Membrane scaffold proteins

An alternative approach consists in reconstitution of the membrane protein in bilayer-mimicking environments, i.e., self-assembling lipid bilayer nanodiscs (Fig. lb). Nanodiscs consist of a small portion of membrane bilayer that has been solubilized by the addition of two amphipathic proteins, the membrane scaffold proteins (MSP) derived from the apolipoprotein A-1 [8-10]. Details of the preparation can also be found at http //sligarlab.life.uiuc.edu/nanodisc/protocols.html. These proteins wrap around the hydrophobic core of the lipids, effectively creating a soluble portion of membrane. 

Grinkova YV, Denisov IG, Sligar SG (2010) Engineering extended membrane scaffold proteins for self-assembly of soluble nanoscale lipid bilayers. Protein Eng Des Select 23 (ll) 843-848... 

Nanodiscs are comprised of a bilayer containing 130-160 lipids, maintained in a discrete, water-soluble state by the association of two copies of the membrane scaffold protein (MSP) from apolipoprotein A-I wrapped around the hydrophobic rim of the bilayer [199-202] (Fig. 3). Originally developed for the solubilization of functionally active integral membrane proteins, they have since been used for solid-state [203], and more recently, solution NMR applications [153, 204-207]. 

Bayburt, T.H., Grinkova, Y.V., SUgar, S.G. (2002) Self-assembly of discoidal phospholipid bfiayer nanoparticles with membrane scaffold proteins. Nano Letters, 2, 853-856. 

Morgan, C.R., Hebling, C.M., Rand, K.D., etal. (2011) Conformational transitions in the membrane scaffold protein of phospholipid hfiayer nanodiscs. Molecular and Cellular Proteomics, 10 (9), Mill 010876. 

Fig. 2 Schematic representation of most common membrane-mimicking environments cmnpatible with solution and/or solid-state NMR studies of integral membrane proteins. Micelle (a), bicelle (b), nanolipoprotein particles also known as nanodiscs (c), and liposomes (d). The membrane protein is depicted in gray, detergents are colored in brown and lipids in blue. The apolipoprotein also known as membrane scaffold protein is represented as a green ring. Liposomes are scaled down by a factor of 2...

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. 

However, not all proteins proceed directly to their eventual destination. Some proteins relocate from one plasma membrane compartment to another by means of trans-cytosis. Transcytosis involves endocytosis of selected proteins in one membrane compartment, followed by subsequent transport through early endosomes to recycling endosomes and finally translocation to a different membrane compartment, for example from the apical to the basolateral surfaces. Sorting at the TGN and endo-some recycling steps appear to have a primary role in the steady state distribution of proteins in different plasma membrane domains [47], However, selective retention of proteins at the plasma membrane by scaffolding proteins or selective removal may also contribute to normal distributions. Finally, microtubule-motor regulatory mechanisms have been discovered that might explain the specific delivery of membrane proteins to discrete plasma membrane domains [48]. 

Okamoto T, Schlegel A, Scherer PE, Lisanti MP. Caveolins, a family of scaffolding proteins for organizing preassembled signaling complexes at the plasma membrane. J Biol Chem 1998 273(10) 5419-5522. 

Fig. 10.1. Principle of signal transduction through intracellular protein kinase cascades. The intracellular protein kinase cascades are organized in modules composed in most cases of three proteinkinases and a scaffold protein. The modules process signals that are registered, integrated and passed on at the inner side of the cell membrane by central switching stations such as the Ras protein or the Rac protein. In the case of the MAP kinase pathway, the cascade includes at least three different protein kinases. Specific regulatory processes may take effect at every level of the cascade in addition, signals may be passed from the different protein kinases to other signaling pathways.

SUMMARY 12.5 Multivalent Scaffold Proteins and Membrane Rafts... 

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