Expression of the TAR RNA binding protein in human testis

In testis, several RNA binding proteins have been shown to play a role in the translational regulation of specific transcripts. The human protein TRBP (TAR RNA binding protein) is the homologue of the mouse Prbp (Prm-1 RNA binding protein) involved in the protamine mRNA translational delay. TRBP is known to activate the HIV-1 long terminal repeat but this protein has never been investigated during spermatogenesis. The aim of this work was to analyse the TRBP expression in human testis. By Northern blot analysis, we demonstrated a major 1.5 kb transcript present at a high level in human testis and, to a lesser extent, in some other tissues. In-situ hybridization revealed that this transcript was present only in elongating spermatids. Antibodies raised against a 27 amino acid TRBP-specific peptide revealed a single protein of 43 kDa expressed in the cytoplasm of elongated spermatids. At the ultrastructural level, quantitative analysis of both TRBP mRNA and protein, using electron microscopy in-situ hybridization and immunocytochemistry, showed that TRBP is expressed mainly in spermatids at steps 3-4 of spermiogenesis. These results are in agreement with the probable role of TRBP in the control of human protamine mRNA translation.