For many scientists, the creation of the long-sought model of tubulin would be the culmination of a career. For biophysicist Eva Nogales it was only the start. As head of her own research team she scored another scientific first: the first 3-D images of the protein complex that initiates transcription of DNA's genetic code to produce new proteins.
These images identify critical components of the transcriptional factor (TF) protein complex including three subunits labeled TFIID, TFIIA, and TFIIB. A 3-D model reconstructed from electron-microscope images shows TFIID as a horseshoe-shaped structure surrounding a central cavity, inside of which recognition and binding to DNA is likely to take place. TFIIA and TFIIB bind to the TFIID in a way that affects the size and shape of the cavity.
"Our 3-D reconstruction gives us a good idea as to how TFIID works in concert with TFIIA and TFIIB to initiate and regulate the transcription of protein coding genes," says Nogales.
Virtual Crystals To create atomic-level 3-D models of proteins, the method of choice is synchrotron-based x-ray crystallography -- provided that the protein to be imaged can be readily crystallized. But if the protein resists crystallization, an alternative method must be found. One alternative that has proven especially effective for determining the structures of large molecular protein complexes is called single-particle image analysis. In this technique, tens of thousands of images of randomly oriented individual protein molecules are recorded with an electron microscope. A computer is then used to create a virtual crystal by bringing these randomly oriented images into proper order and merging them in a 3-D reconstruction. The next step in single-particle image analysis will involve the use of a supercomputer. With extra computational muscle it should be possible to collect and merge as many as a million images of a single non-crystallized protein for reconstruction into a high-resolution 3-D model. This approach has been dubbed "crystallization in silico."
|
