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.

蛋白质从头演化（de novo evolution of proteins）如今被视为生物创新的常见路径，但每种新生成蛋白质背后的遗传与生化过程往往鲜有详实记载。普通向日葵（Helianthus annuus）携带有罕见基因PawS1（Preproalbumin with SFTI-1），该基因编码种子贮藏白蛋白的前体；然而在白蛋白成熟过程中通常被切除的区域，其序列可被加工为成熟的SFTI-1——一种具有蛋白酶抑制活性的环肽（cyclic peptide），其基序与豆科、谷类及蛙类来源的无关抑制剂具有同源性。为解析PawS1如何获得这一具有全新生化功能的额外肽段，我们克隆了PawS1基因，并证实这种兼具双重产物的演化历程已超过1800万年。这一全新的、以主链环化为主的肽家族结构多样，但蛋白酶抑制基序仅存在于向日葵及其所属亚族内的近缘物种所编码的肽段中。我们还发现了一种广泛分布的潜在演化中间物PawS-Like1（PawL1），该基因可被加工为贮藏白蛋白，但无法生成稳定肽段，尽管其携带有PawS1中用于加工与环化所需的关键残基。基于我们克隆得到的序列，我们逆向推演了PawS1如何在简单白蛋白前体的框架下，逐步形成其额外产物命运的过程。我们提出，放松选择（relaxed selection）使得SFTI-1通过趋同演化出成功的序列与结构，从而获得了蛋白酶抑制功能。