Previously in Cell Bio

1
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 Hypothesis one not supported TSH
correct structure, Levels of TSH appropriate
for levels of T3 T4 in system
(decreased) TSH/TSHreceptor affinity normal
2
Now what?
Not supported by data
3
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

4
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, SzkudlinskiMW-Endocrin Rev (4)
18476-501,1997, with permission of the Endocrine
Society).
From The Thyroid manager
5
Even more

• Thyroid plasma membrane 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?
6
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

7
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.
8
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

9
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
10
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?
11
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)
12
Other mechanisms that regulate protein function

• Compartmentalization
• Change in rate of synthesis

Common traits?

• Cleavage
• Phosphorylation/dephosphorylation

Common traits?
13
Receptors 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?
14
2nd Messengers and Signaling Cascades
Getting the signal to where it needs to go

• For Tuesday summarize a cascade involving.
• 1) cGMP 2) RTK (growth factor 3) IP3 (inositol
  triphosphate)
• 4) Ca 5) RTK (insulin) 6) DAG
  (diacyl glycerol)
• Email me one paragraph summary of how it works by
  
• midnight Monday (think handout)