the chemical environments for the hydrogens from each helix would have been slightly different, and so both sets of hydrogens would have appeared on the NMR spectrum. Instead, McIntosh and Oas saw spectral peaks corresponding to only one helix. The zipper hypothesis was becoming increasingly untenable.

X-RAY CRYSTALLOGRAPHY: FINDING THE MOLECULAR DETAILS

Only the high-resolution structure would divulge the location of all the atoms in the protein. This was a job for x-ray crystallography. The researchers began their attempts to crystalize the protein in November 1988. This and the rest of the preparatory work, through collection of x-ray data, took nearly a year. Then, at the end of 1989, O'Shea went to Alber's laboratory to take advantage of his skills and facilities.

It was amazing, says O'Shea, how quickly she generated the structure. Within 9 months she had confirmed her original model. Technological developments in data collection and software made this speed possible, in particular, programs written by Wayne Hendrickson. O'Shea had gotten stuck, but once she began using his program, which exploited the various symmetry relationships that exist in many proteins, she was able to finish the job in 2 months.

The solution of the crystal structure not only proved beyond a doubt that the leucine zipper is a coiled coil but was also a milestone in the study of protein structure. The list of suspected coiled coils had included many important and ubiquitous proteins, including muscle proteins; dynein, which forms molecular motors; intermediate filaments, which make up some of the girders that give cells their structural integrity; and oncogene products. But O'Shea had uncovered the first complete high-resolution structure of a coiled coil. This "made the models of myosin, keratin, and all these other coiled coils much more believable," says Alber.

THE COILED COIL: RULES OF FOLDING

Meanwhile, in 1991, Pehr Harbury, one of Peter Kim's graduate students, set out to determine how structure influences function in the leucine zipper.

Leucine and the other hydrophobic amino acids in the zipper are spaced as evenly as possible within an odd-numbered repeating sequence: Four positions from leucine to the other hydrophobe, and back to leucine again is three, and so on. Since there are 3.6 amino acids in



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