Characterization of an Integral Membrane Glycoprotein Associated with the Microfilaments of Pig Intestinal Microvilli

An integral membrane glycoprotein of pig intestinal microvilli which exists in two polypeptide forms [mol. wt. 140 K and 200 K as measured by SDS-polyacrylamide gel electrophoresis (SDS-PAGE)] was purified to homogeneity and characterized. The 200-K form is probably a precursor of the 140-K species. We have localized the glycoprotein by electron microscope immunochemistry using specific antibodies and determined its topological organization with respect to the membrane bilayer. Triton X-100 treatments which solubilize most other microvillar membrane glycoproteins from purified, closed, right-side out vesicles do not efficiently extract this protein. The protein can be partially solubilized from the detergent-insoluble residue, either by treatment with proteases (trypsin or papain) or by exposure to low ionic strength buffer in the presence of chelating agents and detergents. Once solubilized by papain or trypsin, the protein co-migrates on SDS-PAGE with the protein obtained by low ionic strength extraction. However, the form of the protein released by papain does not bind detergents and exhibits hydrophilic properties. Our observations are consistent with the 140-K protein having a small hydrophobic domain that anchors it to the microvillar membrane. The 140-K glycoprotein binds in vitro to a 110-K protein of the core cytoskeleton residue. These observations suggest that the 140-K glycoprotein may be a transmembrane protein which may in vivo provide attachment sites for direct or indirect association with polypeptides of the microvillus cytoskeleton.