Harnessing cellulose-binding protein domains for the development of functionalized cellulose materials

Cellulosic materials are attracting increasing research interest because of their abundance, biocompatibility, and biodegradability, making them suitable in multiple industrial and medical applications. Functionalization of cellulose is usually required to improve or expand its properties to meet the requirements of different applications. Cellulose-binding domains (CBDs) found in various proteins have been shown to be powerful tools in the functionalization of cellulose materials. In this review, we firstly introduce the structural characteristics of commonly used CBDs belonging to carbohydrate-binding module families 1, 2 and 3. Then, we summarize four main kinds of methodologies for employing CBDs to modify cellulosic materials (i.e., CBD only, genetic fusion, non-covalent linkage and covalent linkage). Via different approaches, CBDs have been used to improve the material properties of cellulose, immobilize enzymes for biocatalysis, and design various detection tools. To achieve industrial applications, researches for lowering the production cost of CBDs, improving their performance (e.g., stability), and expanding their application scenarios are still in need.

Stress-induced protein disaggregation in the endoplasmic reticulum catalysed by BiP

Protein synthesis is supported by cellular machineries that ensure polypeptides fold to their native conformation, whilst eliminating misfolded, aggregation prone species. Protein aggregation underlies pathologies including neurodegeneration. Aggregates' formation is antagonised by molecular chaperones, with cytoplasmic machinery resolving insoluble protein aggregates. However, it is unknown whether an analogous disaggregation system exists in the Endoplasmic Reticulum (ER) where ~30% of the proteome is synthesised. Here we show that the ER of a variety of mammalian cell types, including neurons, is endowed with the capability to resolve protein aggregates under stress. Utilising a purpose-developed protein aggregation probing system with a sub-organellar resolution, we observe steady-state aggregate accumulation in the ER. Pharmacological induction of ER stress does not augment aggregates, but rather stimulate their clearance within hours. We show that this dissagregation activity is catalysed by the stress-responsive ER molecular chaperone - BiP. This work reveals a hitherto unknow, non-redundant strand of the proteostasis-restorative ER stress response.

Functionalization of Cellulose-Based Hydrogels with Bi-Functional Fusion Proteins Containing Carbohydrate-Binding Modules

Materials with novel and enhanced functionalities can be obtained by modifying cellulose with a range of biomolecules. This functionalization can deliver tailored cellulose-based materials with enhanced physical and chemical properties and control of biological interactions that match specific applications. One of the foundations for the success of such biomaterials is to efficiently control the capacity to combine relevant biomolecules into cellulose materials in such a way that the desired functionality is attained. In this context, our main goal was to develop bi-functional biomolecular constructs for the precise modification of cellulose hydrogels with bioactive molecules of interest. The main idea was to use biomolecular engineering techniques to generate and purify different recombinant fusions of carbohydrate binding modules (CBMs) with significant biological entities. Specifically, CBM-based fusions were designed to enable the bridging of proteins or oligonucleotides with cellulose hydrogels. The work focused on constructs that combine a family 3 CBM derived from the cellulosomal-scaffolding protein A from Clostridium thermocellum (CBM3) with the following: (i) an N-terminal green fluorescent protein (GFP) domain (GFP-CBM3); (ii) a double Z domain that recognizes IgG antibodies; and (iii) a C-terminal cysteine (CBM3C). The ability of the CBM fusions to bind and/or anchor their counterparts onto the surface of cellulose hydrogels was evaluated with pull-down assays. Capture of GFP-CBM3 by cellulose was first demonstrated qualitatively by fluorescence microscopy. The binding of the fusion proteins, the capture of antibodies (by ZZ-CBM3), and the grafting of an oligonucleotide (to CBM3C) were successfully demonstrated. The bioactive cellulose platform described here enables the precise anchoring of different biomolecules onto cellulose hydrogels and could contribute significatively to the development of advanced medical diagnostic sensors or specialized biomaterials, among others.

Demonstration of fibrinogen-FcRn binding at acidic pH by means of Fluorescence Correlation Spectroscopy

The neonatal Fc receptor (FcRn) interacts with IgG and albumin at acidic pH within endosomes, thus protecting these plasma proteins from degradation. Recently, we proposed fibrinogen as a new binding partner of FcRn. This work was aimed at providing a direct demonstration of FcRn-fibrinogen binding at acidic pH by Fluorescence Correlation Spectroscopy. The increase in diffusion time between free and fibrinogen-bound FITC-labelled FcRn was assumed as the binding indicator. We observed that, at acidic pH (pH = 5.3), FcRn diffusion time shifted from ≈730 μs (FITC-labelled FcRn alone) to >1200 μs (FITC-labelled FcRn added with fibrinogen). A similar trend was exhibited by albumin, a known FcRn interactor, while no significant variations in diffusion time were observed upon incubation with catalase as negative control. Our results demonstrate a binding interaction between fibrinogen, one of the most abundant plasma proteins, and FcRn, a receptor involved in the regulation of the levels of IgG and albumin. This interaction is likely responsible for fibrinogen protection from intracellular degradation and recycling in plasma. Fibrinogen is crucial not only in haemostasis but also in acute inflammatory response and in some pathological conditions. The interaction with FcRn can influence not only the levels of fibrinogen in plasma and other tissues, but also the levels of other FcRn binding partners, among which are some plasma proteins of clinical relevance.

Colorimetric Detection of DNA Strands on Cellulose Microparticles Using ZZ-CBM Fusions and Gold Nanoparticles

Nucleic acid testing requires skilled personnel and expensive instrumentation. A method for the colorimetric detection of oligonucleotides that combines cellulose microparticles with biomolecular recognition is presented. DNA sequences from Trypanosoma brucei and dengue are used as model targets. Cellulose microparticles (≈20 µm) are bioactived by anchoring anti-biotin antibodies via fusions that combine a carbohydrate-binding module (CBM) with the ZZ fragment of protein A. Samples are prepared by incubating DNA probes immobilized on ≈14 nm gold nanoparticles (AuNPs) with biotin-labeled targets and mixed with bioactive microparticles. The presence of unlabeled targets could also be probed by introducing a second, biotinylated DNA probe. The target:probe-AuNP hybrids are mixed with and captured by the microparticles, which change color from white to red. Depletion of AuNPs from the liquid is also signaled by a decrease in absorbance at 525 nm. It was possible to detect targets with concentrations as low as 50 n m. In the presence of noncomplementary targets, microparticles remain white and the liquid remains red. The system is able to discriminate targets with a high degree of homology (≈53%). Overall, it is demonstrated that simple systems for the visual detection of nucleic acids can be set up by combining cellulose microparticles with biomolecular recognition agents based on CBMs and AuNPs.

Density Gradient Selection of Colloidal Silver Nanotriangles for Assembling Dye-Particle Plasmophores

A simple method based on sucrose density gradient centrifugation is proposed here for the fractionation of colloidal silver nanotriangles. This method afforded particle fractions with surface plasmon resonances, spanning from red to infrared spectral ranges that could be used to tune optical properties for plasmonic applications. This feature was exemplified by selecting silver nanotriangle samples with spectral overlap with Atto-655 dye's absorption and emission in order to assemble dye-particle plasmophores. The emission brightness of an individual plasmophore, as characterized by fluorescence correlation spectroscopy, is at least 1000-fold more intense than that of a single Atto-655 dye label, which renders them as promising platforms for the development of fluorescence-based nanosensors.