A NOVEL TYPE OF DIMERIZATION MOTIF, RELATED TO LEUCINE ZIPPERS, IS PRESENT IN PLANT HOMEODOMAIN PROTEINS

Sunflower HAHR1 is a homeodomain protein presumably involved in some a spects of root development. In the present work, we have studied the o ligomerization properties of HAHR1. A protein containing the entire ho meodomain plus adjacent C-terminal sequences (amino acids 86-325) beha ves as a dimer in gel filtration experiments. When a fragment C-termin al to the homeodomain (amino acids 151-263) is fused to the N-terminal domain of the lambda phage repressor, it is able to confer binding ef ficiency to this domain, as judged by protection from lambda superinfe ction and repression of beta-galactosidase expression under the contro l of the P-R promoter. A smaller fragment (amino acids 151-184) confer s only conditional repression. GSH transferase fusion proteins contain ing the entire homeodomain of HAHR1 plus the above-mentioned adjacent sequences bind with similar efficiency a mixture of oligonucleotides s elected from a random population. The smaller protein, however, loses its binding capacity when separated from the GSH transferase moiety. R etention of a labelled HAHR1 protein synthesized in vitro by GSH trans ferase fusions containing different protein fragments adjacent to the homeodomain and bound to GSH agarose suggests that a portion from amin o acids 151-263 is required for efficient interaction. The results obt ained indicate that HAHR1 interacts with DNA as a dimer and that its d imerization domain is located immediately C-terminal to the homeodomai n. We define two regions, the first of which confers non-efficient dim erization; this region would be stabilized by the presence of the seco nd one through putative mutual interactions. A similar motif is presen t in other related plant homeodomain proteins. (C) 1997 Elsevier Scien ce B.V.