Conserved cysteine residues in Kaposi's sarcoma herpesvirus ORF34 are necessary for viral production and viral pre-initiation complex formation.

Kaposis sarcoma herpesvirus (KSHV) ORF34 plays a significant role as a component of the viral pre-initiation complex (vPIC), which is indispensable for late gene expression across beta and gamma herpesviruses. Although the key role of ORF34 within the vPIC and its function as a hub protein have been recognized, further clarification regarding its specific contribution to vPIC functionality and interactions with other components is required. This study employed a deep-learning algorithm-assisted structural model of ORF34, revealing highly conserved amino acid residues across human beta- and gamma-herpesviruses localized in structured domains. Thus, we engineered ORF34 alanine-scanning mutants by substituting conserved residues with alanine. These mutants were evaluated for their ability to interact with other vPIC factors and restore viral production in cells harboring the ORF34-deficient KSHV-BAC. Our experimental results highlight the crucial role of the 4 cysteine residues conserved in ORF34: a tetrahedral arrangement consisting of a pair of C-Xn-C consensus motifs. This suggests the potential incorporation of metal cations in interacting with ORF24 and ORF66 vPIC components, facilitating late gene transcription, and promoting overall virus production by capturing metal cations. In summary, our findings underline the essential role of conserved cysteines in KSHV ORF34 for effective vPIC assembly and viral replication, thereby enhancing our understanding of the complex interplay between the vPIC components. IMPORTANCEThe initiation of late gene transcription is universally conserved across the gamma- and beta-herpesvirus families. This process employs a viral pre-initiation complex (vPIC), which is analogous to a cellular PIC. Although KSHV ORF34 is a critical factor for viral replication and is a component of the vPIC, the specifics of vPIC formation and the essential domains crucial for its function remain unclear. Structural predictions suggest that the 4 conserved cysteines (C170, C175, C256, and C259) form a tetrahedron that coordinates the metal cation. We further investigated the role of these conserved amino acids in interactions with other vPIC components, late gene expression, and virus production, to demonstrate for the first time that these cysteines are pivotal for these functions. This discovery not only deepens our comprehensive understanding of ORF34 and vPIC dynamics but also lays the groundwork for more detailed studies on herpesvirus replication mechanisms in future research.