Characterization of Sptrx, a novel member of the thioredoxin family specifically expressed in human spermatozoa

Thioredoxins (Trx) are small ubiquitous proteins that participate in differ ent cellular processes via redox-mediated reactions. We report here the ide ntification and characterization of a novel member of the thioredoxin famil y in humans, named Sptrx ( perm-specific trx), the first with a tissue-spec ific distribution, located exclusively in spermatozoa. Sptrx open reading f rame encodes for a protein of 486 amino acids composed of two clear domains : an N-terminal domain consisting of 23 highly conserved repetitions of a 1 5-residue motif and a C-terminal domain typical of thioredoxins. Northern a nalysis and in situ hybridization shows that Sptrx mRNA is only expressed i n human testis, specifically in round and elongating spermatids. Immunostai ning of human testis sections identified Sptrx protein in spermatids, while immunofluorescence and immunogold electron microscopy analysis demonstrate d Sptrx localization in the cytoplasmic droplet of ejaculated sperm. Sptrx appears to have a multimeric structure in native conditions and is able to reduce insulin disulfide bonds in the presence of NADPH and thioredoxin red uctase. During mammalian spermiogenesis in testis seminiferous tubules and later maturation in epididymis, extensive reorganization of disulfide bonds is required to stabilize cytoskeletal sperm structures. However, the molec ular mechanisms that control these processes are not known. The identificat ion of Sptrx with an expression pattern restricted to the postmeiotic phase of spermatogenesis, when the sperm tail is organized, suggests that Sptrx might be an important factor in regulating critical steps of human spermiog enesis.