What is the structure of receptor proteins?

Answer 1

Receptors are plasma membrane proteins that bind specific molecules, e.g., growth factors, hormones, or neurotransmitters, and then transmit a signal to the cell's interior that causes the cell to respond ina aspecific manner, a process called signal transduction cascade.

There are several types of receptor proteins grouped in families of transmembrane proteins. For some families of important membrane proteins, e.g., ion channel proteins, the hydropathy plots (those that help to predict the hydrophobic regions of a membrane protein according to its amino acid sequence and hydropathic index) is often not very reliable because that the membrane-spanning regions of these proteins from channels in the membrane while on the other hand they need hydrophilic residues to line the surface of the channels in contact with the aqueous phase. These are the most important and most well-known receptor-protein families:

• Receptor Protein-Tyrosine Kinases. Known as RPTKs, are type I transmembrane proteins, with their N-termini outside of the cell and single membrane-spanning regions. The structural features that are the transmembrane domain that divides the molecule into a ligand-binding domain and a cytoplasmic domain that contains a conserved protein kinase catalytic domain. On the outside, the N-terminal end, RPTK has a signal peptide that ensures that the protein will be targeted to the secretory pathway. This is followed by an extracellular domain of several hundred amino acids that contain N-linked glycosilation sites, a distintive pattern of cysteine residues, and often a characteristic array of structural motifs. The transmembrane domain consists of about 24 hydrophobic residues that are usually succeeded by several basic residues that function as a stop-transfer signal. On the cytoplasmic side of the membrane there is a juxtamembrane region, usually of around 50 residues long, and which in some cases is known to have important regulatory functions. Next follows the catalytic domain, which is related to the catalytic domains of the cytoplasmic protein-tyrosine kinases and the protein-serine/threonine kinases, and is about 250 residues in length, excluding inserts. The phospho-transfer function lies entirely whithin this region. The region C-terminal to the catalytic domain is of variable length and can be up to 200 residues. The functions of this C-terminal tail vary among members of the RPTKs. Exmaples of this kind of receptors are: EGFR (Epidermial Growth Factor Receptor), PDGFR (Platelet-Derived Growth Factor Receptor), IR (Insulin Receptor), NGFR (Nerve Growth Factor Receptor), FGFR (Fibroblast Growth Factor Receptor), etc.
  

• G proteins. Several important physiological responses like vision, smell, and stress response produce large metabolic effects from a small number of input signals. The receptors for these signals have two things in common, first, they are transmembrane proteins with seven helices spanning the lipid bilayer of the plasma membrane, and second, the signals received by these receptors are amplified and the amplifiers are members of a common family of proteins called G proteins. Most G proteins are heterotrimers consisting of a one copy of alpha (45 kD), beta (35 kD), and gamma (7 kD) subunits. The alpha subunits have GTPase activity and slowly hydrolyze GTP to GDP and inorganic phosphate. The GTP-bound form of the protein is the active form and sends a signal from the receptor to some cellular effector protein. Examples of these kind of receptors are: beta-adrenergic receptor, rhodopsin, mast cell IgE receptor, and muscarinic receptor.

Answer 2

A receptor protein is shaped like a boulder.