The 10 most-cited articles in FEBS Letters, 2023

Perilipins are abundant lipid droplet (LD) proteins present in all metazoans and also in Amoebozoa and fungi. Humans express five perilipins, which share a similar domain organization: an amino-terminal PAT domain and an 11-mer repeat region, which can fold into amphipathic helices that interact with LDs, followed by a structured carboxy-terminal domain. Variations of this organization that arose during vertebrate evolution allow for functional specialization between perilipins in relation to the metabolic needs of different tissues. We discuss how different features of perilipins influence their interaction with LDs and their cellular targeting. PLIN1 and PLIN5 play a direct role in lipolysis by regulating the recruitment of lipases to LDs and LD interaction with mitochondria. Other perilipins, particularly PLIN2, appear to protect LDs from lipolysis, but the molecular mechanism is not clear. PLIN4 stands out with its long repetitive region, whereas PLIN3 is most widely expressed and is used as a nascent LD marker. Finally, we discuss the genetic variability in perilipins in…

The concept of mechanotransduction to the nucleus through a direct force transmission mechanism has fascinated cell biologists for decades. Central to such a mechanism is the linker of nucleoskeleton and cytoskeleton (LINC) complex, which spans the nuclear envelope to couple the cytoplasmic cytoskeleton to the nuclear lamina. In reality, there is not one LINC complex identity, but instead, a family of protein configurations of varied composition that exert both shared and unique functions. Regulated expression of LINC complex components, splice variants, and mechanoresponsive protein turnover mechanisms together shape the complement of LINC complex forms present in a given cell type. Disrupting specific gene(s) encoding LINC complex components therefore gives rise to a range of organismal defects. Moreover, evidence suggests that the mechanical environment remodels LINC complexes, providing a feedback mechanism by which cellular context influences the integration of the nucleus into the cytoskeleton. In particular, evidence for crosstalk between the nuclear and cytoplasmic…

Sar1 is a small GTPase of the ARF family. Upon exchange of GDP for GTP, Sar1 associates with the endoplasmic reticulum (ER) membrane and recruits COPII components, orchestrating cargo concentration and membrane deformation. Many aspects of the role of Sar1 and regulation of its GTP cycle remain unclear, especially as complexity increases in higher organisms that secrete a wider range of cargoes. This review focusses on the regulation of GTP hydrolysis and its role in coat assembly, as well as the mechanism of Sar1-induced membrane deformation and scission. Finally, we highlight the additional specialisation in higher eukaryotes and the outstanding questions on how Sar1 functions are orchestrated.

Polysaccharide‐degrading mono‐copper lytic polysaccharide monooxygenases (LPMOs) are efficient peroxygenases that require electron donors (reductants) to remain in the active Cu(I) form and to generate the H 2 O 2 co‐substrate from molecular oxygen. Here, we show how commonly used reductants affect LPMO catalysis in a pH‐dependent manner. Between pH 6.0 and 8.0, reactions with ascorbic acid show little pH dependency, whereas reactions with gallic acid become much faster at increased pH. These dependencies correlate with the reductant ionization state, which affects its ability to react with molecular oxygen and generate H 2 O 2 . The correlation does not apply to l ‐cysteine because, as shown by stopped‐flow kinetics, increased H 2 O 2 production at higher pH is counteracted by increased binding of l ‐cysteine to the copper active site. The findings highlight the importance of the choice of reductant and pH in LPMO reactions.

RhoGTPases are well known for being controllers of cell cytoskeleton and share common features in the way they act and are controlled. These include their switch from GDP to GTP states, their regulations by different guanine exchange factors (GEFs), GTPase-activating proteins and guanosine dissociation inhibitors (GDIs), and their similar structure of active sites/membrane anchors. These very similar features often lead to the common consideration that the differences in their biological effects mainly arise from the different types of regulators and specific effectors associated with each GTPase. Focusing on data obtained through biosensors, live cell microscopy and recent optogenetic approaches, we highlight in this review that the regulation of RhoA appears to depart from Cdc42 and Rac1 modes of regulation through its enhanced lability at the plasma membrane. RhoA presents a high dynamic turnover at the membrane that is regulated not only by GDIs but also by GEFs, effectors and a possible soluble conformational state. This peculiarity of RhoA regulation may be important for the…

