Tat protein, a transactivating factor of the human immunodeficiency virus type I, acts also as an extracellular molecule. Heparin affects the bioavailability and biological activity of extracellular Tat (Rusnati, M., Coltrini, D., Oreste, P., Zoppetti, G., Albini, A., Noonan, D., D'Adda di Fagagna, F., Giacca, M., and Presta, M. (1997) J. Biol. Chem. 272, 11313-11320). Here, a series of homogen...