Phosphorylation of Prothymosin α. An Approach to Its Biological Significance

Prothymosin α (ProTα), the precursor of the thymosin α1 and thymosin α11, is a 109-111 amino acids protein widely distributed in the mammalian tissues that is essential for the cell proliferation and survival through its implication on chromatin remodeling and in the proapoptotic activity. ProTα is phosphorylated at Thr residues by the M2 isoenzyme of the pyruvate kinase in a process that is dependent on the cell proliferation activity, which constitutes a novel dual functionality of this enzyme. The Thr residues phosphorylated are apparently dependent on the carcinogenic transformation of the cells. Thus, in normal lymphocytes residues Thr11 or Thr12 are phosphorylated in addition to a Thr7 residue, while in tumor cells Thr7 is the only residue phosphorylated. Phosphorylation of ProTα seems to be related to its antiapoptotic activity, although other possibilities cannot be discarded.

Gene and miRNA expression profiles of hematopoietic progenitor cells vary depending on their origin

Hematopoietic progenitor cells (HPCs) from granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood (G-PB), bone marrow (BM), or umbilical cord blood (CB) have differing biological properties and differing kinetics of engraftment post-transplantation, which might be explained, at least in part, by differing gene and miRNA expression patterns. To assess the differences in gene and miRNA expression, we analyzed whole genome expression profiles as well as the expression of 384 miRNAs in CD34(+) cells isolated from 18 healthy individuals (6 individuals per subtype of HPC source). We identified 43 genes and 36 miRNAs differentially expressed in the various CD34(+) cell sources. We observed that CD34(+) cells from CB and BM showed similar gene and miRNA expression profiles, whereas CD34(+) cells from G-PB had a very different expression pattern. Remarkably, 20 of the differentially expressed genes are targets of the differentially expressed miRNAs. Of note, the majority of genes differentially expressed in CD34(+) cells from G-PB are involved in cell cycle regulation, promoting the process of proliferation, survival, hematopoiesis, and cell signaling, and are targets of overexpressed and underexpressed miRNAs in CD34(+) cells from the same source. These data suggest significant differences in gene and miRNA expression among the various HPC sources used in transplantation. We hypothesize that the differentially expressed genes and miRNAs involved in cell cycle and proliferation might explain the differing kinetics of engraftment observed after transplantation of hematopoietic stem cells obtained from these different sources.

Karyopherin alpha2 is essential for rRNA transcription and protein synthesis in proliferative keratinocytes

Karyopherin proteins mediate nucleocytoplasmic trafficking and are critical for protein and RNA subcellular localization. Recent studies suggest KPNA2 expression is induced in tumor cells and is strongly associated with prognosis, although the precise roles and mechanisms of KPNA2 overexpression in proliferative disorders have not been defined. We found that KPNA2 expression is induced in various proliferative disorders of the skin such as psoriasis, Bowen’s disease, actinic keratosis, squamous cell carcinoma, Paget’s disease, Merkel cell carcinoma, and mycosis fungoides. siRNA-mediated KPNA suppression revealed that KPNA2 is essential for significant suppression of HaCaT proliferation under starvation conditions. Ribosomal RNA transcription and protein synthesis were suppressed by starvation combined with knockdown of KPNA (including KPNA2) expression. KPNA2 localized to the nucleolus and interacted with proteins associated with mRNA processing, ribonucleoprotein complex biogenesis, chromatin modification, and transcription, as demonstrated by tandem affinity purification and mass spectrometry. KPNA2 may be an important promoter of ribosomal RNA and protein synthesis in tumor cells.

The influence of phosphorylation of prothymosin α on its nuclear import and antiapoptotic activity

Phosphorylation of prothymosin α (ProTα) appears not to affect its influence on chromatin remodelling. To determine whether it affects nuclear import or cytosolic antiapoptotic activity, cells were transfected with vectors generating tagged recombinant ProTα (rProTα), either wild-type (rProTα-wt), which is partially phosphorylated posttranslation or the nonphosphorylatable rProTα-T7A. Immunofluorescence microscopy showed the predominant location of native ProTα, rProTα-wt, and rProTα-T7A in the nucleus. The activity of caspases 9 and 3 following apoptosis induction treatment (staurosporine) indicated reduction of apoptosis by rProTα-wt but not by rProTα-T7A. It is concluded that phosphorylation of ProTα is required for its antiapoptotic activity, but it does not affect its nuclear import.

The M2-type isoenzyme of pyruvate kinase phosphorylates prothymosin α in proliferating lymphocytes

Prothymosin α (ProTα) is a multifunctional protein that, in mammalian cells, is involved in nuclear metabolism through its interaction with histones and that also has a cytosolic role as an apoptotic inhibitor. ProTα is phosphorylated by a protein kinase (ProTαK), the activity of which is dependent on phosphorylation. ProTα phosphorylation also correlates with cell proliferation. Mass spectrometric analysis of ProTαK purified from human tumor lymphocytes (NC37 cells) enabled us to identify this enzyme as the M2-type isoenzyme of pyruvate kinase. A study on the relationship between ProTαK activity and pyruvate kinase isoforms in NC37 cells and in other cell types confirmed that the M2 isoform is the enzyme responsible for ProTαK activity in proliferating cells. Yet, about 10% of the cellular pool of the M2 isoform shows specific affinity for ProTα and is responsible for ProTαK activity. This pool of M2 protein possesses no observable pyruvate kinase activity and changes its responses to various effectors of pyruvate kinase activity; however, these responses to PK effectors are maintained by the main cellular fraction containing the M2 isoform. Acquisition of ProTαK activity by M2 seems to be due to the phosphorylation of serine and threonine residues, which, besides being essential for its catalytic activity, induces a trimeric association of ProTαK. This association can be shifted to a tetrameric form by fructose 1, 6-bisphosphate, which results in a decrease in ProTαK activity.

