Many protein condensates
can convert to fibrillar aggregates, but
the underlying mechanisms are unclear. Liquid–liquid phase
separation (LLPS) of spider silk proteins, spidroins, suggests a regulatory
switch between both states. Here, we combine microscopy and native
mass spectrometry to investigate the influence of protein sequence,
ions, and regulatory domains on spidroin LLPS. We find that salting
out-effects drive LLPS via low-affinity stickers in the repeat domains.
Interestingly, conditions that enable LLPS simultaneously cause dissociation
of the dimeric C-terminal domain (CTD), priming it for aggregation.
Since the CTD enhances LLPS of spidroins but is also required for
their conversion into amyloid-like fibers, we expand the stickers
and spacers-model of phase separation with the concept of folded domains
as conditional stickers that represent regulatory units.