Abstract
Lysosome-related organelles (LROs) are synthesized in specialized cell types where they largely coexist with conventional lysosomes. Most of the known cellular transport machinery involved in biogenesis are ubiquitously expressed and shared between lysosomes and LROs. Examples of common components are the adaptor protein complex-3 (AP-3) and biogenesis of lysosome-related organelle complex (BLOC)-2. These protein complexes control sorting and transport of newly synthesized integral membrane proteins from early endosomes to both lysosomes and LROs such as the melanosome. However, it is unknown what factors cooperate with the ubiquitous transport machinery to mediate transport to LROs in specialized cells. Focusing on the melanosome, we show that the ubiquitous machinery interacts with cell type-specific Rab proteins, Rab38 and Rab32, to facilitate transport to the maturing organelle. BLOC-2, AP-3, and AP-1 coimmunoprecipitated with Rab38 and Rab32 from MNT-1 melanocytic cell extracts. BLOC-2, AP-3, AP-1, and clathrin partially colocalized with Rab38 and Rab32 by confocal immunofluorescence microscopy in MNT-1 cells. Rab38- and Rab32-deficient MNT-1 cells displayed abnormal trafficking and steady state levels of known cargoes of the BLOC-2, AP-3, and AP-1 pathways, the melanin-synthesizing enzymes tyrosinase and tyrosinase-related protein-1. These observations support the idea that Rab38 and Rab32 are the specific factors that direct the ubiquitous machinery to mediate transport from early endosomes to maturing LROs. Additionally, analysis of tyrosinase-related protein-2 and total melanin production indicates that Rab32 has unique functions that cannot be carried out by Rab38 in melanosome biogenesis.
Highlights
Lysosome-related organelles are a group of cell type-specific compartments with specialized functions, including melanosomes in melanocytes
No evidence has been reported for a link between the biogenesis of lysosome-related organelle complex (BLOC) or AP complexes involved in Lysosome-related organelles (LROs) biogenesis and Rab38 or Rab32
We demonstrate that adaptor protein complex-3 (AP-3), AP-1, and BLOC-2 interact physically and functionally with Rab38 and Rab32 to mediate transport of integral membrane proteins from specialized early endosomal domains to maturing melanosomes
Summary
Lysosome-related organelles are a group of cell type-specific compartments with specialized functions, including melanosomes in melanocytes. Rab38- and Rab32-deficient MNT-1 cells displayed abnormal trafficking and steady state levels of known cargoes of the BLOC-2, AP-3, and AP-1 pathways, the melanin-synthesizing enzymes tyrosinase and tyrosinase-related protein-1 These observations support the idea that Rab and Rab are the specific factors that direct the ubiquitous machinery to mediate transport from early endosomes to maturing LROs. analysis of tyrosinase-related protein-2 and total melanin production indicates that Rab has unique functions that cannot be carried out by Rab in melanosome biogenesis. This result placed Rab and Rab as key players in LRO biogenesis and implied that Rab is able to functionally compensate for Rab, at least in part, or that they operate in parallel pathways [27] This apparently epistatic relationship between Rab and Rab is analogous to the one described between AP-3 and BLOC-2 and raised the exciting possibility that these Rabs could be the specificity factors that work in concert with the ubiquitous trafficking machinery for transport toward LROs [17, 27]. Analysis of the three melanin-synthesizing enzymes, tyrosinase, Tyrp, and Tyrp, and overall melanin production show Rab and Rab have both redundant and nonredundant roles in melanosome biogenesis
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
