Lipid Rafts Exist as Stable Cholesterol-independent Microdomains in the Brush Border Membrane of Enterocytes

Abstract

Glycosphingolipid/cholesterol-rich membranes ("rafts")can be isolated from many types of cells, but their existence as stable microdomains in the cell membrane has been elusive. Addressing this problem, we studied the distribution of galectin-4, a raft marker, and lactase, a protein excluded from rafts, on microvillar vesicles from the enterocyte brush border membrane. Magnetic beads coated with either anti-galectin-4 or anti-lactase antibodies were used for immunoisolation of vesicles followed by double immunogold labeling of the two proteins. A morphometric analysis revealed subpopulations of raft-rich and raft-poor vesicles by the following criteria: 1) the lactase/galectin-4 labeling ratio/vesicle captured by the anti-lactase beads was significantly higher (p < or = 0.01) than that of vesicles captured by anti-galectin-4 beads, 2) subpopulations of vesicles labeled by only one of the two antibodies were preferentially captured by beads coated with the respective antibody (p < or = 0.01), 3) the average diameter of "galectin-4 positive only" vesicles was smaller than that of vesicles labeled for lactase. Surprisingly, pretreatment with methyl-beta-cyclodextrin, which removed >70% of microvillar cholesterol, did not affect the microdomain localization of galectin-4. We conclude that stable, cholesterol-independent raft microdomains exist in the enterocyte brush border.