lecture 5 - General Principles of Cell Signalling

Question 1

Why do cells need to communicate?

Answer 1

Multicellularity requires signalling between cells within
an organism to co-ordinate behaviour.
1) Homeostasis and maintenance of internal state of
the adult.
2) The complex process of development.
i. Cell cycle control
ii. Cell movement
iii. Differentiation
iv. Patterning

Question 2

What are the Basic components of a signalling pathway?

Answer 2

Extracellular signal molecule
Receptor protein
Intracellular signalling proteins
target proteins

Question 3

What are extracellular signalling molecules?

Answer 3

Signalling molecules transmit information between cells.
i) Some signals are secreted from the signalling cells
and diffuse through the extracellular environment
and can signal over large distances.
ii) Some signals remain closely associated with the
signalling cells and signal locally.
a) Signal is membrane bound
b) Signal tightly associated with extracellular
matrix.

There many classes of signalling molecules

i) Peptides e.g. insulin
ii) Small molecules e.g. nitrous oxide
iii) Metabolic products e.g. Steroids
iv) Lipids e.g. phospholipids

Question 4

What are Receptors for Signalling Molecules?

Answer 4

Extracellular signals bind to specific receptor proteins
• Every signal requires a specific receptor
• A cell can only respond to a signal if it has an appropriate receptor.
• 3-D of structures of receptor proteins allow exquisite specificity in signal recognition and response.
• Signalling molecules are present in low concentrations which requires high affinity receptor binding
• Receptors for a signal molecule are typically present at the cell surface
• Receptors for some signals are present within the cell.

Question 5

What are signal transduction pathways?

Answer 5

•Pass the signal from the receptor to the ultimate target proteins
•Signal transduction pathways can be very complex containing multiple intracellular proteins.
•Typically contain a series of proteins that act as molecular switches or proteins that mediate protein-protein interactions.
Function
i) Amplification of the signal
ii) Integration of multiple signalling pathways
iii) Allows multiple levels of regulation

Question 6

What are target proteins?

Answer 6

A typical signalling pathway will affect multiple aspects of cell behaviour.
• A signal will lead to changes in the activity of many target proteins
• These are the target effector proteins which ultimately change cell behaviour

• Metabolic enzyme- Enzymes involved in glucose metabolism
• Gene regulatory protein- Transcription factors
• Cytoskeletal protein- Actin and tubulin

Question 7

Describe how Signalling pathways regulate every aspect of cellular function

Answer 7

i) Metabolic activity- insulin and glucagon
ii) Cell division and growth- growth factors
iii) Architecture of the cytoskeleton affectingaxon
guidance chemotropic factors
a) Cell movement
b) Cell shape
iv) Changes in gene expression- growth factors

Question 8

Describe how hydrophilic cell signalling molecules function

Answer 8

•Hydrophilic molecules (including proteins) are unable to get across the hydrophobic plasma membrane.
•Hydrophilic signalling molecules must bind cell surface receptors.
•Binding of signal to receptor activates an intracellular signal
transduction pathway which passes on the signal to intracellular
target proteins.

Question 9

Describe how hydrophobic cell signalling molecules function

Answer 9

•Hydrophobic signalling molecules are insoluble in water and are
transported bound to carrier proteins.
•At the cell the signal is released and diffuses across the plasma
membrane to bind with a receptor protein in the cytoplasm or
nucleus.
•Intracellular signal transduction pathway is typically less complex

Question 10

What is Contact dependent or juxtacrine signalling?

Answer 10

•Signal may be membrane bound or tightly associated with
extracellular matrix
•Signal requires direct contact between signalling and responding
cell.

Question 11

What is Endocrine signalling (hormone signalling)?

Answer 11

•Signal produced locally in group of cells and transported
systemically to distant responding cells.
•Plant hormones and endocrine hormones act like this.

Question 12

What are Paracrine and autocrine signalling?

Answer 12

Signalling molecules are secreted into the extracellular space
A paracrine signal acts on cells neighbouring the signalling cell.
•An autocrine signal acts back on the signalling cell.