Molecular determinants for cellular uptake of Tat protein of human immunodeficiency virus type 1 in brain cells

We measured the cellular uptake of ¹²⁵I-labeled full-length Tat (amino acids 1 to 86) (¹²⁵I-Tat^1-86) and ¹²⁵I-Tat^1-72 (first exon) in human fetal astrocytes, neuroblastoma cells, and human fetal neurons and demonstrated that the uptake of ¹²⁵I-Tat^1-72 without the second exon was much lower than that of ¹²⁵I-Tat^1-86 (P < 0.01). This suggests an important role for the C-terminal region of Tat for its cellular uptake. ¹²⁵I-Tat uptake could be inhibited by dextran sulfate and competitively inhibited by unlabeled Tat but not by overlapping 15-mer peptides, suggesting that Tat internalization is charge and conformationally dependent. Interestingly, one of 15-mer peptides, Tat^28-42, greatly enhanced ¹²⁵I-Tat uptake. These findings are important for understanding the neuropathogenesis of human immunodeficiency virus type 1 infection and in the potential application of Tat for drug delivery to cells.