Importance of C-terminus of herpes simplex virus type 1 thymidine kinase for maintaining thymidine kinase and acyclovir-phosphorylation activities.

We previously isolated an acyclovir (ACV)-resistant herpes simplex virus type 1 (HSV-1), strain TAR, from a child with Wiskott-Aldrich syndrome. An acyclovir-sensitive HSV-1, strain TAS, had been isolated from the same patient before the isolation of HSV-1 TAR. The TK protein of ACV-sensitive HSV-1 TAS was composed of 376 amino acids, while that of HSV-1 TAR was composed of 407 amino acids with altered amino acid residue between positions 355-407. The elongation of TK was caused by a single nucleotide deletion of cytosine from a homopolymer stretch of 4 cytosine residues between positions 1061-1064. There was no viral TK activity in HSV-1 TAR-infected Vero cells, indicating the importance of the C-terminal portion of TK protein from positions 355-376. Recombinant TK polypeptides with amino acid deletions at the C-terminus were prepared, and TK and ACV-phosphorylation activities were examined. Deletion of 5 and 6 amino acids from the C-terminus of the TK polypeptide of HSV-1 TAS resulted in a reduction of TK activity by approximately 75% and 100%, respectively. These mutant TK polypeptides did not phosphorylate ACV. These results indicate that amino acid residues from positions 371-376 in the C-terminal portion of HSV-1 TK protein are essential for keeping TK and ACV-phosphorylation activities.