No abstract

Thymosin ␣ 1 (T␣ 1 ) and thymosin T␣ 11 (T␣ 11 ) are polypeptides with immunoregulatory properties first isolated from thymic extracts, corresponding to the first 28 and 35 amino acid residues, respectively, of prothymosin ␣ (ProT␣), 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␣, since neither was found in extracts in which proteolysis was prevented. Here we show that a lysosomal asparaginyl endopeptidase is able to process ProT␣ to generate T␣ 1 and T␣ 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 asparaginylglycine residues in the ProT␣ sequence; specifically, Asn 28 -Gly 29 and Asn 35 -Gly 36 residues are cleaved with similar efficiency in vitro to generate T␣ 1 and T␣ 11 , respectively. By contrast T␣ 1 is the main product detected in vivo, free in the cytosol, at concentrations similar to that of ProT␣. The data here reported demonstrate that T␣ 1 is not an artifact but rather is naturally present in diverse mammalian tissues and raise the possibility that it has a functional role.

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.

Prothymosin alpha (ProT alpha) is a nuclear protein that is widely distributed in mammalian tissues, and is thought to play a role in cell proliferation. In an attempt to shed light on this role, affinity chromatography on ProT alpha-Sepharose columns was used to identify proteins in subcellular extracts of transformed human lymphocytes (NC37 cells) that interact with ProT alpha in vitro, and thus may interact with ProT alpha in vivo. Immunoblotting techniques were used to screen the ProT alpha-binding fractions for histones and other proteins involved in nuclear transport and cell-cycle control. The most abundant ProT alpha-binding proteins were histones H2A, H2B, H3, and H4. Of the nuclear-transport proteins, karyopherin beta1, Rch-1, Ran, and RCC1 were detected at high concentrations; NTF2, nucleoporin p62, and Hsp70 were detected at low concentrations; while tranportin, CAS, and Ran BPI were not detected. Of the cell-cycle control proteins, PCNA, Cdk2, and cyclin A were detected at high concentrations; cdc2, Cdk4, and cyclin B were detected at very low concentrations; while cyclin D1, cyclin D3, Cip1, and Kip1 were not detected. These results suggest (i) that ProT alpha is transported into the nucleus by the karyopherin beta1-Rch-1 complex, and (ii) that ProT alpha may interact in the nucleus with proteins involved in DNA metabolism and cell-cycle control.

Prothymosin ␣ (ProT␣) is an acidic protein involved in cell proliferation. Its phosphorylation status is correlated with proliferative activity. Here we report the isolation and characterization of a ProT␣-phosphorylating kinase (ProT␣K) from mouse splenocytes that seems to be responsible for the in vivo phosphorylation of ProT␣ and that differs from other protein kinases reported to date. This enzyme, mainly located in the cytosol, has an molecular mass of 180 kDa and appears to be made up of two proteins of 64 and 60 kDa. Its activity was markedly enhanced by mitogenic activation of cells. The ProT␣ residues phosphorylated by the enzyme in vitro are a Thr at position 7 and another Thr at positions 12 or 13, both located within casein kinase 2 (CK-2) consensus motifs; these are the same residues as are phosphorylated in vivo. The new enzyme shows a number of clear structural and catalytic differences from CK-2. It phosphorylates histones H2B and H3, although with weaker activity than ProT␣. An enzyme with the same characteristics was also found in other murine tissues and cell lines.