The C-terminus of α-Synuclein Regulates its Dynamic Cellular Internalization by Neurexin 1β

The aggregation of the disordered neuronal protein, α-Synuclein (αS), is the primary pathological feature of Parkinson's disease. Current hypotheses favor cell-to-cell spread of αS species as underlying disease progression, driving interest in identifying the molecular species and cellular processes involved in cellular internalization of αS. Prior work from our lab identified the chemically specific interaction between αS and the presynaptic adhesion protein neurexin-1β (N1β) to be capable of driving cellular internalization of both monomer and aggregated forms of αS. Here we explore the physical basis of N1β-driven internalization of αS. Specifically, we show that spontaneous internalization of αS by SH-SY5Y and HEK293 cells expressing N1β requires essentially all of the membrane-binding domain of αS; αS constructs truncated beyond residue 90 bind to N1β in the plasma membrane of HEK cells, but are not internalized. Interestingly, before internalization, αS and N1β codiffuse rapidly in the plasma membrane. αS constructs that are not internalized show very slow mobility themselves, as well as slow N1β diffusion. Finally, we find that truncated αS is capable of blocking internalization of full-length αS. Our results draw attention to the potential therapeutic value of blocking αS-N1β interactions.

Enzyme self-label-bound ATTO700 in single-molecule and super-resolution microscopy

Herein, we evaluate near-infrared ATTO700 as an acceptor in SNAP- and Halo-tag protein labelling for Förster Resonance Energy Transfer (FRET) by ensemble and single molecule measurements. Microscopy of cell surface proteins in live cells is perfomed including super-resolution stimulated emission by depletion (STED) nanoscopy.

Untangling the Conformational Polymorphism of Disordered Proteins Associated With Neurodegeneration at the Single-Molecule Level

A large fraction of the human genome encodes intrinsically disordered proteins/regions (IDPs/IDRs) that are involved in diverse cellular functions/regulation and dysfunctions. Moreover, several neurodegenerative disorders are associated with the pathological self-assembly of neuronal IDPs, including tau [Alzheimer's disease (AD)], α-synuclein [Parkinson's disease (PD)], and huntingtin exon 1 [Huntington's disease (HD)]. Therefore, there is an urgent and emerging clinical interest in understanding the physical and structural features of their functional and disease states. However, their biophysical characterization is inherently challenging by traditional ensemble techniques. First, unlike globular proteins, IDPs lack stable secondary/tertiary structures under physiological conditions and may interact with multiple and distinct biological partners, subsequently folding differentially, thus contributing to the conformational polymorphism. Second, amyloidogenic IDPs display a high aggregation propensity, undergoing complex heterogeneous self-assembly mechanisms. In this review article, we discuss the advantages of employing cutting-edge single-molecule fluorescence (SMF) techniques to characterize the conformational ensemble of three selected neuronal IDPs (huntingtin exon 1, tau, and α-synuclein). Specifically, we survey the versatility of these powerful approaches to describe their monomeric conformational ensemble under functional and aggregation-prone conditions, and binding to biological partners. Together, the information gained from these studies provides unique insights into the role of gain or loss of function of these disordered proteins in neurodegeneration, which may assist the development of new therapeutic molecules to prevent and treat these devastating human disorders.

Conformational switching within dynamic oligomers underpins toxic gain-of-function by diabetes-associated amyloid

Peptide mediated gain-of-toxic function is central to pathology in Alzheimer’s, Parkinson’s and diabetes. In each system, self-assembly into oligomers is observed and can also result in poration of artificial membranes. Structural requirements for poration and the relationship of structure to cytotoxicity is unaddressed. Here we focus on islet amyloid polypeptide (IAPP) mediated loss-of-insulin secreting cells in patients with diabetes. Newly developed methods enable structure-function enquiry to focus on intracellular oligomers composed of hundreds of IAPP. The key insights are that porating oligomers are internally dynamic, grow in discrete steps and are not canonical amyloid. Moreover, two classes of poration occur; an IAPP-specific ligand establishes that only one is cytotoxic. Toxic rescue occurs by stabilising non-toxic poration without displacing IAPP from mitochondria. These insights illuminate cytotoxic mechanism in diabetes and also provide a generalisable approach for enquiry applicable to other partially ordered protein assemblies.

