Membrane localization and topology of leukotriene C4 synthase

Leukotriene C-4 (LTC4) synthase conjugates LTA(4) with GSH to form LTC4. Determining the site of LTC4 synthesis and the topology of LTC4 synthase may uncover unappreciated intracellular roles for LTC4, as well as how LTC4 is transferred to its export carrier, the multidrug resistance protein-1. We have determined the membrane localization of LTC4 synthase by immunoelectron microscopy. In contrast to the closely related five-lipoxygenase-activating protein, LTC4 synthase is distributed in the outer nuclear membrane and peripheral endoplasmic reticulum but is excluded from the inner nuclear membrane. We have combined immunofluorescence with differential membrane permeabilization to determine the topology of LTC4 synthase. The active site of LTC4 synthase is localized in the lumen of the nuclear envelope and endoplasmic reticulum. These results indicate that the synthesis of LTB4 and LTC4 occurs in different subcellular locations and suggests that LTC4 must be returned to the cytoplasmic side of the membrane for export by multidrug resistance protein-1. The differential localization of two very similar integral membrane proteins suggests that mechanisms other than size-dependent exclusion regulate their passage to the inner nuclear membrane.