Multiple tRNA attachment sites in prothymosin alpha

Roles of thymosins in cancers and other organ systems

Thymosins are small peptides, originally identified from the thymus, but now known to be more widely distributed in many tissues and cells. Thymosins are divided into three main groups, alpha-, beta-, : and gamma-thymosins, based on their isoelectric points. alpha-thymosins (ProTalpha, Talphal) have nuclear localization and are involved in transcription and/or DNA replications; whereas beta-thymosins (Tbeta4, Tbeta10, Tbetal5) have cytoplasmic localization and show high affinity to G-actin for cell mobility. Furthermore, it is well known that both alpha- and beta-thymosins play important roles in modulating immune response, vascular biology, and cancer pathogenesis. More importantly, thymosins may have significant clinical applications. They may serve as molecular markers for the diagnosis and prognosis of certain diseases. In addition, they could be molecular targets of certain diseases or be used as therapeutic agents to treat certain diseases. However, the molecular mechanisms of action of thymosins are largely unknown. This review not only presents recent advances of basic science research of thymosins and their clinical applications but provides thoughtful views for future directions of investigation on thymosins.

The thymosins. Prothymosin alpha, parathymosin, and beta-thymosins: structure and function

The studies on thymosins were initiated in 1965, when the group of A. White searched for thymic factors responsible for the physiological functions of thymus. To restore thymic functions in thymic-deprived or immunodeprived animals, as well as in humans with primary immuno-deficiency diseases and in immunosuppressed patients, a standardized extract from bovine thymus gland called thymosin fraction 5 was prepared. Thymosin fraction 5 indeed improved immune response. It turned out that thymosin fraction 5 consists of a mixture of small polypeptides. Later on, several of these peptides (polypeptide beta 1, thymosin alpha 1, prothymosin alpha, parathymosin, and thymosin beta 4) were isolated and tested for their biological activity. The research of many groups has indicated that none of the isolated peptides is really a thymic hormone; nevertheless, they are biologically important peptides with diverse intracellular and extracellular functions. Studies on these functions are still in progress. The current status of knowledge of structure and functions of the thymosins is discussed in this review.

Apoptosis-related fragmentation, translocation, and properties of human prothymosin alpha

Human prothymosin alpha is a proliferation-related nuclear protein undergoing caspase-mediated fragmentation in apoptotic cells. We show here that caspase-3 is the principal executor of prothymosin alpha fragmentation in vivo. In apoptotic HeLa cells as well as in vitro, caspase-3 cleaves prothymosin alpha at one major carboxy terminal (DDVD(99)) and several suboptimal sites. Prothymosin alpha cleavage at two amino-terminal sites (AAVD(6) and NGRD(31)) contributes significantly to the final pattern of prothymosin alpha fragmentation in vitro and could be detected to occur in apoptotic cells. The major caspase cleavage at D(99) disrupts the nuclear localization signal of prothymosin alpha, which leads to a profound alteration in subcellular localization of the truncated protein. By using a set of anti-prothymosin alpha monoclonal antibodies, we were able to observe nuclear escape and cell surface exposure of endogenous prothymosin alpha in apoptotic, but not in normal, cells. We demonstrate also that ectopic production of human prothymosin alpha and its mutants with nuclear or nuclear-cytoplasmic localization confers increased resistance of HeLa cells toward the tumor necrosis factor-induced apoptosis.

First evidence of prothymosin alpha in a non-mammalian vertebrate and its involvement in the spermatogenesis of the frog Rana esculenta

A cDNA clone encoding for a Prothymosin alpha (Prot-alpha) has been isolated and characterized from the testis of the frog Rana esculenta. Frog Prothymosin alpha (fProt-alpha) predicted a 109 amino acid protein with a high homology to the mammalian Prot-alpha. fProt-alpha contains 28 aspartic and 25 glutamic acid residues and presents the typical basic KKQK amino acid sequence in the close carboxyl terminal region. Northern blot analysis revealed that fProt-alpha is highly expressed in the testis. A different expression of fProt-alpha transcript was found during the frog reproductive cycle with a peak in September/October in concomitance with germ cell maturation, strongly suggesting a role for this protein in the testicular activity. In situ hybridization evidenced that the only germ cells expressing fProt-alpha are the primary and secondary spermatocytes; in addition, the hybridization signal was stronger in the October testis. Taken together, our findings indicate that fProt-alpha might contribute to the efficiency of frog spermatogenesis with a role during the meiosis. This study is the first report on the isolation and characterization of a Prot-alpha in a non-mammalian vertebrate. In addition, our results indicate that the testis of the frog R. esculenta may be a useful model to increase the knowledge concerning the physiological role of Prot-alpha in vertebrates.

Fifteen years of prothymosin alpha: contradictory past and new horizons

Prothymosin alpha (ProTalpha) is a highly acidic and small protein of only 111 amino acids with an unusual primary structure. One would expected it to play an essential role in the organism, as it has a wide distribution and is high conserved among mammals, yet its exact function remains elusive. Despite the number of effects described for ProTalpha, intracellular and extracellular, none are accepted as its physiological role. Furthermore, many other aspects of its biology still remain obscure. In this review, we discuss the structural properties, location, gene family, functions and immunomodulatory activities of and cellular receptors for ProTalpha. These topics are addressed in an attempt to reconcile opposing outlooks while emphasizing those points where scant investigations do exist. We have also re-evaluated some previous results in light of the structural properties of ProTalpha and have found that molecular mimetism could be the underlying basis. This molecular mimicry hypothesis provides a clue that must not be overlooked for a realistic appraisal of future results.

