Spiders' silk spinning process revealed

A new research has revealed the way spiders regulate their silk spinning process.

In the work done at the Swedish University of Agricultural Sciences (SLU) and Karolinska Institutet, Anna Rising and Jan Johansson show that to make the impressive lightweight and stretchy material, spiders face quite a challenge.

To produce the silk, silk proteins, called spidroins, must convert from a soluble form to solid fibers at ambient temperatures, with water as a solvent, and at high speed.

Spidroins are big proteins of up to 3,500 amino acids that contain mostly repetitive sequences, but the most important bits for the conversion of spidroins into silk are the ends. Spidroins have a helical and unordered structure when stored as soluble proteins in silk glands, but when converted to silk their structure changes completely to one that confers a high degree of mechanical stability. These changes are triggered by an acidity (pH) gradient present between one end of the spider silk gland (the 'N-terminal domain') and the other (the 'C-terminal domain').

The authors also found that while N-terminal domain tended to pair up with other molecules at the beginning of the duct and became increasingly stable as the acidity increased along the duct, the other end the C-terminal domain destabilized as the acidity increased, and gradually unfolded until it formed the structure characteristic of silk at the acidic pH of 5.5.

The authors proposed a new 'lock and trigger' model for spider silk formation, in which gradual pairing up of the N-terminal domains locks spidroins into a network of many protein molecules, while the changes of structure in the C-terminal domains could trigger the rapid polymerization of spidroins into fibers.

This mechanism elegantly explained how spider silk could form so quickly and smoothly within the spinning duct of these amazing animals.

The research is published in the open access journal PLOS Biology.