Many bivalves synthesize bysus, thin silk-like protein threads. With their help, mollusks such as dreisen are attached to the bottom and stones. The new study is dedicated to the unique protein “silk” of these bivalves: its chemical composition, physical properties and appearance during evolution.

Bivalves are a peculiar class of mollusks that literally lost their heads in the process of evolution and began to lead a sedentary lifestyle in a powerful shell. In addition to pearls, they can also produce thin silk-like protein threads called bysus. It is interesting that the bissus of one of the marine bivalves was previously used to weave expensive clothes from “sea silk”.

This “silk” is very strong and is used by molluscs to cling to hard surfaces such as rocks or the bottom. It is synthesized by a special gland located in one leg of the animal. The “silk” of mollusks consists of protein, but its chemical composition varies greatly: it may consist of creatine, polyphenols or other protein molecules.

Most of what we know about Byssus relates to marine bivalve species such as mussels. Therefore, the authors of the new article PNAS decided to study the “silk” of freshwater inhabitants – Dreissen (Dreissenidae). Of particular importance is the fact that dreysens are an invasive species that actively invades the lakes and rivers of North America.

Scientists combined spectroscopy, X-ray structural analysis and proteomics to learn the chemical composition and structure of the dreissenid bysus. The protein at its core turned out to be unique: it consists mainly of structural units called crystalline beta sheets, which are characteristic of spider silk protein.

At the same time, another protein with a coiled-coil type structure is synthesized in the dreisen leg. At most, it turned out to be the largest of the known proteins of the corresponding group. Then, this precursor protein changes its structure due to mechanical action (processing) and only then turns into something similar to the spider protein. In the future, it helps the mollusks to settle on the bottom.

The origin of the unique Dreisen protein is no less surprising. The authors believe that the mollusks owe the bacteria to them. About 12 million years ago, the gene encoding it entered dreisen DNA via horizontal transfer. The mechanism is usually used by bacteria to exchange genes among themselves, but sometimes plants or animals also participate in it.

A new study is helping to understand how to better “separate” dangerous invasive molluscs and protect ecosystems from them. Dreisen’s unique protein, on the other hand, can be used to create new, strong and water-resistant materials and biocompatible polymers.