Biology subjects

Katranidis, A.

Publications and source records attributed to Katranidis, A..

6 recordsLinked to original sources

bioRxiv2025

Translation-driven temporal control for intertwined protein assembly

Protein complexes are essential to cells. However, how structurally intertwined protein subunits can assemble faithfully is poorly understood. Here, we reveal a "temporal control" mechanism driven by coupled ribosomes to form intertwined dimers. Using Disome Selective Profiling and optical tweezers, we show that the BTB domains of KEAP1, KLHL12, and PATZ1 form stable closed states as monomers, thus impeding proposed domain-swapping assembly routes. By contrast, the timed emergence of nascent chain segments during translation enables alternative folding-assembly pathways that bypass the closed monomeric state. Analysis indicates that this mechanism works in concert with dimerization quality control by the E3 ligase SCF-FBXL17, and is relevant across the BTB domain family. This study shows that ribosome cooperation expands the range of possible protein architectures.

molecular biology↗

bioRxiv2024

Calmodulin acts as a chaperone during co-translational folding of the Kv7.2 channel Calcium Responsive Domain.

In vivo, the majority of nascent protein chains must begin folding during translation in order to obtain their native structure. While the importance of co-translational folding has become increasingly clear, the specific mechanisms underlying the coordination between the ribosome, nascent chain and molecular chaperones are still uncertain. Here, we have constructed a model of the co-translational folding pathway of the calcium responsive domain (CRD) of the human neuronal KV7.2 human neuronal ion channel, showing that calmodulin (CaM) is crucial. By combining Force Profile Analysis and single-molecule force spectroscopy techniques, we found that CaM, in a calcium-dependent manner, affects early folding events involving three key -helices in the CRD. In addition, this study suggests that CaM at early stages induces the formation of metastable hairpins, as a part of the co-translational folding pathway. These findings expand on the role of CaM as a key regulator of folding in eukaryotes: not only as an essential cellular signaling protein, but also as a bona fide co-translational molecular chaperone.

molecular biology↗

bioRxiv2024

Structural organization of p62 filaments and the cellular ultrastructure of calcium-rich p62-enwrapped lipid droplet cargo

The selective autophagy receptor p62/SQSTM1 (from hereon p62) is known to form higher-order filaments in vitro and to undergo liquid-liquid phase separation when mixed with poly-ubiquitin. We determined the full-length cryo-EM structure of p62 and elucidated a structured double helical filament scaffold composed of the PB1-domain associated with the flexible C-terminal part residing in the lumen and the solvent-accessible major groove. At different pH values and upon binding to soluble LC3, LC3-conjugated membranes and poly-ubiquitin, we observed p62 filament re-arrangements in the form of structural unwinding, disassembly, lateral association and bundling, respectively. In the cellular environment, under conditions of ATG5 knockdown leading to stalled autophagy, we imaged high-contrast layers consisting of p62 oligomers enwrapping lipid droplets by cryogenic electron tomography in cellulo, which we identified as calcium phosphate by compositional spectroscopy analysis. Together, we visualized the cellular ultrastructure of p62 oligomers with high calcium content as a potential intermediate step of autophagy initiation.

biophysics↗

bioRxiv2023

Co-translational ribosome pairing enables native assembly of misfolding-prone subunits

Protein complexes are pivotal to most cellular processes. Emerging evidence indicates that pairs of ribosomes ubiquitously drive the synchronized synthesis and assembly of two protein subunits into homodimeric complexes1-5. These observations suggest protein folding mechanisms of general importance enabled by contacts between nascent chains6,7 - which have thus far rather been considered detrimental8,9. However, owing to their dynamic and heterogeneous nature, the folding of interacting nascent chains remains unexplored. Here, we show that co-translational ribosome pairing allows their nascent chains to chaperone each other, thus enabling the formation of coiled-coil homodimers from subunits that misfold individually. We developed an integrated single-molecule fluorescence and force spectroscopy approach to probe the folding and assembly of two nascent chains extending from nearby ribosomes, using the intermediate filament lamin as a model system. Ribosome proximity in early translation stages was found to be critical: when interactions between nascent chains are inhibited or delayed, they become trapped in stable misfolded states that are no longer assembly competent. Conversely, early interactions allow the two nascent chains to nucleate native-like quaternary structures that grow in size and stability as translation advances. We conjecture that protein folding mechanisms enabled by ribosome cooperation are more broadly relevant to intermediate filaments and other protein classes.

molecular biology↗

bioRxiv2020

Accessing mitochondrial protein import in living cells by protein microinjection

Mitochondrial protein biogenesis relies almost exclusively on the expression of nuclear-encoded polypeptides. The current model postulates that most of these proteins have to be delivered to their final mitochondrial destination after their synthesis in the cytoplasm. However, the knowledge of this process remains limited due to the absence of proper experimental real-time approaches to study mitochondria in their native cellular environment. We developed a gentle microinjection procedure for fluorescent reporter proteins allowing a direct non-invasive study of protein transport in living cells. As a proof of principle, we visualized potential-dependent protein import into mitochondria inside intact cells in real-time. We validated that our approach does not distort mitochondrial morphology and preserves the endogenous expression system as well as mitochondrial protein translocation machinery. We observed that a release of nascent polypeptides chains from actively translating cellular ribosomes by puromycin strongly increased the import rate of the microinjected preprotein. This suggests that a substantial amount of mitochondrial translocase complexes were involved in co-translational protein import of endogenously expressed preproteins. Our protein microinjection method opens new possibilities to study the role of mitochondrial protein import in cell models of various pathological conditions as well as aging processes.

cell biology↗

bioRxiv2020

The ribosome modulates folding inside the ribosomal exit tunnel

Proteins commonly fold cotranslationally on the ribosome, while the nascent chain emerges from the ribosomal tunnel. Protein domains that are sufficiently small can even fold while still located inside the tunnel. However, the effect of the tunnel on the folding dynamics of these domains is still not well understood. Here, we combine optical tweezers with single-molecule FRET and molecular dynamics simulations to investigate folding of the small zinc-finger domain ADR1a inside and at the vestibule of the ribosomal tunnel. The tunnel is found to accelerate folding and stabilize the folded state, reminiscent of the effects of chaperonins. However, a simple mechanism involving stabilization by confinement does not reproduce the results. Instead, it appears that electrostatic interactions between the protein and ribosome contribute to the observed folding acceleration and stabilization of ADR1a.Competing Interest StatementThe authors have declared no competing interest.View Full Text

biophysics↗