The key difference between adaptor and scaffold protein is that adaptor protein is usually a small protein that binds only to two proteins involved in the signalling pathway, while scaffold protein is a large protein that binds to a number of different proteins involved in the signalling pathway.
A signalling pathway is a process in which a signal reaches the cells and triggers some reactions or ordered series of events. This results in some changes in the cell, which are normally associated with gene expression or solute uptake. Ultimately, all these changes allow the cell to respond to the signal and adjust its metabolism according to the current environmental conditions. Adaptor protein and scaffold protein are two types of proteins involved in the signalling pathway.
CONTENTS
1. Overview and Key Difference
2. What is an Adaptor Protein
3. What is a Scaffold Protein
4. Similarities Between Adaptor and Scaffold Protein
5. Side by Side Comparison – Adaptor vs Scaffold Protein in Tabular Form
6. Summary
What is an Adaptor Protein?
Adaptor protein is a small protein that usually binds only to two proteins in the signalling pathway to regulate signal transduction. They accomplish this through specific domains such as SH2 and SH3, which recognise specific amino acid sequences in the target protein. Sometimes, it is also called signal-transducing adaptor proteins (STAPs). Adaptor protein typically contains several domains, including Src homology 2 (SH2) and Src homology 3 (SH3) domains. The SH2 domains recognise specific amino acid sequences in the proteins that contain phosphotyrosine residues. On the other hand, SH3 domains recognise proline-rich sequences within specific proteins.
Figure 01: Adaptor Protein
Adaptor proteins lack any intrinsic enzymatic activity. Their function is to mediate specific protein-protein interactions that drive the formation of protein complexes. One of the best-known examples of adaptor protein is GRB2 (growth factor receptor-bound protein 2). This protein sends the signal further down in a signalling pathway by binding through the SH2 domain to another receptor EGF (epidermal growth factor receptor). It attracts the next protein in the pathway (Sos protein in this example) by binding through SH3 domains. MYD88 and SHC1 are two more examples of adaptor proteins.
What is a Scaffold Protein?
Scaffold protein is a large protein that interacts with multiple proteins of a signalling pathway in order to regulate signal transduction. After binding, the scaffold protein tethers these multiple proteins into complexes. The best-known example of scaffold protein is MEKK1 protein. This is present in the MAPK pathway (mitogen-activated protein kinase). This pathway is responsible for the expression of proteins, which affect the cell cycle and cell differentiation. For this purpose, it sends the signal further into the nucleus in order to regulate specific transcription factors.
Figure 02: Scaffold Protein
In such pathways, this protein regulates signal transduction and helps to localise pathway components to a specific area such as plasma membrane, cytoplasm, nucleus, Golgi apparatus, endosomes, and mitochondria. Scaffold protein has the following four functions.
- It is able to tether signalling components.
- It localises signalling components to specific areas of the cell,
- It regulates signal transduction by coordinating positive and negative feedback signals.
- It insulates correct signalling proteins from competing proteins.
What are the Similarities Adaptor and Scaffold Protein?
- Adaptor and scaffold protein are two types of proteins.
- They participate in signalling pathways.
- They are both form complexes with other signalling proteins.
- The functions of both these proteins are very important for cell cycle, cell differentiation and metabolism.
What is the Difference Between Adaptor and Scaffold Protein?
Adaptor protein is normally a small protein that binds only to two proteins involved in the signalling pathway. On the other hand, scaffold protein is a large protein that binds to a number of different proteins involved in the signalling pathway. So, this is the key difference between adaptor and scaffold protein. Moreover, an adaptor protein forms short-lived complexes with other signalling proteins. In contrast, scaffold protein forms stable complexes with other signalling proteins. Thus, this is another significant difference between adaptor and scaffold protein.
The below infographic shows more differences between adaptor and scaffold protein in tabular form.
Summary – Adaptor vs Scaffold Protein
A signalling pathway is a series of chemical reactions in which a group of molecules in a cell work together to control a cell function. A cell receives signals from molecules like growth factors when they bind to cell receptors. After the first molecule in the pathway receives a signal, it activates another molecule. This process is repeated throughout the entire signalling pathway. Adaptor and scaffold proteins are involved in the signalling pathway. Adaptor protein usually binds only to two proteins involved in the signalling pathway. On the other hand, scaffold protein binds to a number of different proteins involved in the signalling pathway and regulate signal transduction. Thus, this is the summary of the difference between adaptor and scaffold protein.
Reference:
1. “Adaptor Protein.” An Overview | ScienceDirect Topics, Available here.
2. “Listeria Spp and Listeria Monocytogenes: A Harmful Bacteria Causing Rare but Severe Foodborne Diseases.” BioMérieux Industrial Microbiology, 2 July 2020, Available here.
Image Courtesy:
1. “AdaptorsOverviewc” By Hirst J, Barlow LD, Francisco GC, Sahlender DA, Seaman MN, Dacks JB, Robinson MS. – figure 10 of “The Fifth Adaptor Protein Complex” DOI: 10.1371/journal.pbio.1001170 (CC BY-SA 4.0) via Commons Wikimedia
2. “Scaffold Function” By Elin9 – Own work. Inspired by: Shaw, A.S. and E.L. Filbert, Scaffold proteins and immune-cell signalling. Nat Rev Immunol, 2009. 9(1): p. 47-56. (CC BY-SA 3.0) via Commons Wikimedia