Lipid Phosphate Phosphatases 1 and 3 Are Localized in Distinct Lipid Rafts

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Abstract
Lipid phosphate phosphatases (LPPs), integral membrane proteins with six transmembrane domains, dephosphorylate a variety of extracellular lipid phosphates. Although LPP3 is already known to bind to Triton X-100–insoluble rafts, we here report that LPP1 is also associated with lipid rafts distinct from those harboring LPP3. We found that LPP1 was Triton X-100–soluble, but CHAPS-insoluble in LNCaP cells endogenously expressing LPP1 and several LPP1 cDNA–transfected cells including NIH3T3 fibroblasts. In addition to the non–ionic detergent insolubility, LPP1 further possessed several properties formulated for raft-localizing proteins as follows: first, the CHAPS-insolubility was resistant to the actin-disrupting drug cytochalasin D; second, the CHAPS-insoluble LPP1 floated in an Optiprep density gradient; third, the CHAPS insolubility of LPP1 was lost by cholesterol depletion; and finally, the subcellular distribution pattern of LPP1 exclusively overlapped with that of a raft marker, cholera toxin B subunit. Interestingly, confocal microscopic analysis showed that LPP1 was distributed to membrane compartments distinct from those of LPP3. Analysis using various LPP1/LPP3 chimeras revealed that their first extracellular regions determine the different Triton X-100 solubilities. These results indicate that LPP1 and LPP3 are distributed in distinct lipid rafts that may provide unique microenvironments defining their non-redundant physiological functions.