Protein polymers proliferate: polymerization of phosphatases and proteases

It has been known for more than half a century that enzymes can polymerize to either activate or repress their catalytic functions. Using electron cryo-microscopy (cryo-EM), we serendipitously found that two commonly used enzymes in molecular microbiology research, alkaline phosphatase and proteinase K, can unexpectedly form filaments. While the physiological significance of fibrillation in alkaline phosphatase and proteinase K is unknown, the atomic models of these polymers show the ability of cryo-EM to discover and determine high-resolution structures of macromolecular assemblies before they have been characterized or purified. IMPORTANCE: Electron cryo-microscopy (cryo-EM) has emerged over the past decade as the dominant technique in structural biology for determining the atomic structures of biological macromolecular complexes. One less appreciated feature of cryo-EM is that, in many cases, complexes do not need to be purified. Thus, the atomic structures of assemblies that can be found in biological samples as "contaminants" can be readily determined. In this paper, the structures of filaments formed by a phosphatase and a protease are described.