Kaposi's Sarcoma-Associated Herpesvirus K8β Is Derived from a Spliced Intermediate of K8 Pre-mRNA and Antagonizes K8α (K-bZIP) To Induce p21 and p53 and Blocks K8α-CDK2 Interaction

Kaposi's sarcoma-associated herpesvirus (KSHV) is a lymphotropic DNA tumor virus that induces Kaposi's sarcoma and AIDS-related primary effusion lymphoma. KSHV open reading frame 50 and K8 genes in early viral lytic infection express, respectively, a tricistronic and a bicistronic pre-mRNA, which undergo alternative splicing to create two major spliced mRNA isoforms, α and β, by inclusion (β) or exclusion (α) of an intron at nucleotides 75563 to 75645. This intron contains some suboptimal features, which cause the intron 5′ splice site (ss) to interact weakly with U1 snRNA and the 3′ ss to bind a U2 auxiliary factor, U2AF, with low affinity. Optimization of this intron in K8 (K8 intron 2) promoted the interaction of the 5′ ss with U1 and the 3′ ss with U2AF, resulting in a substantial increase in intron splicing. Splicing of K8 intron 2 has also been shown to be stimulated by the splicing of a downstream intron. This was confirmed by the insertion of a human β-globin intron into the K8β exon 3-exon 4 splice junction, which promoted splicing of K8β intron 2 and conversion of the K8β mRNA to the K8α mRNA that encodes a K-bZIP protein. Intron 2 contains a premature termination codon, yet the K8β mRNA is insensitive to nonsense-mediated mRNA decay, suggesting that the truncated K8β protein may have a biological function. Indeed, although the truncated K8β protein is missing only a C-terminal leucine zipper domain from the K-bZIP, its expression antagonizes the ability of the K-bZIP to induce p53 and p21 and blocks K-bZIP-CDK2 interaction through interfering K8α mRNA production.