HTLV-I TAX SELF-ASSOCIATION IN OPTIMAL TRANSACTIVATION FUNCTION

HTLV-I Tax protein is a potent transcriptional activator of viral and cellular genes. Tax does not bind DNA directly but interacts through p rotein-protein contact with host cell factors that recognize the viral long terminal repeat (LTR), Domains within Tax needed for protein-pro tein interaction have not been fully characterized, In studying transc riptional function in yeast cells, we unexpectedly found that Tax func tions optimally not as a monomer, but as a homodimer. Here we have use d the one hybrid and two hybrid genetic approaches in yeast to investi gate the region(s) within Tax necessary for self-association. Dimer fo rmation was also confirmed biochemically by using electrophoretic mobi lity shift (EMSA) and supershift assays, Twenty two Tax point mutants were utilized to map relevant residues, Genetic results from this seri es of mutants revealed that a necessary region for dimerization is con tained within a previously characterized zinc finger domain, Two loss- of-function Tax mutants, each poorly active when assayed individually, were found to have complementing activity when co-expressed together, This genetic complementation suggests a mechanism for trans-activatio n resulting from simultaneous but non-identical contact with a respons ive target by each of two Tax monomers in a dimer.