Fractals are patterns that are self-similar across multiple length-scales¹. Macroscopic fractals are common in nature^2,3,4; however, so far, molecular assembly into fractals is restricted to synthetic systems^{5,6,7,8,9,10,11,12}. Here we report the discovery of a natural protein, citrate synthase from the cyanobacterium Synechococcus elongatus, which self-assembles into Sierpiński triangles. Using cryo-electron microscopy, we reveal how the fractal assembles from a hexameric building block. Although different stimuli modulate the formation of fractal complexes and these complexes can regulate the enzymatic activity of citrate synthase in vitro, the fractal may not serve a physiological function in vivo. We use ancestral sequence reconstruction to retrace how the citrate synthase fractal evolved from non-fractal precursors, and the results suggest it may have emerged as a harmless evolutionary accident. Our findings expand the space of possible protein complexes and demonstrate that intricate and regulatable assemblies can evolve in a single substitution.

分形是在多个长度尺度上自相似的模式¹。宏观分形在自然界中很常见^2,3,4；然而，到目前为止，分子组装成分形仅限于合成系统^{5,6,7,8,9,10,11,12}。在这里，我们报告了一种来自蓝细菌细长聚球藻的柠檬酸合酶的发现，它可以自组装成谢尔宾斯基三角形。使用冷冻电子显微镜，我们揭示了分形如何从六聚体构建块组装而成。尽管不同的刺激调节分形复合物的形成，并且这些复合物可以在体外调节柠檬酸合酶的酶活性，但分形在体内可能不发挥生理功能。我们使用祖先序列重建来追溯柠檬酸合酶分形是如何从非分形前体进化而来的，结果表明它可能是作为一种无害的进化事故而出现的。我们的研究结果扩大了可能的蛋白质复合物的空间，并证明复杂且可调节的组装可以在单次替换中进化。