What's the difference between a scaffold protein and an adaptor protein?
These two terms both refer to proteins which associate with other proteins (like receptors or other phosphorylated proteins) and then allow the association of still other proteins onto them. My understanding is that there is not a significant difference between the two terms.
Scaffold proteins are crucial regulators of many key signaling pathways. Although scaffolds are not strictly defined in function, they are known to interact and/or bind with multiple members of a signaling pathway, tethering them into complexes. In such pathways, they regulate signal transduction and help localize pathway components to specific areas of the cell such as the plasma membrane, the cytoplasm, the nucleus, the Golgi, endosomes, and the mitochondria. Adaptor proteins are proteins are accessory to main proteins in a signal transduction pathway. These proteins tend to lack any intrinsic enzymatic activity themselves but instead mediate specific protein–protein interactions that drive the formation of protein complexes. Examples of adaptor proteins include MyD88, Grb2 and SHC1.
For some proteins, both terms apply, and there is no difference. But in general terms, a scaffold protein holds two or more proteins together so that they (or their enzymatic products) can interact with each other. It's usually a fairly large protein. Whereas the typical structure of an adaptor protein is simply two binding domains, one that binds to one protein, and one that binds to another, bringing the two proteins (or protein complexes) together. You can see how the definitions overlap, but when I think of a scaffolding protein I think about the proteins that hold MAP kinase cascades together in order to generate signaling specificity, and when I think of an adaptor protein, I think of something like Grb2, with an SH2 domain binding to an activated receptor and its SH3 domain recruiting the SOS protein to the membrane and activating the Ras signaling pathway.
