control point in the regulation of gene expression (1-6). The general mRNA decay is initiated by removal of the poly(A) tail. The body of the RNA is degraded either from the 5Ј end by exonuclease Xrn1 after decapping or from the 3Ј end by an exoribonucleases complex named the exosome (7).RNA decay mechanisms are diverse in the extent of decay and decay characteristics. This diversity presumably arises from the diverse RNA-binding factors and the diverse RNA-protein and protein-protein interactions in the decay machinery (8-11). Many cis-acting elements and trans-acting factors have been shown to engage in mRNA turnover regulation (11)(12)(13)(14)(15)(16)(17)(18)(19)(20). In mammalian cells, the most common cis element is the AU-rich element (ARE) (21-24), which has been found in the 3Ј UTR of a wide variety of short-lived mRNAs, such as those of growth factors, cytokines, and protooncogenes (25-27). Various ARE binding proteins (AUBPs) have been shown to modulate the stability of ARE-containing RNAs. Some AUBPs such as HuR (13, 28) and NF90 (29) stabilize the RNA, and other AUBPs such as tristetraprolin (TTP) (12,30,31), KRSP (32), and AUF1 (33, 34) destabilize the RNA. It has been reported that the destabilizing AUBPs recruit the exosome to degrade the ARE-containing RNAs (35).The exosome is an evolutionarily highly conserved 3Ј-5Ј exoribonucleases complex existing in both the nucleus and the cytoplasm (36-40). The nuclear exosome is required for the 3Ј processing of many RNA substrates, including prerRNA, snoRNA, snRNA, and premRNA (41-43). The nuclear exosome also functions in the surveillance system, in which the transcripts with defects generated in the RNA processing and exporting pathways are degraded (44-46). The cytoplasmic exosome plays a key role in the degradation of aberrant or unused intermediate mRNAs and AREcontaining mRNAs (1,35,(47)(48)(49)(50)(51)(52)(53). The effects on the target RNA depend on interactions among the exosome, exosome cofactors, the target RNA, and sp...