Human immunodeficiency virus type-1 （HIV-1） regulatory Tat protein plays an essential role in viral replication and infectivity. The HIV-1 TAT is a molecule of 86 aminoacids encoded by two exons. The product of the first exon is sufficient for the transactivation of the HIV-1 promoter. Biotin molecules can be coupled easily to either antigens （such as avidin， streptavidin， Fc binding proteins like Protein A or G） or antibodies and they can be used in a variety of detection systems.
Biotin can be conjugated to many proteins without significantly changing their biological activity. Biotin binds with high affinity to avidin， streptavidin and NeutrAvidin protein.
Every single protein can react with several molecules of biotin， each of which， in turn， can bind a molecule of avidin. This greatly increases the sensitivity of many assay procedures. The avidin-biotin interaction is the strongest known noncovalent biological interaction （Ka = 1015 M-1） between protein and ligand. The bound formation between biotin and avidin is very rapid and， once formed， withstands extremes of pH， organic solvents and other denaturing agents. The avidinbiotin complex can withstand up to 3 M guanidine?HCl. Biotin can be released by 8 M guanidine?HCl at pH 1.5， or by autoclaving.
