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Silk: From Liquid to Solid

A group of scientists in Norway and Sweden reported in the journal Nature last month that they've figured out something new about how spider silk self-assembles. Spider silk, which is a protein, starts out as liquid dope in spiders' silk glands. A protein molecule is a chain of amino acid molecules. As the amino acids up and down the chain interlock with each other in characteristic patterns, the liquid dope transforms into fibers.

The timing of this self-assembly is crucial. If it happened too soon, a spider would be left with balls of silk fiber clogging up its silk glands--useless. Why do the same molecules form a liquid in the glands but form fibers as they emerge from the spinnerets?

Like all proteins, silk protein molecules have two ends and a middle. One end is known as the C-terminus. The middle of silk protein molecules is made up of repeating sequences of amino acids that interlock to form the fiber. And the other end is known as the N-terminus. Silk scientists have known for a while that the C-terminus plays an important role in ensuring correct fiber self-assembly. The new report indicates that the N-terminus determines the timing of self-assembly.

As silk molecules move through the ducts leading from silk glands to spinnerets, they encounter gradually decreasing pH levels--that is, their surroundings become more acidic. The molecular structure of the N-terminus makes it sensitive to such a change, and it in turns influences how the middle, repeating segment of the silk molecule twists back and forth on itself. The researchers found that the N-terminus actually inhibits fiber formation in basic or neutral environments and hastens it at the levels of acidity found out in the spinnerets. So spider silk fibers self-assemble right on time.

One more example of how spider silk proteins may help us understand all sorts of other proteins.
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