ABSTRACT Oral administration of nicotinamide mononucleotide (NMN) is a prominent strategy to elevate nicotinamide adenine dinucleotide (NAD + ) levels to treat age-related pathologies, where it is assumed to be directly incorporated into the NAD + metabolome through the canonical recycling pathway. During oral delivery, NMN is exposed to the gut microbiome, which can modify the NAD + metabolome through enzyme activities that are not present in mammals. Here, we show that orally delivered NMN can undergo direct deamidation and incorporation in mammalian tissue via the de novo pathway, and that this deamidation is reduced in animals treated with antibiotics to ablate the gut microbiome. Further, we show that antibiotics treatment increases the overall availability of NAD + metabolites in the gut epithelium, with one possibility that the gut microbiome could be in competition with the host for dietary NAD + precursors. Together, these data highlight previously undescribed interactions between orally delivered NMN and the gut microbiome.

Nuclear lamins are type-V intermediate filaments that are involved in many nuclear processes. In mammals, A- and B-type lamins assemble into separate physical meshwork underneath the inner nuclear membrane, the nuclear lamina, with some residual fraction localized within the nucleoplasm. Lamins are the major part of the nucleoskeleton, providing mechanical strength and flexibility to protect the genome and allow nuclear deformability, while also contributing to gene regulation via interactions with chromatin. While lamins are the evolutionary ancestors of all intermediate filament family proteins, their ultimate filamentous assembly is markedly different from their cytoplasmic counterparts. Interestingly, hundreds of genetic mutations in the lamina proteins have been causally linked with a broad range of human pathologies, termed laminopathies. These include muscular, neurological and metabolic disorders, as well as premature aging diseases. Recent technological advances have contributed to resolving the filamentous structure of lamins and the corresponding lamina organization. In…

Conserved catabolic pathways operate to remove aberrant polypeptides from the endoplasmic reticulum (ER), the major biosynthetic organelle of eukaryotic cells. The best known are the ER-associated degradation (ERAD) pathways that control the retrotranslocation of terminally misfolded proteins across the ER membrane for clearance by the cytoplasmic ubiquitin/proteasome system. In this review, we catalog folding-defective mammalian, yeast, and plant proteins that fail to engage ERAD machineries. We describe that they rather segregate in ER subdomains that eventually vesiculate. These ER-derived vesicles are captured by double membrane autophagosomes, engulfed by endolysosomes/vacuoles, or fused with degradative organelles to clear cells from their toxic cargo. These client-specific, mechanistically diverse ER-phagy pathways are grouped under the umbrella term of ER-to-lysosome-associated degradation for description in this essay.

The copper histidine brace is a structural unit in metalloproteins ( Proc Natl Acad Sci USA 2011, 108 , 15079). It consists of a copper ion chelated by the NH 2 and π‐N atom of an N‐terminal histidine, and the τ‐N atom of a further histidine, in an overall T‐shaped coordination geometry ( Nat Catal 2018, 1 , 571). Like haem‐containing proteins, histidine‐brace‐containing proteins have peroxygenase and/or oxygenase activity, where the substrates are notable for resistance to oxidation, for example, lytic polysaccharide monooxygenases (LPMOs). Moreover, the histidine brace is an invariant unit around which different protein structures exert different activities. Given the similarities in the diversity of function of proteins that contain either the copper histidine brace or haem, the question arises as to whether the functions of histidine brace‐containing proteins duplicate those containing haem groups.

ATP synthase and four mitochondrial respiratory chain complexes, the terminal enzyme of which is the cytochrome c oxidase (complex IV) that transfers electrons to oxygen, generating water. Complex IV comprises of 14 structural subunits of dual genetic origin: while the three core subunits are mitochondrial encoded, the remaining constituents are encoded by the nuclear genome. Hence, the assembly of complex IV requires the coordination of two spatially separated gene expression machinery. Recent efforts elucidated an increasing number of proteins involved in mitochondrial gene expression, which are linked to complex IV assembly. Additionally, several COX1 biogenesis factors have been intensively biochemically investigated and an increasing number of structural snapshots shed light on the organization of macromolecular complexes such as the mitoribosome or the cytochrome c oxidase. Here, we focus on COX1 translation regulation and highlight the advanced understanding of early steps during COX1 assembly and its link to mitochondrial translation regulation.