chap16

Cells in multicellular organisms communicate through a huge variety of _.

In animals, hormones are carried in the blood to distant target cells, but most other extracellular signal molecules act over only a short distance. Neighboring cells often communicate through _

For an extracellular signal molecule to influence a target cell it must interact with a _ on or in the target cell. Each _ protein recognizes a _ signal molecule.

Most extracellular signal molecules bind to cell-surface receptor proteins that _ the extracellular signal into different _ signals, which are usually organized into _ pathways.

There are three main classes of cell-surface receptors: (1) _, (2) _, and (3) _

GPCRs and enzyme-coupled receptors respond to extracellular signals by _, which, in turn, activate _ that _.

_ signaling pathways is as important as turning them on. Each activated component in a signaling pathway must be subsequently _ for the pathway to function again.

GPCRs activate _ called _ proteins; these act as _, transmitting the signal onward for a short period before switching _ by hydrolyzing their bound GTP to GDP.

G proteins directly regulate _ or _ in the plasma membrane. Some directly activate (or inactivate) the _, which increases (or decreases) the intracellular concentration of the second messenger molecule _; others directly activate the _, which generates the second messenger molecules _ and _.

_ opens Ca2+ channels in the membrane of the endoplasmic reticulum, releasing a flood of free Ca2+ ions into the cytosol. The Ca2+ itself acts as a _, altering the activity of a wide range of _ proteins. These include calmodulin, which activates various target proteins such as _

A rise in cyclic AMP activates _, while Ca2+ and diacylglycerol in combination activate _.

PKA, PKC, and CaM-kinases phosphorylate selected _ proteins on _ and _, thereby altering their activity. Different cell types contain _ sets of _ and are therefore affected in different ways.

Enzyme-coupled receptors have intracellular protein domains that function as _ or are _. Many enzyme-coupled receptors are _, which _ themselves and select intracellular signaling proteins on _. The phosphotyrosines on RTKs then serve as _ for various _ proteins.

Most RTKs activate the monomeric _, which, in turn, activates a _ signaling module that helps relay the signal from the plasma membrane to the _.

Ras mutations stimulate _ by keeping Ras (and, consequently, the Ras–MAP kinase signaling pathway) constantly _ and are a common feature of many human _.

Some RTKs stimulate cell _ and cell _ by activating PI 3-kinase, which phosphorylates specific _ in the cytosolic leaflet of the plasma membrane lipid bilayer. This inositol _ creates lipid _ sites that attract specific signaling proteins from the cytosol, including the protein kinase Akt, which becomes active and relays the signal onward.

Other receptors, such as Notch, have a direct pathway to the _. When activated, part of the receptor migrates from the plasma membrane to the _, where it regulates the _

Some _, such as steroid hormones and nitric oxide, are small or hydrophobic enough to cross the plasma membrane and activate _ proteins, which are usually either _ regulators or _.

Plants, like animals, use enzyme-coupled cell-surface receptors to recognize the extracellular signal molecules that control their _ and _; these receptors often act by _ of specific genes.

Different intracellular signaling pathways _, enabling each cell type to produce the appropriate response to a combination of _. In the absence of such signals, most animal cells have been programmed to _

Animal cells use extracellular signal molecules to communicate with one another in various ways:

Hormones produced in _ glands are secreted into the bloodstream and are distributed widely throughout the body.

_ signals are released by cells into the extracellular fluid in their neighborhood and act locally.

In contact-dependent signaling, a cell-surface-bound _ binds to a _ on an _ cell. Many of the same types of signal molecules are used for endocrine, paracrine, and neuronal signaling. The crucial differences lie in the _ and _ with which the signals are delivered to their targets.