MUTATIONAL ANALYSIS OF A TRANSCRIPTIONAL ACTIVATION REGION OF THE VP16 PROTEIN OF HERPES-SIMPLEX VIRUS

The VP16 protein of herpes simplex virus is a potent transcriptional a ctivator of the viral immediate early genes, The transcriptional activ ation region of VP16 can be divided into two functional subregions, he re designated VP16N (comprising amino acids 413-456) and VP16C (amino acids 450-490), Assays of VP16C mutants resulting from both random and alanine-scanning mutagenesis indicated that the sidechains of three p henylalanines (at positions 473, 475 and 479) and one acidic residue ( glutamate 476) are important for transcriptional activation. Aromatic and bulky hydrophobic amino acids were effective substitutes for each of the three Phe residues, whereas replacement with smaller or polar a mino acids resulted in loss of transcriptional function. In contrast, many changes were tolerated for Glu476, including bulky hydrophobic an d basic amino acids, indicating that the negative charge at this posit ion contributes little to the function of this subregion, Similar rela tive activities for most of the mutants were observed in yeast and in mammalian cells, indicating that the structural requirements for this activation region are comparable in these two species, These results r einforce the hypothesis that bulky hydrophobic residues, not acidic re sidues, are most critical for the activity of this 'acidic' transcript ional activation region.