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Mapping the binding site of thymosin beta4 on actin by competition with G-actin binding proteins indicates negative co-operativity between binding sites located on opposite subdomains of actin.

ABSTRACT: The beta-thymosins are small monomeric (G-)actin-binding proteins of 5 kDa that are supposed to act intracellularly as actin-sequestering factors stabilizing the cytoplasmic monomeric pool of actin. The binding region of thymosin beta4 was determined by analysing the binding of thymosin beta4 to actin complexed with DNase I, gelsolin or gelsolin segment 1. Binding was analysed by determining the increase in the critical concentration of actin polymerization by native gel electrophoresis or chemical cross-linking. The formation of a ternary complex including thymosin beta4 should indicate that the actin-binding proteins attach to different sites on actin. Competition would be indicative of binding to identical or overlapping sites on actin or of a negative co-operative linkage between the two binding sites. Competition of thymosin beta4 for actin binding was observed in the presence of intact gelsolin or the N-terminal gelsolin fragment, segment 1, indicating that thymosin beta4 binds to a site close to or identical with the gelsolin segment 1-binding site. The ternary complex of actin-DNase I-thymosin beta4 was obtained only when using the chemically cross-linked actin-thymosin beta4 complex, indicating that thymosin beta4 is dissociated by the binding of DNase I to actin. It is suggested that the dissociation of thymosin beta4 by DNase I binding to actin is caused by negative co-operativity between their spatially separated binding sites on actin. A similar negative co-operativity was observed between DNase I and gelsolin segment 1 binding to actin. The results therefore indicate that the respective binding sites for DNase I and segment 1 on subdomains 1 and 2 of actin are linked in a negative co-operative manner.

SUBMITTER: Ballweber E

PROVIDER: S-EPMC1218858 | biostudies-other | 1997 Nov

REPOSITORIES: biostudies-other

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Project description:AIM:To investigate whether serum thymosin beta4 can provide diagnostic or prognostic information in liver failure patients caused by chronic hepatitis B virus (HBV) infection. METHODS:Serum thymosin beta4 levels were measured in 30 patients with acute-on-chronic liver failure (ACLF), 31 patients with chronic liver failure (CLF), 30 patients with compensated liver cirrhosis (CR) and 32 patients with chronic hepatitis B and 30 healthy controls. Serum thymosin beta4 levels were measured by enzyme-linked immunosorbent assay and Child-Pugh and model for end-stage liver disease (MELD) scores were calculated for each patient on admission. RESULTS:Compared with healthy controls, serum thymosin beta4 levels in ACLF, CLF, CR and chronic hepatitis B patients were significantly lower, 6.5047 (4.7879-10.5314) microg/mL vs 0.4632 (0.2759-0.8768) microg/mL, 0.6981 (0.5209-1.2008) microg/mL, 1.8053 (0.8110-2.3397) microg/mL, 3.7803 (1.8570-6.4722) microg/mL, respectively (P < 0.001). The levels of thymosin beta4 in liver failure (ACLF or CLF) patients were markedly lower than that in CR (P < 0.001), and a difference was also found between CLF and ACLF patients (P = 0.038). In patients with chronic liver disease, there was a positive relationship between thymosin beta4 levels and albumin, choline esterase, and platelet (P < 0.001), and negative relationship with alanine aminotransferase (P = 0.020), aspartate aminotransferase, total bilirubin, international normalized ratio of prothrombin time, and Child-Pugh and MELD scores (P < 0.001). Of the 61 liver failure patients, the thymosin beta4 levels of non-survivors were significantly lower than that of survivors (P = 0.007). Receiver operating characteristics analysis identified a thymosin beta4 cutoff level of 0.5708 microg/mL for predicting poor prognosis in all liver failure patients. The serial thymosin beta4 values were observed in 13 liver failure inpatients. Lower initial values were observed in the death. While greater improvement in thymosin beta4 value was found in those who recovered from the disease. CONCLUSION:Serum thymosin beta4 can be used as an important potential predictor for liver failure caused by chronic HBV infection.

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Mapping the binding site of thymosin beta4 on actin by competition with G-actin binding proteins indicates negative co-operativity between binding sites located on opposite subdomains of actin.

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