https://phys.org/news/2018-05-scientists-key-mechanism-formation-spider.html

To elucidate this, the CSRS researchers generated silk proteins using genetically modified bacteria that can produce silk from a golden orb-web spider (Nephila clavipes), and then performed complex analyses of the soluble proteins. They looked particularly at the repeating elements that are enclosed between two terminal elements that have been well characterized. They found that the repeating domain is composed of two patterns—random coils and a pattern called polyproline type II helix. It turns out that the second type is crucial for the formation of strong silk.

Essentially, their studies demonstrated that the polyproline type II helix can form a rigid structure which can then be transformed into beta-sheets very quickly, allowing the silk to be quickly woven. Intriguingly, it turned out that pH—which is thought to be important for the molecular interactions of the N- and C-terminal domains—does not play an important part of the folding of the repetitive domains, and that it is rather the removal of water and mechanical forces as the precursor moves through the silk gland.