Prion Fibril Fragmentation and Propagon Number

Propagation Parameters

Elongation rate k₊ 1.0

Fragmentation rate k_frag 0.6

Cell-division interval 14 s

Culture State

Propagons (fibril seeds)

Free monomer pool

240

Mean fibril length

0.0

Generation

Cell is prion-free [psi-]

How it works

Prion propagation is not just one-shot templating (a misfolded monomer touching a normal one). Once a small amyloid fibril (a "propagon") exists, it grows by capturing normal, natively-folded monomers onto its exposed ends and templating their conformation onto the aggregate — nucleated-conformational-conversion polymerization:

dL/dt = k+ · [monomer] · (number of growing ends)
Fragmentation rate per fibril  ≈ k_frag · (L − 1)
New propagons created by a break = +1 per fragmentation event

Long fibrils are fragile. A chaperone system (in yeast, Hsp104; a related surveillance exists for mammalian PrP aggregates) periodically shears a fibril into two shorter ones. Each break creates a brand-new growing end — a new propagon — so fragmentation, not just growth, is what multiplies the infectious/heritable unit. This is the Cox–Tuite / Masel–Jansen–Nowak model for heritable yeast prions such as [PSI+], and the same growth-and-scission logic underlies mammalian PrP^Sc amplification assays (PMCA).

Elongation rate k+ — how fast free monomer is captured onto fibril ends.
Fragmentation rate k_frag — how often fibrils are cut; too low and propagons cannot keep pace with dilution.
Cell-division interval — every division randomly partitions existing propagons roughly 50/50 between "daughters" (only one is rendered); this dilutes propagon number every generation.
Inject Misfolded Seed — adds one short fibril, mimicking exposure to an infectious/misfolded template.

The outcome is a threshold effect: if k_frag is too low relative to the division rate, stochastic loss during partitioning can drive propagon count to zero — the population "cures" itself ([psi-]) even though monomer is still abundant. If k_frag is high enough, propagon number is replenished faster than dilution removes it, and the prion state is mitotically stable indefinitely.

Free monomer (folded PrP-C) Fibril-bound (misfolded, templated)

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