Chapter 9 bio 102

Chapter 9: Cell Communication • Cell communication is about information processing – all cells are capable of sending and receiving signals, either from the environment or from other cells EXAMPLE – in yeast cells when exposed to glucose the glucose will act as a ligand to activate glucose receptors and result in a signal transduction cascade to metabolize the glucose as well as shut down the metabolism Also – in phototropism cells receive signal to grow a certain way in order to get the most amount of light – the light is perceived by the plant and the plant bends towards the light as a result of asymmetric cell growth • Processing is both inter- and intracellular Intracellular receptors – some receptors are inside the cell, so the signal has to pass through the cell membrane before binding to the receptor Example – estrogen binds to the receptor in the nucleus: the estrogen receptor complex then binds to DNA and triggers the production of transcripts that are necessary for the hormone response  estrogen can come through the cell readily in the first place because it is a steroid hormone meaning its hydrophobic, so it can pass through the membranes. • 3 stages: (i) receptor activation, (ii) signal transduction (ST), (iii) physiological response(s) Receptor activation – the external signaling molecule binds to a receptor protein, either on the surface or inside a cell and causes a conformational change in a receptor and activates its function Signal Transduction – the activated receptor stimulates a series of proteins that forms a signal transduction pathway Cellular response – the signal transduction pathway affects the functions and/or amounts of cellular proteins, thereby producing a cellular response. binding of molecule causes conformational change --> domino effect ultimately leads to change in activity of synthesis or others - change in gene expression • There are 5 main classes of intercellular signaling 1. Direct intercellular Signaling – when signaling molecules pass between cells through gap junctions or plasmodesmata (for plants) 2. Contact-dependent signaling – molecules bind to the surface of cells and provide a signal to other cells that possess an appropriate receptor 3. Autocrine Signaling – cell secretes signaling molecules that bind to their own cell surface, stimulating a response  can also affect neighbouring cells of the same cell type. 4. Paracrine Signaling – cells are close by, so when a specific cell secretes a signaling molecule, the signal goes to targets within close proximity (but signaling molecule does not affect its own cell) 5. Endocrine Signaling – this occurs over long distances. In both animal and plants, molecules involved in long distance signaling are typically hormones. SO there is a secretion of hormones into the bloodstream that may affect virtually all the cells in the body  in plants the hormones can flow through the vascular system and move through adjacent cells. • Receptors are at the “top” of the hierarchy (i.e. detection pt) and there are 3 major types 1. Enzyme-linked receptors – this type is found in all living species. Basically when a signaling molecule binds to the extracellular domain, a conformational change is transmitted through the membrane embedded portion of the protein that affects the conformation of the intracellular catalytic domain. This causes the intracellular domain to become functionally active.  majority are protein kinases, which is an enzyme that transfers phosphate from ATP to a specific amino acid.  know that in the absence of a signaling molecule the catalytic domain of the receptor remains inactive, but once signal binds its active. 2. G-Protein-Coupled Reaction (GPCR) – this is common in most eukaryotes (but not plants). They typically contain 7 transmembrane segments that wind back and forth through the plasma membrane. Basically, a ligand comes and binds to the GPCR activating it allowing a G protein to bind. The G protein releases GDP and binds to GTP allowing the G-protein to dissociate into an alpha subunit and a beta/gamma dimer. These subunits interact with other proteins in a signaling pathway. When a signaling molecule and GPCR dissociate, the GPCR is no longer activated and cellular response is reversed. The alpha subunit first hydrolyzes its bound GTP to GDP +P and after this the alpha and beta/gamma subunits reassociate with each other to form an inactive G- protein complex this type is important in vision, smell, immune response and behavior in animals 3. Ligand-Gated Channels – Seen in plant and animal cells. When a ligand binds it causes an ion channel to open and ions flow though the membrane. A lot of the time this involves Ca2+. In general, signals are between nerve and muscle cells, or between 2 nerve cells. In order for this to shut off, the signal must be below the Kd so that the channels will close. • Understand concepts of ligand, Kd A ligand is a signaling molecule. It binds non-covalently t