Polymerisation of chemically cross-linked actin:thymosin β4 complex to filamentous actin: alteration in helical parameters and visualisation of thymosin β4 binding on F-actin

The β-thymosins are intracellular monomeric (G-)actin sequestering proteins forming 1:1 complexes with G-actin. Here, we analysed the interaction of thymosin β4 with F-actin. Thymosin β4 at 200 μM was chemically cross-linked to F-actin. In the presence of phalloidin, the chemically cross-linked actin:thymosin β4 complex was incorporated into F-actin. These mixed filaments were of normal appearance when inspected by conventional transmission electron microscopy after negative staining. We purified the chemically cross-linked actin:thymosin β4 complex, which polymerised only when phalloidin and the gelsolin:2-actin complex were present simultaneously. Using scanning transmission electron microscopy, the mass-per-length of control and actin:thymosin β4 filaments was found to be 16.0(±0.8) kDa/nm and 18.0(±0.9) kDa/nm, respectively, indicating an increase in subunit mass of 5.4 kDa. Analysis of the helical parameters revealed an increase of the crossover spacing of the two right-handed long-pitch helical strands from 36.0 to 40.5 nm. Difference map analysis of 3-D helical reconstruction of control and actin:thymosin β4 filaments yielded an elongated extra mass. Qualitatively, the overall size and shape of the difference mass were compatible with published data of the atomic structure of thymosin β4. The deduced binding sites of thymosin β4 to actin were in agreement with those identified previously. However, parts of the difference map might represent subtle conformational changes of both proteins occurring upon complex formation.