K. Verhoef et al., ON THE ROLE OF THE 2ND CODING EXON OF THE HIV-1 TAT PROTEIN IN VIRUS-REPLICATION AND MHC CLASS-I DOWN-REGULATION, AIDS research and human retroviruses, 14(17), 1998, pp. 1553-1559
Tat is an essential protein of human immunodeficiency virus type 1 (HI
V-1) and activates transcription from the viral long terminal repeat (
LTR) promoter. The tat gene is composed of two coding exons of which t
he first, corresponding to the N-terminal 72 amino acid residues, has
been reported to be sufficient for its transcription function. We intr
oduced a stop codon at the end of the first Tat-coding exon in an expr
ession vector that produces a truncated 71-amino acid Tat protein. Thi
s Q72stop mutant displays reduced transcriptional activity of approxim
ately 54% in transient LTR-CAT transfection assays. To test the contri
bution of the second Tat-coding exon to virus replication, the Q72stop
mutation was also introduced in the infectious pLAI molecular clone.
The effect on virus replication was analyzed in primary cells and in a
transformed T cell line, The fitness of the mutant virus was calculat
ed to be approximately 75% compared with the wild-type control. Thus,
a small contribution of the C-terminal Tat domain to viral fitness was
measured. It has been proposed that the second Tat-coding exon is inv
olved in transcriptional downregulation of the MHC class I gene of the
infected host cell. Cell surface expression of the MHC protein was an
alyzed in T cells infected with the wild-type LAI virus and the replic
ation-competent Q72stop mutant. MHC expression was transiently reduced
on infection with either virus, indicating that the second Tat-coding
exon is not involved in this downregulation.