(a) Depiction of FPP assay. Digitonin selectively permeabilizes the plasma membrane but not internal membranes. Proteinase K degrades cytoplasmic components but membranous compartments remain intact. Under these conditions, MKLP1-GFP-labelled MBds (blue ellipse) in the cytoplasm will be degraded whereas those inside membrane-bound compartments will not. (b) MBds in MBd-poor hRPE-1 cells are largely protected (∼90% in membranous compartments, 10 cells analysed), whereas most MBds in HeLa cells are not (∼27%, 11 cells analysed), and are thus degraded in cytoplasm. Scale bar, 5 μm. 