In vivo aggregation properties of the nuclear poly(A)-binding protein PABPN1

A broad range of degenerative diseases is associated with intracellular inclusions formed by toxic, aggregation-prone mutant proteins. Intranuclear inclusions constitute a pathological hallmark of oculopharyngeal muscular dystrophy (OPMD), a dominantly inherited disease caused by (GCG) repeat expansions in the gene that encodes for nuclear poly(A) binding protein (PABPN1). The mutation results in an extended polyalanine stretch that has been proposed to induce protein aggregation and formation of intranuclear inclusions. Here we show that normal PABPN1 is inherently aggregation-prone when exogenously expressed in either HeLa or myogenic C2 cells. Similar deposits of insoluble PABPN1 are formed by variant forms of the protein containing either a polyalanine expansion or a complete deletion of the polyalanine tract, indicating that the mutation responsible for OPMD is not essential for formation of PABPN1 inclusions. In contrast, interfering with any of the protein domains required for stimulation of poly(A) polymerase prevents the formation of inclusions. Most surprisingly, photobleaching experiments reveal that both normal and expanded PABPN1 molecules are not irreversibly sequestered into aggregates, but rather move rapidly in and out of the inclusions. These findings have important implications for the interpretation of OPMD model systems based on exogenous expression of PABPN1.