Reduced intranuclear mobility of APL fusion proteins accompanies their mislocalization and results in sequestration and decreased mobility of retinoid X receptor α

Acute promyelocytic leukemia (APL) cells contain one of five chimeric retinoic acid et-receptor (RARalpha) genes (X-RARalpha) created by chromosomal translocations or deletion; each generates a fusion protein thought to transcriptionally repress RARalpha target genes and block myeloid differentiation by an incompletely understood mechanism. To gain spatiotemporal insight into these oncogenic processes, we employed fluorescence microscopy and fluorescence recovery after photobleaching (FRAP). Fluorescence microscopy demonstrated that the intracellular localization of each of the X-RARalpha proteins was distinct from that of RARalpha and established which portion(s) of each X-RARalpha protein-X, RAR, or both-contributed to its altered localization. Using FRAP, we demonstrated that the intranuclear mobility of each X-RARalpha was reduced compared to that of RARalpha. In addition, the mobility of each X-RARalpha was reduced further by ligand addition, in contrast to RARalpha, which showed no change in mobility when ligand was added. Both the reduced baseline mobility of X-RARalpha and the ligand-induced slowing of X-RARalpha could be attributed to the protein interaction domain contained within X. RXRalpha aberrantly colocalized within each X-RARalpha; colocalization of RXRalpha with promyelocytic leukemia (PML)-RARalpha resulted in reduced mobility of RXRalpha. Thus, X-RARalpha may interfere with RARalpha through its aberrant nuclear dynamics, resulting in spatial and temporal sequestration of RXRalpha and perhaps other nuclear receptor coregulators critical for myeloid differentiation.