Just a presentation in order to introduce the paper which I found interest for weekly meeting at my laboratory! It's related to pH-dependented transfermation of amyloid mophology! Hope it's useful for someone else!

2. tryptophan Lag phase
Random coil => β-sheet
Β-sheet
Disordered state
Curvy thread-like
fibrilsDendritic fibrils
Final ThT density
lower
=> Weaker internal
packing
Aggregation formed at
pH 5 by nucleation
exhibit more ordered β-
sheet structure than pH
¾
MechanismAggregation
* Widely diverse nanoscale
morphologies.
C-terminal segment of
RPT harboring W423
residue as amyloid
core

3. (a) The isoelectric point (pI) of RPT ~ 4.5 at which presumably both nucleation and isodesmic are operative
(b) Initial oligomers were formed immediately after mixing at pH 3/4, RPT remained monomeric after transfer into pH 5 (hydrodynamic
radius ~ 4nm)
 Early oligomer matures rapidly into dendritic nanostructures in isodesmic mechanism at pH 3/4
(c) Formation of hydrophobic pockets during aggregation followed non-nucleation mechanism at pH 3 and nucleation mechanism at pH
5

4. RPT rapidly assembled
to form spherical
oligomers (pH 3/4)
Slowly transformed
into star-like
nanostructure
Finally matured into
dendritic
nanostructure by
multiple steps of
dissociation and
association
Oligomers were
observed later in lag
phase after several
hours and
transformed into
curvy and straight
fibrils (pH 5)
Aggregation
mechanism
plays a critical
role in
dictating the
RPT amyloid
polymorphism

5. Initial drop
 Disintegration of
dendritic structure
Partially ordered
structure to a highly
ordered β-sheet

6. - Highly ordered fibrils formed at pH 5 did not convert into dendritic
morphology, even after prolonged incubation at pH 3 or 4
 Dendritic nanostructures possessing lower internal packing can be
switched to more ordered fibrils under change of pH
- RPT fibrils undergo rapid disaggregation upon transferring fibrils into
a solution of neutral or mildly alkaline pH
- Both dendrites and fibrils demonstrated exponential decay of ThT
fluorescence upon jumping the pH to neutrality
- Fibrillar aggregates showed much slower disaggregation kinetic
 Higher kinetic stability
- Thermodynamic stability  Denaturing aggregates by Urea (Fig 4d
inset)
=> Fibrillar aggregates have more thermodynamic stability compared to
to dendritic aggregates

7. Conclusion
• Switch in the mechanism of aggregation of the RPT Pmel17 from isodesmic polymerization model to nucleation-
denpendent model as a function of pH
• Aggregation pathway results in nanoscale diversity aggregates from dendritic to fibrillary morphology having varied
internal packing and stability
• Rapid aggregation without lag phase has little or no accumulation of toxic oligomeric species
• The transition in aggregation mechanisms occurred due to protonation/deprotonation of glutamic acid residues (E in
RPT)
 pH 3/4, protonation of glutamates leads to minimal charge repulsion that reinforces intermolecular RPT association
mediated via noncovalent interaction
 pH 5, electrostatic repulsions between polypeptide chains prevent instantaneous oligomerization
=> overcome the electrostatic repulsions during nucleation that eventually results in the formation of a critical nucleus
• pH modulation within melanosomes allows the optimal conditions for the formation of functional amyloids that dictate
the template-assisted melanin biosynthesis