Spider silk is an impressive material: Light weight and stretchy yet stronger than steel. Silk proteins, called spidroins, rapidly convert from a soluble form to solid fibers at ambient temperatures and with water as solvent. How the spiders regulate this process is to a large extent unknown.
Now, Anna Rising and Jan Johansson at the Swedish University of Agricultural Sciences (SLU) and Karolinska Institutet show how the silk formation process is regulated. The work was done in collaboration with colleagues in Latvia, China and USA.
Spidroins are big proteins of up to 3,500 amino acids that contain mostly repetitive sequences. The non-repetitive N- and C-terminal domains at opposite ends are thought to regulate conversion to silk. These terminal domains are unique to spider silk and are highly conserved among spiders.
Spidroins have a helical and unordered structure when stored as soluble proteins in silk glands, but when converted to silk they contain β-sheets that confer mechanical stability. We know that there is a pH gradient across the spider silk gland, which narrows from a tail to a sac to a slender duct, and that silk forms at a precise site in the duct. But further details of spider silk production have been elusive.
By using ion-selective microelectrodes to measure the pH of the glands we could show that the pH fall from 7.6 to 5.7 between the beginning of the tail and half-way down the duct. This pH gradient is much steeper than previously thought.
The microelectrodes also showed that bicarbonate ions and carbon dioxide pressure simultaneously rise along the gland. Taken together, these patterns suggested that the pH gradient is due to carbonic anhydrase, an enzyme that converts carbon dioxide and water to bicarbonate and hydrogen ions. We used a histological method, developed at SLU, to identify active carbonic anhydrase in the distal part of the gland. Carbonic anhydrase is responsible for generating the pH gradient since an inhibitor called methazolamide collapsed the pH gradient.
We also found that pH had opposite effects on the two domains' stability, which was a surprise given that the domains had been suggested to have a similar impact on silk formation. The N-terminal dimerized at pH 6 (i.e. in the beginning of the duct) and became increasingly stable as the pH dropped along the duct.
In contrast, the C-terminal domain destabilized as the pH dropped, gradually unfolding until it formed the β-sheets characteristic of silk at pH 5.5. These findings show that both terminals undergo structural changes at the pH found in the beginning of the duct. Importantly, this is also where carbonic anhydrase activity is concentrated.
These findings led us to propose a new "lock and trigger" model for spider silk formation. Gradual dimerization of the N-terminal domains lock spidroins into multimers, while the β-sheet fibrils at the C-terminals could serve as nuclei that trigger rapid polymerization of spidroins into fibers. Interestingly, the C-terminal β-sheets are similar to those in the amyloid fibrils characteristic of diseases such as Alzheimer's disease. This mechanism elegantly explains how spider silk
can form so quickly as well as how its formation can be confined to the spinning duct. Besides being essential to producing biomimetic spidroin fibers, knowing how spiders spin silk could give insights into natural ways of hindering the amyloid fibrils associated with disease.
Anna Rising, Researcher SLU and KI, ph +46-70 974 48 88 firstname.lastname@example.org
Jan Johansson, Professor, SLU and KI, +46-70-34 570 48
SLU:s vision: SLU är ett universitet i världsklass inom livs- och miljövetenskaper.
David Stephansson | www.mynewsdesk.com
An innovative high-performance material: biofibers made from green lacewing silk
20.01.2017 | Fraunhofer-Institut für Angewandte Polymerforschung IAP
Treated carbon pulls radioactive elements from water
20.01.2017 | Rice University
An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by...
Yersiniae cause severe intestinal infections. Studies using Yersinia pseudotuberculosis as a model organism aim to elucidate the infection mechanisms of these...
Researchers from the University of Hamburg in Germany, in collaboration with colleagues from the University of Aarhus in Denmark, have synthesized a new superconducting material by growing a few layers of an antiferromagnetic transition-metal chalcogenide on a bismuth-based topological insulator, both being non-superconducting materials.
While superconductivity and magnetism are generally believed to be mutually exclusive, surprisingly, in this new material, superconducting correlations...
Laser-driving of semimetals allows creating novel quasiparticle states within condensed matter systems and switching between different states on ultrafast time scales
Studying properties of fundamental particles in condensed matter systems is a promising approach to quantum field theory. Quasiparticles offer the opportunity...
Among the general public, solar thermal energy is currently associated with dark blue, rectangular collectors on building roofs. Technologies are needed for aesthetically high quality architecture which offer the architect more room for manoeuvre when it comes to low- and plus-energy buildings. With the “ArKol” project, researchers at Fraunhofer ISE together with partners are currently developing two façade collectors for solar thermal energy generation, which permit a high degree of design flexibility: a strip collector for opaque façade sections and a solar thermal blind for transparent sections. The current state of the two developments will be presented at the BAU 2017 trade fair.
As part of the “ArKol – development of architecturally highly integrated façade collectors with heat pipes” project, Fraunhofer ISE together with its partners...
19.01.2017 | Event News
10.01.2017 | Event News
09.01.2017 | Event News
20.01.2017 | Awards Funding
20.01.2017 | Materials Sciences
20.01.2017 | Life Sciences