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Azurin is a small periplasmic blue copper protein found in bacterial strains such as Pseudomonas and Alcaligenes where it facilitates denitrification. Azurin is extensively studied for its ability to mediate electron-transfer processes, but it has also sparked interest of the pharmaceutical community as a potential antimicrobial or anticancer agent. Here we offer a novel approach for expression and single-step purification of azurin in Escherichia coli with high yields and optimal metalation. A fusion tag strategy using an N-terminal GST tag was employed to obtain pure protein without requiring any additional purification steps. After the on-column cleavage by HRV 3C Protease, azurin is collected and additionally incubated with copper sulphate to ensure sufficient metalation. UV-VIS absorption, mass spectroscopy, and circular dichroism analysis all validated the effective production of azurin, appropriate protein folding and the development of an active site with an associated cofactor. MD simulations verified that incorporation of the N-terminal GPLGS segment does not affect azurin structure. In addition, the biological activity of azurin was tested in HeLa cells.

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Perforation of the outer mitochondrial membrane triggered by BAX and facilitated by its main activator cBID is a fundamental process in cell apoptosis. Here, we employ a newly designed correlative approach based on a combination of a fluorescence cross correlation binding with a calcein permeabilization assay to understand the involvement of BAX in pore formation under oxidative stress conditions. To mimic the oxidative stress, we enriched liposomal membranes by phosphatidylcholines with truncated sn-2 acyl chains terminated by a carboxyl or aldehyde moiety. Our observations revealed that oxidative stress enhances proapoptotic conditions involving accelerated pore-opening kinetics. This enhancement is achieved through increased recruitment of BAX to the membrane and facilitation of BAX membrane insertion. Despite these effects, the fundamental mechanism of pore formation remained unchanged, suggesting an all-or-none mechanism. In line with this mechanism, we demonstrated that the minimal number of BAX molecules at the membrane necessary for pore formation remains constant regardless of BAX activation by cBID or the presence of oxidized lipids. Overall, our findings give a comprehensive picture of the molecular mechanisms underlying apoptotic pore formation and highlight the selective amplifying role of oxidized lipids in triggering formation of membrane pores.

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Fibroblast growth factor 2 (FGF2) exits cells by direct translocation across the plasma membrane, a type I pathway of unconventional protein secretion. This process is initiated by phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2)-dependent formation of highly dynamic FGF2 oligomers at the inner plasma membrane leaflet, inducing the formation of lipidic membrane pores. Cell surface heparan sulfate chains linked to glypican-1 (GPC1) capture FGF2 at the outer plasma membrane leaflet, completing FGF2 membrane translocation into the extracellular space. While the basic steps of this pathway are well understood, the molecular mechanism by which FGF2 oligomerizes on membrane surfaces remains unclear. In the current study, we demonstrate the initial step of this process to depend on C95-C95 disulfide-bridge-mediated FGF2 dimerization on membrane surfaces, producing the building blocks for higher FGF2 oligomers that drive the formation of membrane pores. We find FGF2 with a C95A substitution to be defective in oligomerization, pore formation, and membrane translocation. Consistently, we demonstrate a C95A variant of FGF2 to be characterized by a severe secretion phenotype. By contrast, while also important for efficient FGF2 secretion from cells, a second cysteine residue on the molecular surface of FGF2 (C77) is not involved in FGF2 oligomerization. Rather, we find C77 to be part of the interaction interface through which FGF2 binds to the α1 subunit of the Na,K-ATPase, the landing platform for FGF2 at the inner plasma membrane leaflet. Using cross-linking mass spectrometry, atomistic molecular dynamics simulations combined with a machine learning analysis and cryo-electron tomography, we propose a mechanism by which disulfide-bridged FGF2 dimers bind with high avidity to PI(4,5)P2 on membrane surfaces. We further propose a tight coupling between FGF2 secretion and the formation of ternary signaling complexes on cell surfaces, hypothesizing that C95-C95-bridged FGF2 dimers are functioning as the molecular units triggering autocrine and paracrine FGF2 signaling.

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Several peripheral membrane proteins are known to form membrane pores through multimerization. In many cases, in biochemical reconstitution experiments, a complex distribution of oligomeric states has been observed that may, in part, be irrelevant to their physiological functions. This phenomenon makes it difficult to identify the functional oligomeric states of membrane lipid interacting proteins, for example, during the formation of transient membrane pores. Using fibroblast growth factor 2 (FGF2) as an example, we present a methodology applicable to giant lipid vesicles by which functional oligomers can be distinguished from nonspecifically aggregated proteins without functionality. Two distinct populations of fibroblast growth factor 2 were identified with (i) dimers to hexamers and (ii) a broad population of higher oligomeric states of membrane-associated FGF2 oligomers significantly distorting the original unfiltered histogram of all detectable oligomeric species of FGF2. The presented statistical approach is relevant for various techniques for characterizing membrane-dependent protein oligomerization.

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Neuroblastoma cell line SH-SY5Y, due to its capacity to differentiate into neurons, easy handling, and low cost, is a common experimental model to study molecular events leading to Alzheimer's disease (AD). However, it is prevalently used in its undifferentiated state, which does not resemble neurons affected by the disease. Here, we show that the expression and localization of amyloid-ß protein precursor (AßPP), one of the key molecules involved in AD pathogenesis, is dramatically altered in SH-SY5Y cells fully differentiated by combined treatment with retinoic acid and BDNF. We show that insufficient differentiation of SH-SY5Y cells results in AßPP mislocalization.

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In-membrane oligomerization is decisive for the function (or dysfunction) of many proteins. Techniques were developed to characterize membrane-inserted oligomers and the hereby obtained oligomerization states were intuitively related to the function of these proteins. However, in many cases, it is unclear whether the obtained oligomerization states are functionally relevant or are merely the consequence of nonspecific aggregation. Using fibroblast growth factor 2 (FGF2) as a model system, we addressed this methodological challenge. FGF2 oligomerizes in a PI(4,5)P2-dependent manner at the inner plasma membrane leaflet. This process results in membrane insertion and the formation of a lipidic membrane pore, the key intermediate in unconventional secretion of FGF2. To tackle the problem of discriminating functional oligomers from irrelevant aggregates, we present a statistical single molecule and single vesicle assay determining the brightness of individually diffusing in-membrane oligomers and correlating their oligomerization state with membrane pore formation. Importantly, time-dependent membrane pore formation was analyzed with an ensemble of single vesicles providing detailed statistics. Our findings demonstrate that quantifying oligomeric states alone does not allow for a deep understanding of the structure-function relationship of membrane-inserted oligomers.

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Plasma membranes of living cells are compartmentalized into small submicroscopic structures (nanodomains) having potentially relevant biological functions. Despite this, structural features of these nanodomains remain elusive, primarily due to the difficulties in characterizing such small dynamic entities. It is unclear whether nanodomains found in the upper bilayer leaflet are transversally registered with those found in the lower leaflet. Experiments performed on larger microscopic domains indicate that the coupling between the leaflets is strong, forcing the domains to be in perfect registration, but can the same thing be said about the biologically more relevant nanodomains? This work provides experimental evidence that even small nanodomains of variable sizes between 10 and 160 nm are interleaflet coupled. Importantly, the alternative scenarios of partially registered, independent, or antiregistered nanodomains could be excluded.