Cryo-EM structure of the chain-elongating E3 ubiquitin ligase UBR5

UBR5 is a nuclear E3 ligase that ubiquitinates a vast range of substrates for proteasomal degradation. This HECT domain-containing ubiquitin ligase has recently been identified as an important regulator of oncogenes, e.g., MYC, but little is known about its structure or mechanisms of substrate engagement and ubiquitination. Here, we present the cryo-EM structure of human UBR5, revealing an α-solenoid scaffold with numerous protein-protein interacting motifs, assembled into an antiparallel dimer that adopts further oligomeric states. Using cryo-EM processing tools, we observe the dynamic nature of the UBR5 catalytic domain, which we postulate is important for its enzymatic activity. We characterise the proteasomal nuclear import factor AKIRIN2 as an interacting protein and propose UBR5 as an efficient ubiquitin chain elongator. This preference for ubiquitinated substrates and several distinct domains for protein-protein interactions may explain how UBR5 is linked to several different signalling pathways and cancers. Together, our data expand on the limited knowledge of the structure and function of HECT E3 ligases.

Binding Mode Characterization of Osteopontin on Hydroxyapatite by Solution NMR Spectroscopy

Extracellular matrix glycoproteins play a major role in bone mineralization and modulation of osteogenesis. Among these, the intrinsically disordered protein osteopontin (OPN) is associated with the inhibition of formation, growth and proliferation of the bone mineral hydroxyapatite (HAP). Furthermore, post-translational modifications like phosphorylation can alter conformations and interaction properties of intrinsically disordered proteins (IDPs). Therefore, the actual interaction of OPN with a HAP surface on an atomic level and how this interaction is affected by phosphorylation is of great interest. Here, we study the interaction of full-length OPN on the surface of suspended HAP nanoparticles by solution NMR spectroscopy. We report the binding modes of this IDP and provide evidence for the influence of hyperphosphorylation on the binding character and an explanation for the differing roles in biomineralization. Our study moreover presents an easy and suitable option to measure interaction of nanoparticles in a stable suspension with full-length proteins.

Effect of hydroxypropyl-β-cyclodextrin in fluid and semi-solid submicron emulsions on physiological skin parameters during regular in vivo application

Objective

The aim of the present study was to evaluate the effect of hydroxypropyl‐β‐cyclodextrin (HP‐β‐CD) in cosmetic submicron emulsions and submicron emulsion gels on physiological skin parameters during regular application in a clinical set‐up.

Methods

Formulation morphology was investigated using cryo‐transmission electron microscopy. Stability of the employed formulations was determined by photon correlation spectroscopy, measurement of pH and rheological properties. Effect on physiological skin parameters was evaluated during regular application over four weeks in a parallel group study (n = 15, healthy forearm skin) with a Corneometer, Sebumeter, skin‐pH‐Meter, Aquaflux and an Epsilon sensor. Confocal Raman spectroscopy was employed to monitor urea and NMF levels.

Results

Both submicron emulsions and gels showed satisfying storage stability irrespective of cyclodextrin incorporation. No statistically significant effects on skin barrier function and any of the observed parameters were obtained, indicating good skin tolerability of all tested formulations.

Conclusion

Results suggest good skin tolerability of the developed cosmetic submicron emulsions and gels with HP‐β‐CD.

Size analysis of nanoparticles extracted from W/O emulsions

Nanosized particles are frequently used in many different applications, especially TiO2 nanoparticles as physical filters in sunscreens to protect the skin from UV radiation. However, concerns have arisen about possible health issues caused by nanoparticles and therefore, the assessment of the occurrence of nanoparticles is important in pharmaceutical and cosmetic formulations. In a previous work of our group, a method was presented to extract nanoparticles from O/W emulsions. But to respond to the needs of dry and sensitive skin, sunscreens of the water-in-oil emulsion type are available. In these, assessment of present nanoparticles is also an important issue, so the present study offers a method for extracting nanoparticles from W/O emulsions. Both methods emanate from the same starting point, which minimizes both effort and cost before the beginning of the assessment. By addition of NaOH pellets and centrifugation, particles were extracted from W/O emulsions and measured for their size and surface area by laser diffraction. With the simple equation Q=A/S a distinction between nanoparticles and microparticles was achieved in W/O emulsions, even in commercially available samples. The present method is quick and easy to implement, which makes it cost-effective.

Differential selection of Golgi proteins by COPII Sec24 isoforms in procyclic Trypanosoma brucei

The Sec24 subunit of the coat protein complex II (COPII) has been implicated in selecting newly synthesized cargo from the endoplasmic reticulum (ER) for delivery to the Golgi. The protozoan parasite, Trypanosoma brucei, contains two paralogs, TbSec24.1 and TbSec24.2, which were depleted using RNA interference in the insect form of the parasite. Depletion of either TbSec24.1 or TbSec24.2 resulted in growth arrest and modest inhibition of anterograde transport of the putative Golgi enzyme, TbGntB, and the secretory marker, BiPNAVRG-HA9. In contrast, depletion of TbSec24.1, but not TbSec24.2, led to reversible mislocalization of the Golgi stack proteins, TbGRASP and TbGolgin63. The latter accumulated in the ER. The localization of the COPI coatomer subunit, TbεCOP, and the trans Golgi network (TGN) protein, TbGRIP70, was largely unaffected, although the latter was preferentially lost from those Golgi that were not associated with the bilobe, a structure previously implicated in Golgi biogenesis. Together, these data suggest that TbSec24 paralogs can differentiate among proteins destined for the Golgi.

Tubulohelical membrane arrays: novel association of helical structures with intracellular membranes

A novel organelle-like membrane specialization has been found in an epithelial cell line. Characteristically, the helical membrane arrays (referred to hereafter as TUHMAs) are organized around tubular, proteinaceous electron-dense cores of 80 nm in diameter. Depending on the cell status, up to 8 of these cores provide the basis for an intermingled membrane scaffold of an overall length of 3-5 microm. TUHMAs exist as single organelles in transient association with the nucleus, the rough endoplasmic reticulum, patches of annulate lamellae, and the Golgi complex. While most of the constituents are still unknown, evidence for an involvement of nucleoporins in TUHMA organization is presented, as shown by fluorescence immunochemistry. This should make TUHMAs more easily accessible for future studies on their structure and function.

Rapid microwave fixation of cell monolayers preserves microtubule-associated cell structures

Microwave (MW) fixation has been suggested as a method to rapidly immobilize cellular dynamics for fine structural studies in the electron microscope. To show its suitability for studies on cell monolayers, one has to apply MW fixation systematically in correlation with samples on the light microscopy level. Examples for MW fixation of cell monolayers, however, are still rare. MW-accelerated fixation for relatively long periods of time (1-2 min) has been reported without showing its suitability at the fine structural level. Here, we provide a rapid MW fixation protocol for cell monolayers on a subminute time scale. The impact of the MW-accelerated glutaraldehyde fixation on temperature-sensitive cytoskeletal components such as microtubules was evaluated. For testing the effectiveness of MW-assisted primary fixation, saponin treatment of the monolayers was included. Simultaneous MW-accelerated fixation and extraction by saponin was necessary to achieve a gradual improvement in visualization of cytoskeletal aspects in association with cell junctions, mitochondria, and centrioles. To establish a valuable routine program for fine structural studies of resin-embedded cell models on substrata, a protocol combining MW fixation with automatic processing in a tissue processor is provided.