The F→S mutant of MBP loses its ability for protein extrusion, but not for membrane binding.

<p>(A) Sequence of the 14 kDa isoform of MBP showing positions of phenylalanine residues that are mutated to serines in the F→S mutant. (B) Distribution of cell surface glycoproteins as visualized by fluorophore-conjugated ConA staining in PtK2 cells expressing mCherry-TM-MBP or mCherry-TM-MBP F→S. Enlarged view of the selected regions in merged images is shown on the right side. (C) Assessment of plasma membrane distribution of soluble MBP and MBP F→S in Oli-neu cells, an oligodendroglial precursor cell line. Scale bar, 10 µm. Relative intensity profile plots along the plasma membrane is shown on the right side (<i>n</i> = 20 cells). (D) Typical images of SLBs (with the inner myelin leaflet lipid composition) stained for MBP after incubation with 7 µM of either purified wild-type (MBP) or mutant (MBP F→S). Scale bar, 10 µm. (E) Western blot analysis of supernatant (S) and pellet (P) fractions after incubation of LUVs (large unilamellar vesicles with the inner myelin leaflet lipid composition) with either 3.5 µM of WT MBP, MBP F→S, or recombinant GFP.