CONSERVED SER RESIDUES IN THE BASIC REGION OF THE BZIP-TYPE TRANSCRIPTION FACTOR HBP-1A(17) - IMPORTANCE IN DNA-BINDING AND POSSIBLE TARGETS FOR PHOSPHORYLATION

HBP-1a(17) is representative of a group of plant bZIP-type transcripti on factors which includes HBP-1a proteins and G-box-binding factors. W e found kinase activity in wheat nuclear extract that phosphorylated H BP-1a(l7). Experiments using recombinant HBP-1a(l7) derivatives as sub strates revealed that all three of the Ser residues in the basic regio n, Ser-261, Ser-265, and Ser-269, were phosphorylated in a Ca2+-stimul ated manner. DNA-binding analysis of mutants with a Ser-to-Glu change, prepared to mimic the phosphorylated proteins, indicated that introdu ction of a negative charge at position 265 or 269 prevents HBP-1a(l7) from binding DNA not only in the homodimer of mutants but also in hete rodimers with a wild-type protein. It is therefore suggested that the phosphorylation regulates the function of HBP-1a(17) at least at the l evel of DNA binding. Since Ser-265 and Ser-269 are highly conserved am ong the plant bZIP-type factors known to date, a common Ca2+-mediated regulatory mechanism may exert an effect on the bZIP-type factors thro ugh phosphorylation of these conserved Ser residues.