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By the end of this section, you will be able to:
  • Describe how signaling pathways direct protein expression, cellular metabolism, and cell growth
  • Identify the function of PKC in signal transduction pathways
  • Recognize the role of apoptosis in the development and maintenance of a healthy organism

Inside the cell, ligands bind to their internal receptors, allowing them to directly affect the cell’s DNA and protein-producing machinery. Using signal transduction pathways, receptors in the plasma membrane produce a variety of effects on the cell. The results of signaling pathways are extremely varied and depend on the type of cell involved as well as the external and internal conditions. A small sampling of responses is described below.

Gene expression

Some signal transduction pathways regulate the transcription of RNA. Others regulate the translation of proteins from mRNA. An example of a protein that regulates translation in the nucleus is the MAP kinase ERK. ERK is activated in a phosphorylation cascade when epidermal growth factor (EGF) binds the EGF receptor (see [link] ). Upon phosphorylation, ERK enters the nucleus and activates a protein kinase that, in turn, regulates protein translation ( [link] ).

This illustration shows the pathway by which ERK, a MAP kinase, activates protein synthesis. Phosphorylated ERK phosphorylates MNK1, which in turn phosphorylates eIF-4E, which is associated with mRNA. When eIF-4E is phosphorylated, the mRNA unfolds and protein synthesis begins.
ERK is a MAP kinase that activates translation when it is phosphorylated. ERK phosphorylates MNK1, which in turn phosphorylates eIF-4E, an elongation initiation factor that, with other initiation factors, is associated with mRNA. When eIF-4E becomes phosphorylated, the mRNA unfolds, allowing protein synthesis in the nucleus to begin. (See [link] for the phosphorylation pathway that activates ERK.)

The second kind of protein with which PKC can interact is a protein that acts as an inhibitor. An inhibitor    is a molecule that binds to a protein and prevents it from functioning or reduces its function. In this case, the inhibitor is a protein called Iκ-B, which binds to the regulatory protein NF-κB. (The symbol κ represents the Greek letter kappa.) When Iκ-B is bound to NF-κB, the complex cannot enter the nucleus of the cell, but when Iκ-B is phosphorylated by PKC, it can no longer bind NF-κB, and NF-κB (a transcription factor) can enter the nucleus and initiate RNA transcription. In this case, the effect of phosphorylation is to inactivate an inhibitor and thereby activate the process of transcription.

Increase in cellular metabolism

The result of another signaling pathway affects muscle cells. The activation of β-adrenergic receptors in muscle cells by adrenaline leads to an increase in cyclic AMP (cAMP) inside the cell. Also known as epinephrine, adrenaline is a hormone (produced by the adrenal gland attached to the kidney) that readies the body for short-term emergencies. Cyclic AMP activates PKA (protein kinase A), which in turn phosphorylates two enzymes. The first enzyme promotes the degradation of glycogen by activating intermediate glycogen phosphorylase kinase (GPK) that in turn activates glycogen phosphorylase (GP) that catabolizes glycogen into glucose. (Recall that your body converts excess glucose to glycogen for short-term storage. When energy is needed, glycogen is quickly reconverted to glucose.) Phosphorylation of the second enzyme, glycogen synthase (GS), inhibits its ability to form glycogen from glucose. In this manner, a muscle cell obtains a ready pool of glucose by activating its formation via glycogen degradation and by inhibiting the use of glucose to form glycogen, thus preventing a futile cycle of glycogen degradation and synthesis. The glucose is then available for use by the muscle cell in response to a sudden surge of adrenaline—the “fight or flight” reflex.

Questions & Answers

what is oxidation?
Rose Reply
 the state or result of being oxidized
Emmanuel
hahahaha thanks, but my teachers requires a thorough meaning about that
Rose
Is the process of oxidizing ,the addition of oxygen to a compound with a loss of electrons, always accompanied by reduction
Korletey
loss of electron....
Anwar
thank you. 😊
Rose
thank you. 😊
Rose
thank you. 😊
Rose
what is genetic
Chibawa Reply
name the enzymes that i found in the saliva
Valuables Reply
draw a bacterium cell and label
Kadijah Reply
What are the osmoregulatory functions of the kidney?
bisi Reply
filter
Meenu
What is ecology
Hebert Reply
what is cell
Etama Reply
cell is the basic unit of life
Asiatou
cell is the basic structural and functional unit of an living organism
Darshan
a cell is the smallest and most basic unit of a living thing
John
cell is the basic unit of life. we are made up of 60,000 billions of cells.Each cell carry out a specific function in the body.
Pallavi
A cell is the smallest basic functioning unit of life.
Ali
where is the pectoral gridle located?
Tiania Reply
What is hypotonic
Bright Reply
what is hypotonic
Dangaya
Hypotonic means weak solution
Ali
the difference between the two cells
Obeng Reply
explain the courses and the correction of lon term sightedness and short term sightedness
Isaac Reply
long sightedness is said to be like someone that can see far object clearly why short sightedness is someone that only can see near obect
SHEDRACK
why drinking excess alcohol causes thirst and dehydration
uwikuzo Reply
Can we chat about nutrition please?
Elia
yes
Uzair
sure
Uzair
Uhm why is it so important to follow the nutritional process?
Elia
BC it contribute to the source of life
SHEDRACK
what is reproduction
smart Reply
it is d act of bringing young ones to life
Oyebanji
to ensure survival of a species🚴‍♀️
Michelle
what is a genotype
Collins
what is hazardous
smart
a cell is the smallest unit of a living thing. so we all have cell
smart
It is the formation of a zygote resulting from the fusion of the sperm cell with the ovum.Thus,this results in the production of new species which are genetically dissimilar from their parent cells.
Pallavi
yes we all have cell round our body without the existances of cell them they will be no life in us as human
SHEDRACK
what is size of cell
Mohd Reply
what is size of Hart
Mohd
nanometers=um sign thingie
Michelle
microns=nanometers
Michelle
monomers and polymers of nucleic acids?
Jyrl Reply
dna and rna involvement
Michelle

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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