Abstract

We used the acetic acid bacterium Gluconacetobacter hansenii to produce bacterial cellulose. Because of its fibrous, tough, water-insoluble properties [1] bacterial cellulose is the perfect substrate for biOrigami. After making the cellulose, we refined it into a flat, paper-like sheet using a DIY paper-making protocol. One unique aspect of cellulose is the existence of a class of proteins known as cellulose binding domains (CBDs) that can attach to a cellulose sheet. We designed a universal CBD (uCBD) that allows for the attachment of any protein onto a cellulose sheet. Using our uCBD device, we can extend our self-folding system to serve additional functions, such a sensor for the detection of inorganic molecules, or binding enzymes to catalyze reactions on the surface of the sheet. We have contributed the BioBricks of our uCBD system to the registry. See our biobricks below:

See our BioBricks

Introduction

We used G. hansenii to produce cellulose as an alternative substrate for our biOrigami. Combine with cellulose binding domain and spore coat protein, cotZ, we could attach spores onto the cellulose surface for self-folding. Lastly, we collaborated with the Edinburgh 2015 IGEM team by providing them with cellulose sheets to test their protein sensors. You can learn more about the collaboration here

Cellulose is a polysaccharide made up of the monomer, B-D glucose, linked by (1->4) glycosidic bond [1]. It is an important structural component of the plant cell walls and has been used in various textile applications such as paper production [2]. Cellulose is also produced by a Gram-negative bacterium, Gluconacetobacter hansenii, which secretes highly-crystalline cellulose [3]. Because of the few requirements needed to produce bacterial cellulose, G. hansenii is seen as a model organism for the study of cellulose synthesis in plants [3]. We grew a large culture of G. hansenii in growth medium to produce large batch of bacterial cellulose. After retrieving the cellulose from the medium, we used a paper-making protocol to turn them into cellulose sheets (see protocol below). The cellulose would be an important substrate for our biOrigami when combined with the spores to make bioHYDRAS (link to bioHYDRA page). We can use CBDs to attach the spores to the cellulose sheet. This can be accomplished by using a DNA linker connecting the CBD sequence to a protein expressed on the spore coat, Cotz. Since Cotz localizes to the spore coat, the hybrid Cotz-CBD will allow the spores to attach to the sheet. Furthermore, we can expand on the unique properties of this CBD to allow attachment of any protein onto a cellulose surface. We call our CBD system uCBD to note its universal protein attachment abilities.
The system is inspired by the previous 2014 Stanford-Brown-Spelman iGEM team (link to page); however, it has several improvements. We used the cellulosomal-scaffolding protein A (cipA) of Clostridium thermocellum as our CBD. We chose cipA because of its high cellulose binding affinity (tested by the Imperial 2014 IGEM team); furthermore, we improved on the Imperial cipA by removing the illegal EcoRI site within the gene. CipA is attached to a monomeric streptavidin instead of the wild-type tetramer streptavidin to prevent the chance of protein aggregation, which can inhibit protein function [4]. We thus, sacrifice a small decrease in binding affinity from the monomeric streptavidin for an increase in functional activity of the overall hybrid protein. Together, the monomeric streptavidin and cipa (mSA-cipA) gene made up the cellulose attachment part of our uCBD sytem (Part I). The second part of the system involves molecular cloning any protein of interest onto the bifunctional ligase/repressor (birA) protein attached to an acceptor peptide. BirA is found in E. coli and it catalyzes attachment of biotin onto the biotin acceptor peptide such as the Avitag [5]. Because the acceptor peptide is connected to the protein of interest, this protein can be extracted and purified from the cells and can then attach to Part I via streptavidin-biotin interaction. The overall second construct would be called protein of interest-birA (POI-birA)

Data and Results

We made bacterial cellulose using G. hansenii and processed them into flat cellulose sheet as shown here.
We also designed and cloned the mSA-cipA and POI-birA into NEB-5 alpha competent E. coli . Sequencing result agrees is consistent for mSA-cipA; however, our cloning for POI-birA is unsucessful. This is because of a pstI site within the gene that causes difficulty in cloning. Further experiment will be done to resolve this problem.

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Protocols

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References

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