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

Spermatogenesis is accompanied by a remarkable reorganization of the chromatin in post-meiotic stages, characterized by a near genome-wide displacement of histones by protamines and a transient expression of transition proteins. In Drosophila, the transition-protein-like protein Tpl94D contains an HMG-box domain and is expressed during chromatin reorganization. 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-protamine transition. 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 chromatin of the male germ line, with the most abundant levels observed during post-meiotic chromatin reorganization. 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-box domain, 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 spermatogenesis is characterized by continuous and overlapping expression of different HMG-box-containing proteins. We hypothesize that the mechanism of chromatin reorganization is a process highly secured by redundancies.