Role of phospholipase A<sub>2</sub>-dependent membrane tubules and platelet-activating factor acetyl-hydrolase isoform IB alpha-1 and alpha-2 subunits in membrane trafficking pathways
Proper functioning of the secretory and endocytic pathways is critical to the structural and functional organization of eukaryotic cells. Transport of material within these pathways maintains a necessary balance of membrane and protein. Vesicle-mediated transport has been studied extensively. However, the molecular mechanisms of membrane tubule-mediated transport are not well understood. Evidence suggests that cytoplasmic calcium-independent phospholipase A2 (PLA2) activity is involved in the direct formation of membrane tubules from the Golgi complex, trans Golgi network (TGN), and endosomes. The focus of this work is to examine the role of PLA 2 enzymes in various membrane trafficking pathways and to demonstrate a role for platelet-activating factor acetyl-hydrolase isoform Ib (PAF-AH Ib) α1 and α2 subunits in tubule formation.
Previous studies have shown that export of transferrin and transferrin receptor from early sorting endosomes (ESEs) and the endocytic recycling compartment (ERC) is mediated in part by PLA2-dependent membrane tubules. Studies here show that tubules are a general feature of intracellular trafficking pathways. I show that PLA2 activity is likely involved in trafficking pathways from ESEs to late endosomes (LEs), from LEs to lysosomes, and from the ERC to the TGN.
I have also found a novel role for the Ca2+-independent PLA2 PAF-AH Ib α1 and α2 subunits. These proteins are found in bovine brain cytosol and are capable of inducing tubulation of Golgi membranes in vitro. In addition, over-expression of α1 or α2 disrupts the normal localization of proteins found in the Golgi complex, TGN, late endosomes, and lysosomes. Furthermore, the normal trafficking of cargo through the recycling and degradative endocytic pathways is altered by over-expression of α1.
These results are consistent with the model that a cytoplasmic Ca 2+-independent PLA2 is involved in tubulation of Golgi and endosomal membranes, and that tubules play a general role in the trafficking of cargo through the secretory and endocytic pathways. These studies also suggest a novel role for PAF-AH Ib in membrane trafficking. Understanding the exact role of the PAF-AH Ib subunits in membrane tubulation and how the actions of these subunits are regulated will help to elucidate the mechanisms underlying intracellular trafficking pathways.
0307: Molecular biology