The HIV protease that you analyzed in the previous part was composed of two identical subunits. In biochemistry, such molecules are called homodimeric. However, many proteins are heteromultimeric, i.e. they are composed of many different functional subunits. One interesting example of a heteromultimeric protein is provided by bacterial toxins such as the heat-labile enterotoxin in pathogenic E. coli strain O157:H7 (fresh California spinach, anybody). Such toxins typically combine a subunit that inactivates a key protein in the cell with subunits that facilitate the entry of the toxin into the cell. You will now visualize the structure of the heat-labile enterotoxin from pathogenic E. coli.
Search the Protein Data Bank for the PDB code 1LTS. Click on the Download Files menu item and save the Biological Unit Coordinates in your directory. The Biological Unit Coordinates option is preferred over the PDB file option in the case of oligomeric proteins because the latter often contains only one subunit. The file that you downloaded (1lts.pdb1.gz) is compressed in the gzip format and has a nonstandard extension (pdb1), which visualization programs do not recognize. Thus before analyzing this file, you need to uncompress it with the Unix gunzip program and rename it with the Unix mv command:
gunzip name_of_the_compressed_file.gz
mv name_of_the_file 1LTS.pdb
The chain A in heat-label enterotoxin is the catalytic subunit that is responsible for the potentially deadly effects of E. coli poisoning. The chains D, E, F, G, H bind to the membrane of the victim cell and facilitate the entry of the toxic subunit A with the help of a spike-shaped chain C. Analyze the architecture of the E. coli heat-labile enterotoxin after coloring different subunits in different color.
Assignments (Enterotoxin):