Protein tetramerization

While a number of proteins have been crystallized in this manner, the majority of studies have focused on a robust system comprising the tetrameric protein streptavidin and the vitamin biotin. The choice of this system is primcirily motivated by the strong bond between biotin and streptavidin (having an association equilibrium constant, Ka Tbe binding properties were recently... 

The polypeptide chain of the lac repressor subunit is arranged in four domains (Figure 8.21) an N-terminal DNA-hinding domain with a helix-turn-helix motif, a hinge helix which binds to the minor groove of DNA, a large core domain which binds the corepressor and has a structure very similar to the periplasmic arablnose-binding protein described in Chapter 4, and finally a C-terminal a helix which is involved in tetramerization. This a helix is absent in the PurR subunit structure otherwise their structures are very similar. 

Back A hand-drawn image of the potassium channel, in the same view as on the front cover, with each subunit of the tetrameric protein shown in a different color. 

Biochemical characterization of clathrin-coated vesicles revealed that their major coat components are clathrin and various types of adaptor complexes. Clathrin assembles in triskelions that consist of three heavy chains of approximately 190 kDa and three light chains of 30 40 kDa. Four types of adaptor complexes have been identified to date, AP-1, AP-2, AP-3 and AP-4 (AP for adaptor protein). Whereas AP-1, AP-3 and AP-4 mediate sorting events at the TGN and/or endosomes, AP-2 is involved in endocytosis at the plasma membrane. Each adaptor complex is a hetero-tetrameric protein complex, and the term adaptin was extended to all subunits of these complexes. One complex is composed of two large adaptins (one each of y/a/S/s and [31-4, respectively, 90-130 kDa), one medium adaptin (pi -4, <50 kDa), and one small adaptin (ol-4, <20 kDa). In contrast to AP-1, AP-2 and AP-3, which interact directly with clathrin and are part of the clathrin-coated vesicles, AP-4 seems to be involved in budding of a certain type of non-clathrin-coated vesicles at the TGN. 

All Kir channels are tetrameric proteins (see Fig. 3) of one-pore/two-transmembrane (1P/2TM) domain subunits which equally contribute to the formation of highly selective K+ channels. Most Kir channels can be assembled in functional homotetramers while some require heteromeric assembly (see Fig. 3). For example, functional GIRK channels underlying DCAch (Acetyl-choline-activated) current in atria are heteromultimers of two members ofKir3 subfamily Kir3.1 andKir3.4. 

Kir Channels are a class of potassium channels generated by tetrameric arrangement of one-pore/two-transmembrane helix (1P/2TM) protein subunits, often associated with additional beta subunits. Kir Channels serve to modulate cell excitability, being involved in repolarization of Action Potentials, setting the resting potential of the cell and contributing to potassium homeostasis. 

Fig. 3. The tetrameric structure of the MoFe protein (Kpl) from Klebsiella pneumoniae (7). The two FeMoco clusters and the P clusters are depicted by space-filling models and the polypeptides by ribbons diagrams (MOLSCRIPT (196)). The FeMoco clusters are bound only to the a subunits, whereas the P clusters span the interface of the a and j8 subunits.

Although biotin-fluorophores have been described in the detection of biomolecules, namely proteins, peptides, or DNA, most of these loose part of their fluorescence when binding to the tetrameric proteins avidin and streptavidin. 

In E. coli cells, DNA replication starts at a specific site called oriC. The oriC locus contains only 245 base pairs. Similar sequences are responsible for initiating the synthesis of plasmid and bacteriophage DNA. The oriC nucleotide sequence binds several units of the tetrameric form of the dnaA protein. This protein is named for the gene that encodes it. The dnaB and dnaC proteins then bind to the complex. As a result of binding these proteins, a portion of the helical DNA is unwound. This forces the rest of the DNA into a left-handed double helix that wraps around the proteins to give a structure... 

---