Small GTPase Rab12 regulates transferrin receptor degradation: Implications for a novel membrane trafficking pathway from recycling endosomes to lysosomes

Abstract

Plasma membrane receptor proteins play a key role in signal transduction and nutrient uptake, thereby controlling quality of receptor proteins is one of the most important issues in cellular logistics. After endocytosis, receptor proteins are generally delivered to lysosomes for degradation or recycled back to the plasma membrane for recycling. Transferrin receptor (TfR) is a well-known representative of recycling receptor proteins, which are traveled between plasma membrane and recycling endosomes. Although the molecular mechanism of the TfR recycling pathway has been extensively investigated in the literature, almost nothing is known about its degradation mechanism. We have recently shown that small GTPase Rab12 and its upstream activator Dennd3 regulate the constitutive degradation of TfR without modulating a conventional endocytic degradation pathway or TfR recycling pathway. Our findings suggest that Rab12 regulates membrane trafficking of TfR from recycling endosomes to lysosomes. In this addendum, we discuss the physiological significance of TfR degradation and the fate of determination of TfR (recycling or degradation).

Figure 1

(A) TfR is constitutively degraded by lysosomes in MEF cells. MEF cells were treated with DMSO (left part), 50 µg/ml cycloheximide (middle part) for 4 h or 100 nM bafilomycin A1 (bafilomycin; right part) for 9 h. The cells were fixed and immunostained with anti-TfR antibody. Note that TfR signals in cycloheximide-treated and bafilomycin-treated cells were weaker and stronger, respectively, than those in control cells. (B) Rab11 and Rab12 differently regulate the trafficking of TfR protein from recycling endosomes. MEF cells transfected with control siRNA (si-Control; left part), Rab11B siRNA (si-Rab11; middle part) or Rab12 siRNA (si-Rab12; right part) were immunostained with anti-TfR antibody. Note that TfR signals in Rab11B-knockdown and Rab12-knockdown cells were weaker and stronger, respectively, than those in control cells. Scale bar, 20 µm.

Figure 2

Rab12 is not involved in iron-induced TfR degradation. MEF cells transfected with control siRNA or Rab12 siRNA were treated with 50 µg/ml FAC (ferric ammonium citrate, an iron supplier) for the indicated times. Cell lysates were analyzed by 10% SDS-PAGE followed by immunoblotting with anti-Rab12 antibody (top part), anti-TfR antibody (middle part), and anti-β-actin antibody (bottom part). The positions of the molecular mass markers (in kDa) are shown on the left.

Figure 3

A model for two distinct TfR degradation pathways. Internalized TfR is recycled back to the plasma membrane through Rab11-depdenent recycling pathway or delivered to lysosomes through either iron-induced “selective” pathway or Rab12-dependent “constitutive” pathway. Adapted with permission from reference .