Mussels are aquatic (freshwater and marine) invertebrate animals. They are in phylum mollusca under class Bivalve called so after their shell which comprises of two hinged valves. One of their distinguishing character is the byssus thread which they use to attach or adhere themselves to substrata. They are sessile and thus have to be firmly established in position and these threads serve this purpose. Research has been conducted, mainly in the common mussel Mytilus edulis in order to characterise these threads. These threads can withstand strong water currents at different temperatures and can adhere to different kinds of surfaces like glass, metal, rock etc even if the surface is wet. These characters led to a scientific interest on these biopolymers especially in material sciences.
Byssus threads are protein biopolymers. Biopolymers are chains of repetitive monomeric units which can be protein, sugars or nucleotides. They are of great interest today as research has shown that they can be of outmost importance in industry and in medicine. This is because they are renewable, sustainable, carbon neutral, biodegradable and composable, making them ideal to be used to make materials such as surgical threads, adhesive and other durable non-synthetic products.
They are made of collagen which is secreted by glands positioned on the foot of the mussel. They are made one-by-one and the whole process of making and laying the threads can take as little as 5 minutes. The threads are constructed from anisotropically oriented bundles of collagen fibrils and have a nonperiodic microstructure, shrinkage and melting point in excess of 90oC. Research report that they have a high energy absorption capacity and can emit as much as 70% of the total energy that they absorbed. This makes these threads to be very unique as they are stiff at one end and elastic at the other end, making them to be tough and flexible at the same time. They have been reported to be five times tougher and sixteen times stretchier than the human tendon which is also made of collagen. They can repair themselves after being damaged. All these characters are due to the proteins that makes the byssi threads.
The byssus is divided into three regions which are the stem which supports each thread at the foot of the mussel, the thread itself and the plaque which is the adhesive pad located at the end of the thread that attaches to substrata.Proteins precursors that make up the byssus have been isolated and are of three kinds. These are byssal precollagen-P (preCol-P) dominating at the proximal end, byssal precollagen-C (preCol-D), which dominates at the distal end and lastly byssal precollagen-NG (preCol-NG) which is evenly distributed along the length of the thread and is suspected to be an adapter that joins preCol-P and preCol-D (Qin and Waite, 1997).The proteins are referred to as Col-P, Col-D and Col-NG respectively.
The plaque at the end of the thread can adhere to surfaces by an adhesive that is secreted by the mussel. Studies have been carried out to isolate and characterise and define the make-up of the adhesive (Deming. 1999) .The adhesive has been reported to be made of five distinct types of proteins which are Mefp-1, Mefp-2, Mefp-3, Mefp-4 and Mefp-5 , all containing high levels of the amino acid 3,4-dihydroxyphenyl-L-alanine (DOPA) (Deming, 1999).Researchers have proposed that DOPA residues allows the protein molecules to cross link each other and studies have shown that proteins with less or without DOPA have reduced adhesion ability. Recently, a mussel foot protein type 5 (fp-5) have been identified from the mussel adhesive protein pad Mefp-5 and it showed to have more DOPA residues compared to the other 4 proteins.
Looking at the said potential uses of biopolymers and the qualities related with the byssus and the adhesion protein, it will therefore be of great importance if they can be produced in bulk in order to be used in industry for the different applications. This then calls in ways that can be devised to ensure that production of these threads and adhesive can meet demand. Biotechnological techniques can be used to transform other systems (bacterial, planta fungal etc), that can be used to produce even more of the desired product. As the proteins have been isolated and characterised this then makes it easy to come up with ways that the recombinant techniques can be used.
Agrobacterium-mediated transformation can be used to clone the identified genes involved in the synthesis and proteins of this important biopolymer into tobacco which is an easy plant to transform, thus ensuring continuous production in a renewable system. After successfully transforming the plants and growing them, proteins can be isolated from the plants so that they can be used to for different applications such as dental fillings, make tough elastic ropes, surgical adhesives etc.
This approach does not seem to be far fetched as scientists in Israel have successfully transformed tobacco plants to produce type-1 human collagen (Ruggiero etal., 2000) by inserting different genes in the same plant via Agrobacterium-mediated transformation, thus there is a high chance that the transformation using byssus thread and adhesive protein genes will be successful using this transformation technique and the tobacco plant system. However it is true that other systems i.e. fungal (Yeast), might be able to produce more of the desired products and there is currently a study that is being under-carried to produce the byssal proteins in Yeast and it is of great interest to get the outcome of this invention, because then other proteins can be produced in the same way.
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