The Leucine Zipper Motif

Careful analysis of a 30-amino-acid sequence in the car-boxyl terminal region of the enhancer binding protein C/EBP revealed a novel structure. As illustrated in Figure 39-15, this region of the protein forms an a helix in which there is a periodic repeat of leucine residues at every seventh position. This occurs for eight helical turns and four leucine repeats. Similar structures have been found in a number of other proteins associated with the regulation of transcription in mammalian and yeast cells. It is thought that this structure allows two identical monomers or heterodimers (eg, Fos-Jun or Jun-Jun) to "zip together" in a coiled coil and form a tight dimeric complex (Figure 39-15). This proteinprotein interaction may serve to enhance the association of the separate DNA binding domains with their target (Figure 39-15).

THE DNA BINDING & TRANS-ACTIVATION DOMAINS OF MOST REGULATORY PROTEINS ARE SEPARATE & NONINTERACTIVE

DNA binding could result in a general conformational change that allows the bound protein to activate transcription, or these two functions could be served by separate and independent domains. Domain swap experiments suggest that the latter is the case.

The GAL1 gene product is involved in galactose metabolism in yeast. Transcription of this gene is positively regulated by the GAL4 protein, which binds to an upstream activator sequence (UAS), or enhancer, through an amino terminal domain. The amino terminal 73-amino-acid DNA-binding domain (DBD) of GAL4 was removed and replaced with the DBD of LexA, an E coli DNA-binding protein. This domain swap resulted in a molecule that did not bind to the GAL1 UAS and, of course, did not activate the GAL1 gene (Figure 39-16). If, however, the lexA operator—the DNA sequence normally bound by the lexA DBD—was inserted into the promoter region of the GAL gene, the hybrid protein bound to this promoter (at the lexA operator) and it activated transcription of GAL1. This experiment, which has been repeated a number of times, affords solid evidence that the carboxyl terminal region of GAL4 causes transcriptional activation. These data also demonstrate that the DNA-binding DBD and trans-activation domains (ADs) are independent and noninteractive. The hierarchy involved in assembling gene transcription activating complexes includes proteins that bind DNA and trans-activate; others that form protein-protein complexes which bridge DNA-binding proteins to trans-activating proteins; and others that form protein-protein complexes with components of the basal transcription

DNA binding protein C/EBP. The amino acid sequence is displayed end-to-end down the axis of a schematic a-helix. The helical wheel consists of seven spokes that correspond to the seven amino acids that comprise every two turns of the a-helix. Note that leucine residues (L) occur at every seventh position. Other proteins with "leucine zippers" have a similar helical wheel pattern. B is a schematic model of the DNA binding domain of C/EBP. Two identical C/EBP polypeptide chains are held in dimer formation by the leucine zipper domain of each polypeptide (denoted by the rectangles and attached ovals). This association is apparently required to hold the DNA binding domains of each polypeptide (the shaded rectangles) in the proper conformation for DNA binding. (Courtesy of S McKnight.)

apparatus. A given protein may thus have several surfaces or domains that serve different functions (see Figure 39-17). As described in Chapter 37, the primary purpose of these complex assemblies is to facilitate the assembly of the basal transcription apparatus on the cis-linked promoter.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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