Association of brefeldin A-inhibited guanine nucleotide-exchange protein 2 (BIG2) with recycling endosomes during transferrin uptake

ADP-ribosylation factors (ARFs) are critical in vesicular trafficking. Brefeldin A-inhibited guanine nucleotide-exchange protein (BIG)1 and BIG2 activate ARFs by accelerating replacement of bound GDP with GTP. Additional and differing functions of these approximately 200-kDa proteins are now being recognized, as are their independent intracellular movements. Here, we describe the localization in COS7 cells by immunofluorescence microscopy of BIG2, but not BIG1, with structures that have characteristics of recycling endosomes during transferrin (Tfn) uptake and Tfn receptor (TfnR) recycling. Cell content of BIG2 and Rab11, but not TfnR, BIG1, Rab4, or Exo70, was increased after 60 min of Tfn uptake. BIG2, but not BIG1, appeared in density-gradient fractions containing TfnR, Rab11, and Exo70 after 60 min of Tfn uptake. Treatment of cells with BIG2 small interfering RNA (siRNA), but not BIG1 or control siRNAs, decreased BIG2 protein >90% without affecting BIG1, ARF, or actin content, whereas TfnR was significantly increased as was its accumulation in perinuclear recycling endosomes. Tfn release appeared unaffected by BIG1 siRNA but was significantly slowed from cells treated with BIG2 siRNA alone or plus BIG1 siRNA. We suggest that BIG2 has an important role in Tfn uptake and TfnR recycling, perhaps through its demonstrated interaction with Exo70 and the exocyst complex.