Understanding Types of Cellular Communication - AP Biology

G protein-coupled receptors are part of a large class of receptors involved in intercellular signaling. Structurally, G protein-coupled receptors have an extracellular N terminus, seven transmembrane helices, three intracellular loops, three extracellular loops, and an intracellular C terminus. The ligand-binding domain is within the transmembrane helices.

When a protein is active in it’s native state, it is said to be constitutively active. In the case of protein receptors, a receptor is constitutively active when it is active without binding to a ligand.

Calcium is a widely used second messenger of signal transduction. Calcium ions can function as a second messenger because its concentration within cell cytosol is much lower than outside the cell, and it is actively transported out of the cell by protein pumps. Modulation in calcium levels is used to transmit signals from both G protein and receptor tyrosine kinase signaling cascades and is involved in such functions as muscle contractions and synaptic signaling.

The other common second messenger molecule is cAMP.

After a ligand binds to receptor tyrosine kinases, the receptors need to form a dimer to foster activation of their tyrosine kinase activity. After dimerization, a phosphate is transferred from ATP to the amino acid tyrosine at specific sites on the cytoplasmic region of receptor. The phosphorylation of the tyrosines provides a site where other cellular proteins can bind and further relay the signal from the receptor to the cell.

Intracellular receptors are found in the cytoplasm of the cell. Ligands for intracellular receptors are usually small molecules that can pass through the cell membrane, and include substances such as steroid hormones. Upon binding and activation, intracellular receptors bind specific DNA motifs in the nucleus and function as transcription factors, directly changing expression of genes.

In contrast, transmembrane receptors are embedded in the plasma membrane and bind extracellular ligands to mediate intracellular responses. Ligand binding to transmembrane receptors often initiates a signal cascade or mediates channel activity within the membrane of the cell.

Seeing as peptide hormones are generally large, water-soluble molecules, they cannot transverse the phospholipid membrane. Instead, they must act through a membrane-bound protein receptor. Steroid hormones are generally small, fat-soluble organic molecules that can easily travel through the phospholipid membrane and the nuclear membrane. They can then act on transcription factors or interact directly with DNA. Both peptide and steroid hormones initiate changes within the cell; they simply do so by different mechanisms.

Cell signaling is the process used by cells to communicate and control cellular activities. It can occur both within and between cells. The correct sequence of events that takes place during cell signaling is as follows: reception, transduction, and response. The reception stage is the detection of a signal, typically by a receptor on the cell surface. Next, transduction is characterized by the transmission of signals from the cell’s exterior to its interior by way of proteins. Finally, the response is the subsequent cellular reaction to the signaling. Cell signaling is critically important in normal cell function and widely diversified.

G protein-coupled receptors are only found in eukaryotes including yeast cells and animal cells. Bacteria cells are prokaryotes, and therefore do not contain G protein-coupled receptors. Even though yeast cells are single-celled, they possess all the characteristics of eukaryotic cells.

G proteins are a class of protein signaling molecules that are activated by G protein-coupled receptors (GPCRs). When a ligand binds to the transmembrane domain of GPCRs, the GPCR undergoes a conformational change. This conformational change activates the G protein, which binds to GTP rather than lower energy GDP. The active G protein can then dissociate and transmit the signal by interacting with other proteins.

Tyrosine kinase receptors exist as single monomers but possess the capability to polymerize. Tyrosine kinase receptors have a transmembrane domain, an extracellular N terminus, and an intracellular C terminus. When a ligand binds to the extracellular N terminus, the tyrosine kinase receptor dimerizes. There are multiple models of receptor dimerization. One of the models is that dimerization is aided by the ligand itself, which binds to the N termini of both tyrosine kinase receptors. Another is that dimerization occurs after each tyrosine kinase receptor monomer binds to a ligand. A final model postulates that the binding of a ligand induces a conformation change that allows dimerization.

Tyrosine kinase receptors are composed of a transmembrane domain, an extracellular N terminus, and an intracellular C terminus. Ligand binding to the extracellular N terminus stimulates receptor dimerization. The dimerization stimulates kinase activity on the intracellular C terminus, leading to the autophosphorylation of the tyrosine residues on the C terminus. This phosphorylation of tyrosine residues creates binding sites for relay proteins, which are then phosphorylation by the tyrosine kinase receptors. The phosphorylated relay proteins then transmit the signal to other cellular pathways.

Tyrosine kinase receptors are fully activated when they bind to an extracellular ligand, dimerize, and then autophosphorylate at tyrosine residues on the C terminus. The signal is not transduced until relay proteins are phosphorylated by the tyrosine kinases. These relay proteins can then stimulate a phosphorylation cascade that initiates signaling pathways, which regulate nuclear gene transcription

When inactive, ion gated receptors are closed. When a ligand binds, the channel undergoes a conformational change and opens: creating a tunnel. This conformational change does not last for a long period of time; the ligand soon dissociates and the ion channel closes.

In animal cells, hormones mediate long distance cell signaling. Hormones are chemical messengers secreted by cells that travel through the circulatory system to the target cell receptors. Hormones communicate between diverse cell types and initiate diverse transduction pathways. Hormones are used for long distance cell signaling in both plant and animal cells.

Agonists activate receptors; therefore, the receptor is activated when the agonist binds to it.

Hormones are chemical messengers responsible for long distance cell signaling. Hormones are involved in diverse signaling pathways and have a variety of effects on cellular activities; therefore, there are many types of hormones. Animal hormones can be organized into classes based on their chemical makeup—peptide hormones, steroid hormones, lipid-based hormones, and amino acid-derived hormones. Insulin and growth hormones are both in the class of peptide hormones, and they are responsible for promoting the absorption of glucose from the bloodstream and promoting cell growth and reproduction, respectively. Testosterone is a steroid hormone that controls the development of male reproductive features.

Plant hormones, like animal hormones, are involved in long distance cell signaling. Most plant hormones are involved in regulating plant growth and are secreted by plant cells. Because plants lack a circulatory system, plant hormones move through cells via passive transport. This demands that the chemical composition of plant hormones must be simple. Auxin is a plant hormone whose distribution controls plant growth in response to environmental conditions. Other common plant hormones include abscisic acid, cytokinin, ethylene, and gibberellin.

Receptors are proteins embedded in the plasma membrane that receive and transmit signals from extracellular and intracellular sources. Receptors can also be embedded in the plasma membrane of the nucleus. Receptors bind to ligands, which can elicit a variety of responses including: activation, partial activation, and inhibition.

Ligands are molecules that form complexes with other molecules, proteins, or genetic material. The binding of a ligand causes a conformational change to the complex that leads to a response. This response by associated molecules is not limited to activation, but also includes inhibition, partial activation, and inhibition of constitutive activity. Ligands can be either selective or non-selective, meaning that they can bind to only specific molecules or to many receptors, respectively.