Significance Human ALIX is involved in many essential cellular processes. The carboxy (C)-terminal proline-rich domain (PRD) of ALIX, which interacts with multiple partners, has eluded analysis owing to the presence of polyproline stretches that hinder recombinant protein production. By dividing it into two fragments, the amino (N)- and the C-terminal portions, we carry out a detailed biophysical investigation of ALIX-PRD. The disordered N-terminal fragment comprises three proline-rich motifs that compete for binding to its partner, TSG101-UEV. In contrast, the C-terminal fragment forms reversible amyloids modulated by tyrosine phosphorylation, the first known example where posttranslational modifications dictate the dissolution of amyloid fibrils. This study illustrates the mechanistic interdependence of multivalency brought out by polymerization of ALIX-PRD and its effect on ALIX function.