Functional characterization of all-trans retinoic acid-induced differentiation factor (ATRAID).

All-trans retinoic acid-induced differentiation factor (ATRAID) was first identified in HL60 cells. Several mRNA isoforms exist, but the respective proteins have not been fully characterized. In transfected cells expressing Myc-Flag-tagged ATRAID Isoform (Iso) A, B and C, Iso C was found to be expressed at high levels, Iso A was found to be expressed at low levels due to rapid degradation, and the predicted protein expressed from Iso B was not detected. Iso C was present mainly in an N-glycosylated form. In subcellular fractionation experiments, Iso C localized to the membranous and nuclear fractions, while immunofluorescence analysis revealed that Iso C is located close to the plasma membrane, mainly in cytoplasmic vesicles and in the Golgi area. We confirm that Iso C co-localizes to some extent with both endosomal/lysosomal markers LAMP1 and LAMP2. Furthermore, we show that ATRAID co-localizes with RAB11, a GTPase associated with recycling endosomes (REs) and implicated in regulating vesicular trafficking.