Prothymosin α Interacts with Free Core Histones in the Nucleus of Dividing Cells

The acidic protein prothymosin alpha (ProTalpha), with a broad presence in mammalian cells, has been widely considered to have a role in cell division, through an unrevealed mechanism in which histones may be involved in view of their ability to interact with ProTalpha in vitro. Results of co-immunoprecipitation experiments presented here demonstrate that ProTalpha interacts in vivo with core histones in proliferating B-lymphocytes (NC-37 cells). This interaction occurs with histones H3, H2A, H2B and H4 located free in the nucleoplasm, whereas no interaction was detected with histone H1, mono-nucleosome particles or chromatin. Moreover, the core histones form part of a nuclear multiprotein complex of about 700 kDa separated by ProTalpha-Sepharose affinity, with components including H3 and H4 acetyltranferases, H3 methyltransferases, hnRNP isotypes A3, A2/B1 and R, ATP-dependent and independent DNA helicases II, beta-actin and vimentin, all co-purifying by gel filtration. This indicates that the interaction of ProTalpha with core histones in the nucleus may be related to the structural modification of histones H3 and H4, and hence to chromatin activity, raising the possibility that the other proteins in the nuclear complex may play a role in this process.

Novel function of prothymosin alpha as a potent inhibitor of human immunodeficiency virus type 1 gene expression in primary macrophages

CD8(+) T lymphocytes control human immunodeficiency virus type 1 (HIV-1) infection by a cytotoxic major histocompatibility complex-restricted pathway as well as by secretion of noncytotoxic soluble inhibitory factors. Several components of CD8(+) cell supernatants have been identified that contribute to the latter activity. In this study we report that prothymosin alpha (ProTalpha), a protein found in the cell culture medium of the herpesvirus saimiri-transformed CD8(+) T-cell line, K#1 50K, has potent HIV-1-inhibitory activity. Depletion of native ProTalpha from an HIV-1-inhibitory fraction of CD8(+) cell supernatants removes the inhibitory activity, supporting its role in inhibition via soluble mediators. ProTalpha is an abundant, acidic peptide that has been reported to be localized in the nucleus and associated with cell proliferation and activation of transcription. In this report we demonstrate that ProTalpha suppresses HIV-1 replication, its activity is target cell specific, and inhibition occurs following viral integration. Native and recombinant ProTalpha protein potently inhibit HIV-1 long terminal repeat (LTR)-driven gene expression in macrophages. Furthermore studies using different promoters in lentiviral vectors (cytomegalovirus and phosphoglycerate kinase) revealed that suppression of viral replication by ProTalpha is not HIV LTR specific.

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.

Prothymosin alpha is processed to thymosin alpha 1 and thymosin alpha 11 by a lysosomal asparaginyl endopeptidase

Thymosin alpha(1) (T alpha(1)) and thymosin T alpha(11) (T alpha(11)) are polypeptides with immunoregulatory properties first isolated from thymic extracts, corresponding to the first 28 and 35 amino acid residues, respectively, of prothymosin alpha (ProT alpha), a protein involved in chromatin remodeling. It has been widely supposed that these polypeptides are not natural products of the in vivo processing of ProT alpha, since neither was found in extracts in which proteolysis was prevented. Here we show that a lysosomal asparaginyl endopeptidase is able to process ProT alpha to generate T alpha(1) and T alpha(11). In view of its catalytic properties and structural and immunological analyses, this protease was identified as mammalian legumain. It selectively cleaves some of the asparaginyl-glycine residues in the ProT alpha sequence; specifically, Asn(28)-Gly(29) and Asn(35)-Gly(36) residues are cleaved with similar efficiency in vitro to generate T alpha(1) and T alpha(11), respectively. By contrast T alpha(1) is the main product detected in vivo, free in the cytosol, at concentrations similar to that of ProT alpha. The data here reported demonstrate that T alpha(1) is not an artifact but rather is naturally present in diverse mammalian tissues and raise the possibility that it has a functional role.

Utility of peptide-protein affinity complexes in proteomics: identification of interaction partners of a tumor suppressor peptide

We used a N-biotinylated peptide analog of the C-terminal domain of the tumor suppressor protein, p21cip1/waf1 to elucidate peptide/protein interacting partners. The C-terminal domain of p21cip1/waf1 protein spanning 141-160 amino acid residues is known to bind PCNA and this interaction is important in many biological processes including cell-cycle control. This C-terminal 20-mer efficiently extracts PCNA in the presence of a variety of N- or C-terminally attached affinity tags. Using difference silver stained 2D gels combined with in-gel tryptic digests, we identified the difference spots using MALDI-TOF mass spectrometry-based peptide mass fingerprinting followed by a database search using PROFOUND against NCBIs human nonredundant protein sequence data bank. Identified spots include the p48 subunit of chromatin assembly factor-1, the heat shock 70 protein analog BiP, calmodulin, nucleolin and a spot similar in size to dimeric PCNA. In contrast, microcapillary ion-trap LC-MS/MS analysis of a tryptic digest of entire affinity extracts derived from both control and experimental runs followed by database searches using SEQUEST confirmed the presence of most of the above proteins. This strategy also identified hnRNPA1, HPSP90alpha, HSP40 and T-complex protein 1, a protein similar to prothymosin, and a possible allelic variant of the p21cip1/waf1 protein. The use of N-biotinylated peptide derived from the C-terminal domain of p21cip1/waf1 protein in proteomic analysis exemplified here suggests that peptides obtained from intracellular functional screens could also potentially serve as efficient baits to discover new drug targets.