Membrane proteins, ligand interactions

All the long-range forces discussed in this chapter play a role in biological processes. Interactions between membranes, proteins, ligands, antibodies... 

Protein-ligand interactions are important phenomena that touch upon every facet of biological functions. These include such examples as enzyme-substrate interactions in biochemical transformations, transducer-membrane interactions in signal transduction, protein-nucleic acid interactions in genetic transmission, protein-carbohydrate interactions in cell adhesion as well as protein-protein interactions in biochemical regulations and defense (immune response). Databases of interacting proteins are available respectively at DIP (http //dip.doe-mbi.ucla.edu) and IntAct project of EBI (http //ebi.ac.uk/intact). 

Simone, S., Curcio, E., Di Profio, G., Ferraroni, M. and Drioli, E. 2006. Polymeric hydrophobic membranes as a tool to control polymorphism and protein-ligand interactions. Memb. Sci. 283 123-132. 

In particular, steady-state and time-resolved fluorescence as well anisotropy are powerful tools for studying function and conformation of sensitizer in vesicular membrane systems, in order to provide structural information on sensitizer-colloid binding or to evaluate protein-ligand interactions. 

The general types of protein-protein interactions that occur in cells include receptor-ligand, enzyme-substrate, multimeric complex formations, structural scaffolds, and chaperones. However, proteins interact with more targets than just other proteins. Protein interactions can include protein-protein or protein-peptide, protein-DNA/RNA or protein-nucleic acid, protein-glycan or protein-carbohydrate, protein-lipid or protein-membrane, and protein-small molecule or protein-ligand. It is likely that every molecule within a cell has some kind of specific interaction with a protein. 

Figure 8 Ubiquitin and endocytosis. Receptors on the plasma membrane undergo monoubiquitination as a result of ligand (e.g., neurotransmitter). Ubiquitinated receptors bind to proteins called epsins, which in turn interact with adaptor proteins (adaptin) bound to clathrin-coated pits. Ubiquitination also functions to sort the internalized membrane protein into early endosomes, which directs them to degradation by lysosome through the multivesicular body. If ubiquitin from the endocytosed receptors is removed by an UBP, the receptor recycles back to the membrane. Proteasome inhibitors block endocytotic degradation of some proteins such as glutamate receptor subunits indicating a possible role for the proteasome.

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