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Lecture 16

BIO241 Lecture 16

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Jennifer Harris

Thursday March 5 2009 We discussed anchoring junctions and how they can overcome the challenges to cellcell adhesion by clustering and by making linkages to the cytoskeleton We specifically talked about adherens junctions and how they can form clusters and how they link to the actin cytoskeleton by means of the catenin adaptor proteins and then we talked about adherens junction functions in a number of processes Today we are going into these things in the slide We will go through a few more types of anchoring junctions then well talk about occluding junctions and finally communicating junctionscarbohydrate binding proteins The first anchoring junction he wants to discuss is adhesion mediated by these selectin receptors Like all cell adhesion molecules to mediate cellcell adhesion these receptors have to reach out into the extracellular space so these are transmembrane proteins and we can see an example of a selectin here its transmembrane domain and its cytoplasmic tail as well as its extracellular region These proteins are lectins so that means they bind to carbohydrates they bind to sugars and on the end of this is the lectin domain which will reach out and bind to sugars that are on proteins or in the membrane of other cells to mediate cellcell adhesion So here we have this protein reaching out and binding to sugars or carbohydrates on another cell Is this homophilic or heterophilic cell adhesion This is heterophilic binding It is not like binding like it is binding to something different it is binding to a carbohydrate it is different from the cadherin binding mechanismcarbohydrate binding proteinsOligosaccharides on other cells This lectin domain then binds to oligosaccharides on other cellsthe fact that it is binding to oligosaccharides so multiple sugars means that this will have a clustering effect This is how it meets that one main criterion about junctions that the receptors tend to cluster to give higher binding strength through avidity of binding Binding to the oligosaccharides is calcium dependent and like other cell adhesion complexes this one is also linked to the actin cytoskeleton beneath through specific adaptor proteins that bind to these receptors and mediate their linkage to the actin cytoskeleton There are 3 main types of selectin some in white blood cells others in blood platelets and endothelial cells and others that are found only in endothelial cells What do these selectins do
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