Large subunit-binding protein

The genes that code for ribulose- 1,5-bisphosphate carboxylase (rubisco) are found within the chloroplast (the L subunit) and the nucleus (the S subunit). The activation of these genes is mediated by an increase in light intensity (illumination). Phytochrome also appears to play a role in this activating process. Once the S subunit is transported from the cytoplasm into the chloroplast, both subunits assemble to form the L8S8 holoenzyme. A protein called the large subunit-binding protein appears to assist in the assembly of the holoenzyme. When illumination is low, the synthesis of both subunits is rapidly depressed. 

In plant chloroplasts the newly synthesized L can bind to another chloroplast protein complex called large subunit binding protein (BP) (1). The intermediate steps of the assembly of higher plant Rubisco as well as the function of BP are still largely unknown. 

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. 

Adenosine deaminase (ADA) is an amino hydrolase that catalyzes the deamination of adenosine and 2 -deoxyadenosine to inosine and 2 -deoxyinosine, respectively. High activity of ADA is seen in thymus and other lymphoid tissues. ADA has been shown in many different physical forms. A small form of the enzyme predominates in the spleen, stomach, and red blood cells, whereas the large form predominates in the kidney, liver, and skin fibroblasts. The small form of the catalytic subunit can be converted to the large form by complexing with a protein termed binding protein or complexing protein. 

By combining bacterial expression and chemical synthesis Ras constructs with the properties of the post-translationally modified protein can be generated. These hybrid proteins can insert into artifical and biological membranes, have been proven to be efficient tools for biochemical, biophysical and biological experiments and can be synthesized in large amounts. Principally the same method is applicable to many of the Ras-related GTP-binding proteins or the y-subunit of heterotrimeric G proteins. 

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