How Spiders Spin Silk
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.