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What kind of function does the longest gene in the human genome code for? The answer is a
bit mundane: a very long molecular spring that provides muscle with passive elasticity.
Nature adjusts the protein, called titin, for many types of muscle, e.g., skeletal or
cardiac muscle, as well as for other cellular functions. The molecular spring contains
hundreds of elastic elements in series like beads on a string. One type of bead is
the immunoglobulin domain, which can stretch to ten times its normal length. For a long time
only one of the immunoglobulin domains was structurally known, permitting only a single
peek into nature's design library. Recently, a second domain became structurally known
and protein crystallographers and modelers joined forces to discover how nature
designs its beads along titin, as described in a recent publication.
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