Anastellin impacts on the processing of extracellular matrix fibronectin and stimulates release of cytokines from coronary artery smooth muscle cells.

Anastellin, a recombinant protein fragment from the first type III module of fibronectin, mimics a partially unfolded intermediate implicated in the assembly of fibronectin fibrils. Anastellin influences the structure of fibronectin and initiates in vitro fibrillation, yielding "superfibronectin", a polymer with enhanced cell-adhesive properties. This ability is absent in an anastellin double mutant, L37AY40A. Here we demonstrate that both wild-type and L37AY40A anastellin affect fibronectin processing within the extracellular matrix (ECM) of smooth muscle cells. Fibronectin fibrils are diminished in the ECM from cells treated with anastellin, but are partially rescued by supplementation with plasma fibronectin in cell media. Proteomic analyses reveal that anastellin also impacts on the processing of other ECM proteins, with increased collagen and decreased laminin detected in media from cells exposed to wild-type anastellin. Moreover, both anastellin forms stimulate release of inflammatory cytokines, including interleukin 6. At the molecular level, L37AY40A does not exhibit major perturbations of structural features relative to wild-type anastellin, though the mutant showed differences in heparin binding characteristics. These findings indicate that wild-type and L37AY40A anastellin share similar molecular features but elicit slightly different, but partially overlapping, responses in smooth muscle cells resulting in altered secretion of cytokines and proteins involved in ECM processing.

Copper binding leads to increased dynamics in the regulatory N-terminal domain of full-length human copper transporter ATP7B.

ATP7B is a human copper-transporting P1B-type ATPase that is involved in copper homeostasis and resistance to platinum drugs in cancer cells. ATP7B consists of a copper-transporting core and a regulatory N-terminal tail that contains six metal-binding domains (MBD1-6) connected by linker regions. The MBDs can bind copper, which changes the dynamics of the regulatory domain and activates the protein, but the underlying mechanism remains unknown. To identify possible copper-specific structural dynamics involved in transport regulation, we constructed a model of ATP7B spanning the N-terminal tail and core catalytic domains and performed molecular dynamics (MD) simulations with (holo) and without (apo) copper ions bound to the MBDs. In the holo protein, MBD2, MBD3 and MBD5 showed enhanced mobilities, which resulted in a more extended N-terminal regulatory region. The observed separation of MBD2 and MBD3 from the core protein supports a mechanism where copper binding activates the ATP7B protein by reducing interactions among MBD1-3 and between MBD1-3 and the core protein. We also observed an increased interaction between MBD5 and the core protein that brought the copper-binding site of MBD5 closer to the high-affinity internal copper-binding site in the core protein. The simulation results assign specific, mechanistic roles to the metal-binding domains involved in ATP7B regulation that are testable in experimental settings.

The two-domain elevator-type mechanism of zinc-transporting ZIP proteins.

Zinc is essential for all organisms and yet detrimental at elevated levels. Hence, homeostasis of this metal is tightly regulated. The Zrt/Irt-like proteins (ZIPs) represent the only zinc importers in metazoans. Mutations in human ZIPs cause serious disorders, but the mechanism by which ZIPs transfer zinc remains elusive. Hitherto, structural information is only available for a model member, BbZIP, and as a single, ion-bound conformation, precluding mechanistic insights. Here, we elucidate an inward-open metal-free BbZIP structure, differing substantially in the relative positions of the two separate domains of ZIPs. With accompanying coevolutional analyses, mutagenesis, and uptake assays, the data point to an elevator-type transport mechanism, likely shared within the ZIP family, unifying earlier functional data. Moreover, the structure reveals a previously unknown ninth transmembrane segment that is important for activity in vivo. Our findings outline the mechanistic principles governing ZIP-protein transport and enhance the molecular understanding of ZIP-related disorders.

Assessing water permeability of aquaporins in a proteoliposome-based stopped-flow setup.

Aquaporins (AQPs) are water channels embedded in the cell membrane that are critical in maintaining water homeostasis. We describe a protocol for determining the water permeation capacity of AQPs reconstituted into proteoliposomes. Using a stopped-flow setup, AQP embedded in proteoliposomes are exposed to an osmogenic gradient that triggers water flux. The consequent effects on proteoliposome size can be tracked using the fluorescence of an internalized fluorophore. This enables controlled characterization of water flux by AQPs. For complete details on the use and execution of this protocol, please refer to Kitchen et al. (2020).

High-yield overproduction and purification of human aquaporins from Pichia pastoris.

Aquaporins (AQPs) are membrane-bound water channels that play crucial roles in maintaining the water homeostasis of the human body. Here, we present a protocol for high-yield recombinant expression of human AQPs in the methylotropic yeast Pichia pastoris and subsequent AQP purification. The protocol typically yields 1-5 mg AQP per g of yeast cell at >95% purity and is compatible with any membrane protein cloned into Pichia pastoris, although expression levels may vary. For complete details on the use and execution of this protocol, please refer to Kitchen et al. (2020) and Frick et al. (2014).

Characterization of human aquaporin protein-protein interactions using microscale thermophoresis (MST).

Aquaporin water channels (AQPs) are membrane proteins that maintain cellular water homeostasis. The interactions between human AQPs and other proteins play crucial roles in AQP regulation by both gating and trafficking. Here, we describe a protocol for characterizing the interaction between a human AQP and a soluble interaction partner using microscale thermophoresis (MST). MST has the advantage of low sample consumption and high detergent compatibility enabling AQP protein-protein interaction investigation with a high level of control of components and environment. For complete details on the use and execution of this protocol, please refer to Kitchen et al. (2020) and Roche et al. (2017).

Cyclohexyl-α maltoside as a highly efficient tool for membrane protein studies.

Membrane proteins (MPs) constitute a large fraction of the proteome, but exhibit physicochemical characteristics that impose challenges for successful sample production crucial for subsequent biophysical studies. In particular, MPs have to be extracted from the membranes in a stable form. Reconstitution into detergent micelles represents the most common procedure in recovering MPs for subsequent analysis. n-dodecyl-ß-D-maltoside (DDM) remains one of the most popular conventional detergents used in production of MPs. Here we characterize the novel DDM analogue 4-trans-(4-trans-propylcyclohexyl)-cyclohexyl α-maltoside (t-PCCαM), possessing a substantially lower critical micelle concentration (CMC) than the parental compound that represents an attractive feature when handling MPs. Using three different types of MPs of human and prokaryotic origin, i.e., a channel, a primary and a secondary active transporter, expressed in yeast and bacterial host systems, respectively, we investigate the performance of t-PCCαM in solubilization and affinity purification together with its capacity to preserve native fold and activity. Strikingly, t-PCCαM displays favorable behavior in extracting and stabilizing the three selected targets. Importantly, t-PCCαM promoted extraction of properly folded protein, enhanced thermostability and provided negatively-stained electron microscopy samples of promising quality. All-in-all, t-PCCαM emerges as competitive surfactant applicable to a broad portfolio of challenging MPs for downstream structure-function analysis.