Prothymosin alpha selectively enhances estrogen receptor transcriptional activity by interacting with a repressor of estrogen receptor activity

We find that prothymosin alpha (PTalpha) selectively enhances transcriptional activation by the estrogen receptor (ER) but not transcriptional activity of other nuclear hormone receptors. This selectivity for ER is explained by PTalpha interaction not with ER, but with a 37-kDa protein denoted REA, for repressor of estrogen receptor activity, a protein that we have previously shown binds to ER, blocking coactivator binding to ER. We isolated PTalpha, known to be a chromatin-remodeling protein associated with cell proliferation, using REA as bait in a yeast two-hybrid screen with a cDNA library from MCF-7 human breast cancer cells. PTalpha increases the magnitude of ERalpha transcriptional activity three- to fourfold. It shows lesser enhancement of ERbeta transcriptional activity and has no influence on the transcriptional activity of other nuclear hormone receptors (progesterone receptor, glucocorticoid receptor, thyroid hormone receptor, or retinoic acid receptor) or on the basal activity of ERs. In contrast, the steroid receptor coactivator SRC-1 increases transcriptional activity of all of these receptors. Cotransfection of PTalpha or SRC-1 with increasing amounts of REA, as well as competitive glutathione S-transferase pulldown and mammalian two-hybrid studies, show that REA competes with PTalpha (or SRC-1) for regulation of ER transcriptional activity and suppresses the ER stimulation by PTalpha or SRC-1, indicating that REA can function as an anticoactivator in cells. Our data support a model in which PTalpha, which does not interact with ER, selectively enhances the transcriptional activity of the ER but not that of other nuclear receptors by recruiting the repressive REA protein away from ER, thereby allowing effective coactivation of ER with SRC-1 or other coregulators. The ability of PTalpha to directly interact in vitro and in vivo with REA, a selective coregulator of the ER, thereby enabling the interaction of ER with coactivators, appears to explain its ability to selectively enhance ER transcriptional activity. These findings highlight a new role for PTalpha as a coregulator activity-modulating protein that confers receptor specificity. Proteins such as PTalpha represent an additional regulatory component that defines a novel paradigm enabling receptor-selective enhancement of transcriptional activity by coactivators.

Divalent metal cation binding properties of human prothymosin alpha

The divalent cation binding properties of human prothymosin alpha, an abundant nuclear protein involved in cell proliferation, were evaluated. By using prothymosin alpha retardation on a weak cation chelating resin charged with various divalent cations, specific binding of Zn2+ ions by prothymosin alpha was observed. This finding was further confirmed by the equilibrium dialysis analysis which demonstrated that, within the micromolar range of Zn2+ concentrations, prothymosin alpha could bind up to three zinc ions in the presence of 100 mM NaCl and up to 13 zinc ions in the absence of NaCl. Equilibrium dialysis analysis also revealed that prothymosin alpha could bind Ca2+, although the parameters of Ca2+ binding by prothymosin alpha were less pronounced than those of Zn2+ binding in terms of the number of metal ions bound, the KD values, and the resistance of the bound metal ions to 100 mM NaCl. The effects of Zn2+ and Ca2+ on the interaction of prothymosin alpha with its putative partners, Rev of HIV type 1 and histone H1, were examined. We demonstrated that Rev binds prothymosin alpha, and that prothymosin alpha binding to Rev but not to histone H1 was significantly enhanced in the presence of zinc and calcium ions. Our data suggest that the modes of prothymosin alpha interaction with Rev and histone H1 are distinct and that the observed zinc and calcium-binding properties of prothymosin alpha might be functionally relevant.

Nuclear distribution of prothymosin alpha and parathymosin: evidence that prothymosin alpha is associated with RNA synthesis processing and parathymosin with early DNA replication

Prothymosin alpha and parathymosin are two ubiquitous small acidic nuclear proteins that are thought to be involved in cell cycle progression, proliferation, and cell differentiation. In an effort to investigate the molecular function of the two proteins, we studied their spatial distribution by indirect immunofluorescence labeling and confocal scanning laser microscopy in relation to nuclear components involved in transcription, translation, and splicing. Results indicate that both proteins exhibit a punctuated nuclear distribution and are excluded by nucleoli. The distribution of prothymosin alpha in the nucleus is related to that of transcription sites, whereas the distribution of parathymosin correlates with early replication sites. This implies that prothymosin alpha and parathymosin are involved in transcription and replication, respectively. In addition to the punctate distribution, prothymosin alpha also is found concentrated in 1-6 nuclear domains per cell. These domains are found in more than 80% of randomly growing T24 human bladder carcinoma cells. They have a diameter of 0.2-2.5 microm, their size being inversely related to the number of domains per cell. The domains disappear during mitosis and the protein is excluded from the metaphase chromosomes. Double-labeling experiments associate these prothymosin alpha domains with PML and CstF64 containing nuclear bodies, but not with hnRNP-I containing domains or coiled bodies.