Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 8

Cell adhesion molecules were originally identi- fied using antibodies raised against cell-surface compo- nents to block cell aggregation. In the adhesion-blocking assays, the researchers found it necessary to use antibody fragments, each with a single binding site (so-called Fab fragments), rather than intact IgG antibodies, which are Y-shaped molecules with two identical binding sites. The Fab fragments were generated by digesting the IgG anti- bodies with papain, a protease, to separate the two bind- ing sites (Figure Q19-1). Why do you suppose it was nec- essary to use Fab fragments to block cell aggregation?
A. How can it be that two tight-junction strands remain, even though all of the claudin-4 has disappeared? b. Why do you suppose the toxin works when it is added to the basolateral side of the epithelial sheet, but not when added to the apical side?