Identification and expression of an autosomal paralogue of ribosomal protein S4, X-linked, in mice: Potential involvement of testis-specific ribosomal proteins in translation and spermatogenesis

• A new paralogue of ribosomal protein S4 was identified in the mouse testis ribosome.
• The autosomal gene encoding this paralogue exhibited testis-prominent expression.
• This paralogue was included in translating ribosomes as a small subunit component.
• This paralogue was expressed ubiquitously in spermatogenic cells.
• This expression pattern was partially different from that of ribosomal L10-like.

Recent studies have revealed heterogeneity in the structure of eukaryotic cytoplasmic ribosomes, including a difference in protein composition. It has been proposed that this heterogeneity, or the specialized ribosome, contributes to tissue development and homeostasis through selective mRNA translation, although this remains largely unclear. Our previous proteomic survey of rodent ribosomes found the testis-specific ribosomal proteins L10-like and L39-like, which are paralogues of the X-linked ribosomal proteins L10 and L39, respectively. We have hypothesized that the rodent testis provides a good model for examining the possible functional importance of ribosome heterogeneity. In the present study, a new paralogue of X-linked ribosomal protein S4 has been identified in the mouse testis. The gene encoding this paralogue was autosomal, intronless and expressed predominantly in the testis. It appeared that this paralogue was included in polysomes as a component of the ribosome. Although these properties were similar to those of the ribosomal proteins L10-like and L39-like, this S4 paralogue and L10-like showed partially different expression patterns in spermatogenic cells. These findings are discussed in relation to the unique evolution of genes encoding a paralogue of ribosomal protein S4 in mammals and to the significance of testis-specific paralogues of ribosomal proteins in active ribosomes during spermatogenesis.