Transcript Previously in Cell Bio

Previously in Cell Bio
Signals are detected via binding interactions
Binding interactions governed by protein folding
Protein folding dictated by amino acid sequence
(molecular models as link from index page)
Hypotheses for ‘problem’ in Graves’ Disease
Positive signals (TRH or TSH) altered to increase
amount or affinity for their receptor
Signal receptor altered to increase their affinity
for hormone
Problem with signal relay ‘inside’ thyroid
Graves’ hypothesis 1: TSH, TSH-Receptor
interaction ‘too strong’
According to this hypothesis and what we
now know about protein binding……
T3 and T4 levels should be _?_ in Graves’
vs. normal.
TSH levels should be __?__ in Graves’ vs.
normal
TSH/TSH receptor interactions should show
__?___ binding constant vs. normal.
Blood tests show
T3 and T4 levels are elevated
TSH levels are decreased
TSH/TSH receptor interactions have same
binding constant vs. normal.
Therefore: Perfectly logical hypothesis…….
Not supported by data
Now what?
Hypothesis 2: Mutation in signaling within
cell leading to increase in thyroid hormone
production
Normal activation is the result of signal
transduction second messenger cascade
How does signal transduction work?
What could have gone wrong?
What do we know so far?
•Thyroid is ‘overacting’
•Pituitary normally responsible for thyroid
stimulation through levels of TSH
•Graves’ patients have normal/decreased levels
of TSH in blood
•Binding affinity between TSH and TSH-R normal
More
of
what
we
know
•TSH is water soluble hormone (why is this important?)
Figure 4-1. Schematic drawing of
human TSH, based on a
molecular homology model built
on the template of a hCG
model14. The a-subunit is shown
as checkered, and the b-subunit
as a solid line. The two hairpin
loops in each subunit are marked
L1, L3; each subunit has also a
long loop (L2), which extends from
the opposite site of the central
cystine knot. The functionally
important a-subunit domains are
boxed. Important domains of the
b-subunit are marked directly
within the line drawing (crossed
line, beaded line and dashed line):
For further details the reader is
referred to Grossman et al.2.
(Reproduced from Grossman,M,
Weintraub BD, SzkudlinskiMWEndocrin Rev (4) 18:476501,1997, with permission of the
Endocrine Society).
From “The Thyroid manager”
•Thyroid
Evenplasma
moremembrane is barrier
to polar molecules
•TSH interacts with a receptor on
the surface of thyroid cells
HOW and WHY is the thyroid responding as
though over-stimulated?
And to get to the answer of that question:
How do signals get passed across membranes?
Characteristics of Transmembrane Proteins
•Hydrophobic face of protein in transmembrane region
-one continuous structure or multiple regions of 2° structure
•Charges ‘anchor’ transmembrane region
•Asymmetric orientation
Peripheral Membrane proteins
Characteristics
•Associations with membrane not as strong
•Various means of attachment
-Protein-protein
-Protein-phospholipid head
Fig 3-32 Molecular Cell Biology by Lodish et al.
Membranes and membrane proteins
How can a polar signal gain access to the cytosol
Direct access: From the ‘outside’
•Pores
•Channels
•Pumps
From cytosol to cytosol
•Gap junctions
Membrane proteins
Indirect access: Receptors
If signaling molecule never
gains access to cytosol how can
the information be transmitted?
Extracellular domain
Plasma Membrane
Cytoplasmic Domain
TSH Receptor: from “The Thyroid Manager” Ch16
Transmembrane receptors
•Same general structure as other transmembrane proteins
•Able to bind specific ligand
•Ligand binding causes conformational change
What change in the TSH receptor could cause
overproduction of T3 and T4
How could you test your hypothesis?
Allosteric transitions
What are they, why are they important,
How do they relate to signal transduction
•R
T
state transitions
•Cooperative binding
Examples DNA helicase and ras (links from index page)
Other mechanisms that regulate protein
function
•Compartmentalization
•Change in rate of synthesis
Common traits?
•Cleavage
•Phosphorylation/dephosphorylation
Common traits?
Receptor’s role (summary)
Able to transduce signal because of:
•Placement in membrane (span it)
•Ability to bind ligand
•Ligand -induced conformational changes
So the signal ‘gets in’ without
physically crossing membrane
BUT How do you go from a shape change to causing a
change in gene expression?
2nd Messengers and Signaling Cascades
Getting the signal to where it needs to go