Immediate early protein IE63 of herpes simplex virus type 1 binds RNA directly

The herpes simplex virus type 1 (HSV-1) immediate early protein IE63 acts post-transcriptionally to affect RNA 3'-processing and splicing. Functional domains such as the RGG box and zinc-finger motifs potentially provide the protein with RNA binding capacity. Here, IE63 protein expressed in E. coli, purified by affinity chromatography and used in RNA binding assays, demonstrated similar binding to RNA substrates containing poly(A) sites from different temporal classes of HSV-1 genes, RNA containing splice site recognition sequences and RNA containing no recognized processing motifs. Competition binding assays showed that IE63 binding could be competed out, suggesting that IE63 binds RNA weakly. HSV-1 infection results in an increase or stabilization in vitro of protein binding to poly(A) site-containing RNAs; IE63 is required for this effect. RNA binding assays combining purified IE63 with protein from mock-infected and HSV-1 infected nuclear extracts demonstrated no effect on protein-RNA binding patterns.