Most vertebrate oocytes contain a Balbiani body, a
large, non-membrane-bound compartment packed
with RNA, mitochondria, and other organelles. Little
is known about this compartment, though it specifies
germline identity in many non-mammalian vertebrates.
We show Xvelo, a disordered protein with
an N-terminal prion-like domain, is an abundant constituent
of Xenopus Balbiani bodies. Disruption of the
prion-like domain of Xvelo, or substitution with a
prion-like domain from an unrelated protein, interferes
with its incorporation into Balbiani bodies
in vivo. Recombinant Xvelo forms amyloid-like networks
in vitro. Amyloid-like assemblies of Xvelo recruit
both RNA and mitochondria in binding assays.
We propose that Xenopus Balbiani bodies form by
amyloid-like assembly of Xvelo, accompanied by
co-recruitment of mitochondria and RNA. Prion-like
domains are found in germ plasm organizing proteins
in other species, suggesting that Balbiani
body formation by amyloid-like assembly could be
a conserved mechanism that helps oocytes function
as long-lived germ cells.