Prothymosin alpha (PTMA) is a highly acidic, intrinsically disordered protein that was first extracted from rat thymus and characterized as an immunogenic factor but soon detected in a variety of mammalian tissues. The presence of a nuclear localization signal and the adoption of a peculiar random-coil conformation are among the reasons behind its interaction with several molecular partners, hence at this time PTMA is known to be a very conserved and widely expressed molecule, involved in numerous and diverse biological processes. Only few studies have tried to weigh its possible involvement in reproduction, specifically in male gametogenesis: first reports have suggested that PTMA might be associated with the proliferative and early-meiotic phases of mammal spermatogenesis. Some years later, a comparative project on vertebrate spermatogenesis reported the isolation, for the first time, of prothymosin in a non-mammalian species, the amphibian Pelophylax esculentus. PTMA transcript and protein are localized in the germinal compartment, from spermatocytes to spermatozoa. A congruent pattern has been highlighted in studies on the fish Torpedo marmorata and Danio rerio, and in the mammal Rattus norvegicus, in which the expression of PTMA has been found in meiotic and post-meiotic germ cells inside testicular cysts and tubules. Moreover, its presence has been confirmed in rat and human spermatozoa (associated with the acrosome); its retention in the apical region of the head after the acrosome reaction revealed a striking conservation of the pattern during phylogenesis and suggested a possible role for the protein in gametogenesis and in fertilization.

Prothymosin α (PTMA) is a highly acidic, intrinsically disordered protein, which is widely expressed and conserved throughout evolution; its uncommon features are reflected by its involvement in a variety of processes, including chromatin remodelling, transcriptional regulation, cell proliferation and death, immunity. PTMA has also been implicated in spermatogenesis: during vertebrate germ cell progression in the testis the protein is expressed in meiotic and post-meiotic stages, and it is associated with the acrosome system of the differentiating spermatids in mammals. Then, it finally localizes on the inner acrosomal membrane of the mature spermatozoa, suggesting its possible role in both the maturation and function of the gametes. In the present work we studied PTMA expression during the spermatogenesis of the adult zebrafish, a species in which two paralogs have been described. Our data show that ptma transcripts are expressed in the testis, and localize in meiotic and post-meiotic germ cells, namely spermatocytes and spermatids. Consistently, the protein is expressed in spermatocytes, spermatids, and spermatozoa: its initial perinuclear distribution is extended to the chromatin region during cell division and, in haploid phases, to the cytoplasm of the developing and final gametes. The nuclear localization in the acrosome-lacking spermatozoa suggests a role for PTMA in chromatin remodelling during gamete differentiation. These data further provide a compelling starting point for the study of PTMA functions during vertebrate fertilization.

Prothymosin alpha (PTA) was detected by immunocytological and biochemical methods in oocytes at different stages of oogenesis, and in early embryos of the amphibian Bufo arenarum. In all cases PTA was detected in the nucleus and was absent from the cytoplasm. This indicates that this protein could act at the level of regulating transcription. Western blots were carried out using polyclonal antibodies with extracts of embryos at different stages of development from early fertilisation up to neural tube. With this method PTA was detected in all the samples under study.