Major ampullate silk is secreted by the major ampullate gland of spiders. In the best studied systems two silk proteins differing in amino acid sequence and structure are secreted by the gland. The ratio of the two proteins is thought to affect the silk properties.
MA silk is produced in the MA gland which has a distal tail, a storage reservoir, and a duct to the outside that ends at a spigot to control release. Silk proteins are secreted by cells lining the narrow tail of the MA gland. The proteins are stored in the reservoir at a pH between 6.8 and 7.0. pH affects the folding and structure of proteins. When a silk strand is released, protein from the reservoir travels down the duct to the spigot. Within the duct the pH of the silk solution acidifies from a neutral pH at the reservoir end to an acidic pH of about 5 near the spigot. The change in pH helps the silk to assemble into strands when they are released from the spigot.
Vollrath F, Knight DP. 2001. Liquid crystalline spinning of spider silk. Nature 410:541–48