Intrinsically disordered proteins play causative roles in many human diseases. Their overexpression is toxic in many organisms, but the causes of toxicity are opaque. In this paper, we exploit yeast technologies to determine the root of toxicity for one such protein, the yeast prion Rnq1. This protein is profoundly toxic when overexpressed but only in cells carrying the endogenous Rnq1 protein in its RNQ(+) prion (amyloid) conformation. Surprisingly, toxicity was not caused by general proteotoxic stress. Rather, it involved a highly specific mitotic arrest mediated by the Mad2 cell cycle checkpoint. Monopolar spindles accumulated as a result of defective duplication of the yeast centrosome (spindle pole body SPB). This arose from selective Rnq1-mediated sequestration of the core SPB component Spc42 in the insoluble protein deposit (IPOD). Rnq1 does not normally participate in spindle pole dynamics, but it does assemble at the IPOD when aggregated. Our work illustrates how the promiscuous interactions of an intrinsically disordered protein can produce highly specific cellular toxicities through illicit, yet highly specific, interactions with the proteome.