Toxic gain-of-function by islet amyloid polypeptide (IAPP) is thought to be mediated by membrane poration. Here the authors develop diluted-FRET to show that changes in pore structure correlate with onset of toxicity inside insulin secreting cells.

Foldamer scaffolds suggest distinct structures are associated with alternative gains-of-function in a preamyloid toxin

An oligoquinoline foldamer library was synthesized and screened for antagonism of lipid bilayer catalysed assembly of islet amyloid polypeptide (IAPP). One tetraquinoline, ADM-116, showed exceptional potency not only in this assay, but also in secondary assays measuring lipid bilayer integrity and rescue of insulin secreting cells from the toxic effects of IAPP. Structure activity studies identified three additional oligoquinolines, closely related to ADM-116, which also have strong activity in the primary, but not the secondary assays. This contrasts work using an oligopyrdyl foldamer scaffold in which all three assays are observed to be correlated. The results suggest that while there is commonality to the structures and pathways of IAPP conformational change, it is nevertheless possible to leverage foldamers to separately affect IAPP's alternative gains-of-function.

Structure and function of MPN (Mpr1/Pad1 N-terminal) domain-containing proteins

MPN (Mpr1/Pad1 N-terminal) domain-containing proteins are present throughout all domains of life. In eukaryotes, MPN domain-containing proteins are commonly found in association with other molecules in large protein complexes, where examples comprise; the 26S proteasome and the COP9 (Constitutive photomorphogenesis 9) signalosome complexes, including the MPN subunits, POH1 and Mov34, CSN5 and CSN6, respectively. Examples of MPN domaincontaining proteins that are not incorporated in a large multi-protein complex have also been reported and include AMSH (for associated molecule with the SH3 domain of STAM) and the AMSH-Like Protein (AMSH-LP). Within the MPN domain super-family, two main subclasses have been characterised: the MPN⁺ and MPN⁻ domain-containing proteins. MPN⁺ domain-containing proteins are classified as metalloenzymes responsible for isopeptidase activity. These proteins display a JAMM (JAB1-MPN-MOV34) metalloisopeptidase motif, typically consisting of a canonical sequence (E-x[2]-H-S/T-Hx[7]-S-x[2]-D) and coordinating a zinc ion. The JAMM motif specifies a catalytic centre essential for selective hydrolysis of linkages, contained between ubiquitin/ubiquitin-like proteins and target proteins or between ubiquitin monomers within a polymeric chain. The MPN⁻ family classifies proteins, which lack the key residues present in the typical JAMM motif. These MPN⁻ proteins are void of catalytic activity, but recent studies have proposed a role in mediating protein-protein interactions, in acting as a scaffold or in activity regulation. In light of recent structural and functional studies, a more detailed understanding of these proteins has been gained and is given in the present review.

Structural and biochemical characterization of the Cop9 signalosome CSN5/CSN6 heterodimer

The Cop9 signalosome complex (CSN) regulates the functional cycle of the major E3 ubiquitin ligase family, the cullin RING E3 ubiquitin ligases (CRLs). Activated CRLs are covalently modified by the ubiquitin-like protein Nedd8 (neural precursor cell expressed developmentally down-regulated protein 8). CSN serves an essential role in myriad cellular processes by reversing this modification through the isopeptidase activity of its CSN5 subunit. CSN5 alone is inactive due to an auto-inhibited conformation of its catalytic domain. Here we report the molecular basis of CSN5 catalytic domain activation and unravel a molecular hierarchy in CSN deneddylation activity. The association of CSN5 and CSN6 MPN (for Mpr1/Pad1 N-terminal) domains activates its isopeptidase activity. The CSN5/CSN6 module, however, is inefficient in CRL deneddylation, indicating a requirement of further elements in this reaction such as other CSN subunits. A hybrid molecular model of CSN5/CSN6 provides a structural framework to explain these functional observations. Docking this model into a published CSN electron density map and using distance constraints obtained from cross-linking coupled to mass-spectrometry, we find that the C-termini of the CSN subunits could form a helical bundle in the centre of the structure. They likely play a key scaffolding role in the spatial organization of CSN and precise positioning of the dimeric MPN catalytic core.