Macroautophagy is a catabolic pathway that targets large, long-lived structures such as protein aggregates, damaged organelles and invading pathogens to cellular degradation by double-membrane vesicles referred to as autophagosomes.It has recently emerged that several adaptor proteins, termed autophagy receptors, support the selective sequestration and transport of specifically marked cargo to nascent autophagosomes.In this process, autophagy receptors recognize ubiquitylated cargo and interact with Atg8/LC3 to bring the cargo-receptor complex in contact with the autophagosomal membrane.The autophagy receptors p62 in mammals and Nbr1 in plants both contain a Phox and Bem1p (PB1) domain that supports homomeric self-interaction.Apart from their function in autophagy, heteromeric interactions with PB1 domain-containing protein kinases link these autophagy receptors to several important signaling pathways.We show that the PB1 domain of A.thaliana Nbr1 (AtNbr1-PB1) assembles into long and regular helical polymers.We present an atomic model of these PB1 polymers based on a 3.9 (A) density map obtained by three-dimensional image reconstruction from electron micrographs of AtNbr1-PB1 polymers embedded in vitreous ice.Our model, supported by biochemical data, demonstrates that the helical structure is formed by electrostatic interactions between PB1 domains via its canonical basic and acidic charge cluster motifs.Mutation of critical residues in these interfaces abolishes polymerization.We suggest that PB1-domain mediated polymers of p62/Nbr1 function as helical scaffolds for cargo recruitment in selective autophagy and in signaling complexes.Comparison with related interaction motifs of other signaling proteins in plants and mammals suggests that molecular organization into helical scaffolds by self-polymerization may be a more general theme.