Mechanistic inhibition of herpes simplex virus-1 UL21 immune-evasion function by natural-product scaffolds: A multi-tier docking, dynamics, and energetic profiling approach.

Alpha-herpesviruses, particularly herpes simplex virus type 1 (HSV-1), establish lifelong latency and employ multiple strategies to evade host immunity, thereby sustaining infection. A key mediator of this immune evasion is the viral tegument protein, unique long 21 (UL21), which disrupts the host cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) signaling pathway and suppresses antiviral type-I interferon responses, facilitating viral replication. Despite its important role in HSV-1 pathogenesis, UL21 remains an underexplored therapeutic target. This study aimed to identify natural-product scaffolds capable of targeting the UL21 N-terminal domain and potentially interfering with UL21-mediated immune evasion. Using comprehensive virtual screening of East and South African natural-product libraries, we employed a multi-tier computational workflow comprising molecular docking, molecular dynamics (MD) simulations, and binding free-energy calculations. Four compounds, Saundersioside C, kaempferol 3,7,4'-tri-O-β-glucoside, soyasaponin II, and OSW‑1, emerged as promising UL21-binding candidates with favorable docking scores and stable interaction profiles. Docking scores for saundersioside C, kaempferol 3,7,4'-tri-O-β-glucoside, soyasaponin II, and OSW-1 were -9.05, -8.94 kcal/mol, -7.45 kcal/mol, and -7.30 kcal/mol, respectively. Molecular dynamics (MD) trajectory analyses demonstrated stable conformational behavior as indicated by consistent root mean square deviation (RMSD), limited structural fluctuations, and persistent structural compactness. Total binding free-energy calculations further identified kaempferol 3,7,4'-tri-O-β-glucoside (-40.9 kcal/mol MM/GBSA; -33.9 kcal/mol MM/PBSA) as the compound with the most favorable predicted binding affinity toward UL21. Collectively, these findings identify natural-product scaffolds with high potential to modulate UL21-mediated immune suppression, with kaempferol 3,7,4'-tri-O-β-glucoside emerging as the most promising candidate. The findings provide a foundation for experimental validation and future antiviral drug development targeting HSV-1.