The HMG-box-containing proteins tHMG-1 and tHMG-2 interact during the histone-to-protamine transition in Drosophila spermatogenesis.

Abstract Spermatogenesisis accompanied by a remarkable reorganization of the chromatinin post-meiotic stages, characterized by a near genome-wide displacement of histones by protaminesand a transient expression of transition proteins. In Drosophila , the transition-protein-like protein Tpl94D contains an HMG-boxdomain and is expressed during chromatinreorganization. Here, we searched for additional HMG-box-containing proteins with a similar expression pattern. We identified two proteins specifically expressed in the testis, tHMG-1 and tHMG-2, whose expression levels were highest during the histone-to- protaminetransition. Protein–protein interaction studies revealed that tHMG-1 and tHMG-2 form heterodimers in vivo . We demonstrated that Tpl94D, tHMG-1 and tHMG-2 localize to chromatinof the male germ line, with the most abundant levels observed during post-meiotic chromatinreorganization. Analysis of a tpl94D mutant showed that the C-terminal region of Tpl94D is dispensable for fertility. These data strongly suggested either that the truncated protein, which still contains the N-terminal HMG-boxdomain, is functional or that other proteins act in functional redundancy with Tpl94D during spermiogenesis. A thmg-1/thmg-2 null mutant also had no detectable specific phenotype, but hmgz , which encodes the major somatic HMG-box-containing protein HMGZ, was transcriptionally up-regulated. Our results showed that Drosophila spermatogenesisis characterized by continuous and overlapping expression of different HMG-box-containing proteins. We hypothesize that the mechanism of chromatinreorganization is a process highly secured by redundancies.