An Amyloid Organelle: Solid State NMR Evidence for Cross-Beta Assembly of Gas Vesicles

The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Background: The gas vesicles of aquatic microorganisms are hollow proteinaceous shells with remarkable physical properties that enable them to function as floatation organelles. Result: The gas vesicle subunits associate in a cross-beta arrangement. Conclusion: The gas vesicle wall constitutes a functional amyloid. Significance: This new category of functional amyloid broadens our understanding of the diverse roles of the amyloid fold. SUMMARY Functional amyloids have been identified in a wide range of organisms, taking on a variety of biological roles, and being controlled by remarkable mechanisms of directed assembly. Here we report that amyloid fibrils constitute the ribs of the buoyancy organelles of Anabaena flos-aquae. The walls of these gas-filled vesicles are known to comprise a single protein, GvpA, arranged in a low-pitch helix. However, the tertiary and quaternary structures have been elusive. Using solid-state NMR correlation spectroscopy we find detailed evidence for an extended cross-β structure. This amyloid assembly helps to account for the strength and amphiphilic properties of the vesicle wall. Buoyancy organelles thus dramatically extend the scope of known functional amyloids. Introduction Found in a wide variety of organisms and associated with pathological and functional states, amyloids are open-ended protein assemblies characterized by a highly ordered cross-β architecture in which the β-strands are oriented transverse to the polymerization axis (1-3). Amyloid fibrils have been observed in a large number of pathologies, including neurodegenerative disorders and systemic amyloidoses (1). However, the discovery of in vitro fibril formation by proteins unrelated to disease has led to the notion that amyloid is a generic and accessible state in the protein folding landscape (4). Moreover, proteins in organisms ranging from bacteria to mammals have been found to form amyloid structures that carry out specific functions and are referred to as functional amyloids (5). While functional amyloids share the general morphology of disease-related amyloids, they possess key distinctive features related to the control of fibril formation. These include sequestration of the proteins to avoid aberrant fibril formation, rapid fibril elongation that avoids the accumulation of intermediates, and regulation of both assembly and disassembly (6). Thus the discovery and study of functional amyloid systems have provided new insights into the diverse ways in which proteins adopt amyloid structures (7).