Sample Preparation.

Mussels of the species M. edulis were acquired from farms near Prince Edward Island and prior to preparation were kept in a marine aquarium with artificial seawater at 15 °C. Mussels were dissected open and were induced to secrete thread proteins via injection of 0.56 M potassium chloride solution in the base of the mussel foot at the bottom of the pedal groove, as described previously (20). After 5 to 10 min, the mussel foot was cut out and immediately frozen in optimal cutting temperature (OCT) medium with liquid nitrogen-cooled isopentane. Afterward, 5-µm thick longitudinal or transverse cross-sections were cut with a cryotome at −20 °C.