Data from: Evolutionary origins of a bioactive peptide buried within preproalbumin

The de novo evolution of proteins is now considered a frequented route for
biological innovation, but the genetic and biochemical processes that lead
to each newly created protein are often poorly documented. The common
sunflower (Helianthus annuus) contains the unusual gene PawS1
(Preproalbumin with SFTI-1) that encodes a precursor for seed storage
albumin; however, in a region usually discarded during albumin maturation,
its sequence is matured into SFTI-1, a protease-inhibiting cyclic peptide
with a motif homologous to unrelated inhibitors from legumes, cereals, and
frogs. To understand how PawS1 acquired this additional peptide with novel
biochemical functionality, we cloned PawS1 genes and showed that this dual
destiny is over 18 million years old. This new family of mostly
backbone-cyclic peptides is structurally diverse, but the
protease-inhibitory motif was restricted to peptides from sunflower and
close relatives from its subtribe. We describe a widely distributed,
potential evolutionary intermediate PawS-Like1 (PawL1), which is matured
into storage albumin, but makes no stable peptide despite possessing
residues essential for processing and cyclization from within PawS1. Using
sequences we cloned, we retrodict the likely stepwise creation of PawS1’s
additional destiny within a simple albumin precursor. We propose that
relaxed selection enabled SFTI-1 to evolve its inhibitor function by
converging upon a successful sequence and structure.