I-Shaped Dimers of a Plant Chloroplast FOF1-ATP Synthase in Response to Changes in Ionic Strength

Small-angle X-ray scattering structural insights into alternative pathway of actin oligomerization associated with inactivated state

In addition to the well-known monomeric globular (G-actin) and polymeric fibrillar (F-actin) forms, actin can exist in the so-called inactivated form (I-actin). Hsp70 chaperon, prefoldin, and CCT chaperonin are required to obtain native globular state. In contrast, I-actin is spontaneously formed in the absence of intracellular folding machinery. I-actin can be obtained from G-actin by elimination of divalent ion, incubation in presence of small concentrations of denaturants, and by heat exposure. Since G-actin is a quasi-stationary, thermodynamically unstable form, it can gradually transform into inactivated state in the absence of chelating/denaturating agents or heat exposure, but the transition is much slower. I-actin was shown to associate into oligomers up to the molecular weight of 14-16 G-actin monomers, though the structure of these oligomers remains uncharacterized. This study employs small-angle X-ray scattering to reveal novel insights into the oligomerization process of such spontaneously formed inactivated actin. These oligomers are differentiated from F-actin through comparative analysis, highlighting a unique oligomerization pathway.

Ferritin-based fusion protein shows octameric deadlock state of self-assembly

Ferritin is a universal protein complex responsible for iron perception in almost all living organisms and has applications from fundamental biophysics to drug delivery and structure-based immunogen design. Different platforms based on ferritin share similar technological challenges limiting their development - control of self-assembling processes of ferritin itself as well as ferritin-based chimeric recombinant protein complexes. In our research, we studied self-assembly processes of ferritin-based protein complexes under different expression conditions. We fused a ferritin subunit with a SMT3 protein tag, a homolog of human Small Ubiquitin-like Modifier (SUMO-tag), which was taken to destabilize ferritin 3-fold channel contacts and increase ferritin-SUMO subunits solubility. We first obtained the octameric protein complex of ferritin-SUMO (8xFer-SUMO) and studied its structural organization by small-angle X-ray scattering (SAXS). Obtained SAXS data correspond well with the high-resolution models predicted by AlphaFold and CORAL software of an octameric assembly around the 4-fold channel of ferritin without formation of 3-fold channels. Interestingly, three copies of 8xFer-SUMO do not assemble into 24-meric globules. Thus, we first obtained and structurally characterized ferritin-based self-assembling oligomers in a deadlock state. Deadlock oligomeric states of ferritin extend the known scheme of its self-assembly process, being new potential tools for a number of applications. Finally, our results might open new directions for various biotechnological platforms utilizing ferritin-based tools.