Arginine methylation of the RGG box does not appear to regulate ICP27 import during herpes simplex virus infection.

Arginine methylation can regulate protein import and export and can modulate protein interactions. Herpes simplex virus 1 (HSV-1) ICP27 is a shuttling protein involved in viral mRNA export. We previously reported that ICP27 is methylated on three arginines within its RGG box and that arginine methylation regulates ICP27 export and its interaction with SRPK1 and Aly/REF. Here, we report that ICP27 was efficiently imported into the nucleus when hypomethylated as determined by Fluorescence Recovery After Photobleaching (FRAP). Furthermore, coimmunoprecipitation of ICP27 with ß-importin was not significantly affected by ICP27 hypomethylation. Thus, ICP27 import does not appear to be regulated by arginine methylation.

Three arginine residues within the RGG box are crucial for ICP27 binding to herpes simplex virus 1 GC-rich sequences and for efficient viral RNA export.

ICP27 is a multifunctional protein that is required for herpes simplex virus 1 mRNA export. ICP27 interacts with the mRNA export receptor TAP/NXF1 and binds RNA through an RGG box motif. Unlike other RGG box proteins, ICP27 does not bind G-quartet structures but instead binds GC-rich sequences that are flexible in structure. To determine the contribution of arginines within the RGG box, we performed in vitro binding assays with N-terminal proteins encoding amino acids 1 to 160 of wild-type ICP27 or arginine-to-lysine substitution mutants. The R138,148,150K triple mutant bound weakly to sequences that were bound by the wild-type protein and single and double mutants. Furthermore, during infection with the R138,148,150K mutant, poly(A)(+) RNA and newly transcribed RNA accumulated in the nucleus, indicating that viral RNA export was impaired. To determine if structural changes had occurred, nuclear magnetic resonance (NMR) analysis was performed on N-terminal proteins consisting of amino acids 1 to 160 from wild-type ICP27 and the R138,148,150K mutant. This region of ICP27 was found to be highly flexible, and there were no apparent differences in the spectra seen with wild-type ICP27 and the R138,148,150K mutant. Furthermore, NMR analysis with the wild-type protein bound to GC-rich sequences did not show any discernible folding. We conclude that arginines at positions 138, 148, and 150 within the RGG box of ICP27 are required for binding to GC-rich sequences and that the N-terminal portion of ICP27 is highly flexible in structure, which may account for its preference for binding flexible sequences.

Arginine methylation of the ICP27 RGG box regulates the functional interactions of ICP27 with SRPK1 and Aly/REF during herpes simplex virus 1 infection.

The herpes simplex virus 1 protein ICP27 is methylated on arginine residues within an RGG box, and arginine methylation regulates ICP27 export to the cytoplasm. Arginine methylation can regulate protein-protein interactions; therefore, we examined the effect of hypomethylation on ICP27's interactions with cellular proteins SRPK1 and Aly/REF, which bind to ICP27 through the RGG box region. During infections with viral mutants containing lysine substitutions or the methylation inhibitor adenosine dialdehyde, the interaction of ICP27 with SRPK1 and Aly/REF was decreased, as determined by coimmunoprecipitation and colocalization studies, indicating that ICP27 RGG box methylation regulates interaction with these proteins.

Arginine methylation of the ICP27 RGG box regulates ICP27 export and is required for efficient herpes simplex virus 1 replication.

The herpes simplex virus 1 (HSV-1) multifunctional regulatory protein ICP27 shuttles between the nucleus and cytoplasm in its role as a viral mRNA export factor. Arginine methylation on glycine- and arginine-rich motifs has been shown to regulate protein export. ICP27 contains an RGG box and has been shown to be methylated during viral infection. We found by mass spectrometric analysis that three arginine residues within the RGG box were methylated. Viral mutants with substitutions of lysine for arginine residues were created as single, double, and triple mutants. Growth of these mutants was impaired and the viral replication cycle was delayed compared to wild-type HSV-1. Most striking was the finding that under conditions of hypomethylation resulting from infection with arginine substitution mutants or treatment of wild-type HSV-1-infected cells with the methylation inhibitor adenosine dialdehyde, ICP27 export to the cytoplasm occurred earlier and was more rapid than wild-type ICP27 export. We conclude that arginine methylation of the ICP27 RGG box regulates its export activity and that early export of ICP27 interferes with the performance of its nuclear functions.