Colocalization of sterol isomerase and sigma(1) receptor at endoplasmic reticulum and nuclear envelope level

SR31747A is a sigma ligand previously described as having original immunosu ppressive properties. Two SR31747A targets were recently identified and ter med sigma(1) or SR-BP-1 (SR31747A-binding protein-1) and hSI (human sterol isomerase). In order to characterize these proteins further, we examined th eir expression and localization at the subcellular level. Based on the amin o acid sequence deduced from the cloned hSI, anti-hSI polyclonal antibody w as raised against the N-terminal fragment of the protein. Using this antibo dy, we performed Western-blot experiments to demonstrate the presence of hS I in various B and T cell Lines, and hSI expression was quantified in these cell lines by flow cytometry and estimated at 15 000-30 000 sites per cell . Subcellular localization studies by both confocal and electron microscopy , performed on THP1 cells with anti-hSI antibody and with the previously de scribed anti-(SR-BP-1) monoclonal antibody, demonstrated that: (a) hSI was colocalized with SR-BP-1; (b) hSI and SR-BP-1 were associated with the endo plasmic reticulum and with the outer and inner membranes of the nuclear env elope; (c) both proteins were delocalized during the cell cycle at the mito sis step when the nuclear membranes disappeared. Taken together our results suggest that both SR31747A-binding proteins not only play a role in sterol metabolism but indirectly affect lipoprotein functions.