Apical membrane localization of the adenomatous polyposis coli tumor suppressor protein and subcellular distribution of the beta-catenin destruction complex in polarized epithelial cells

The adenomatous polyposis coli (APC) protein is implicated in the majority of hereditary and sporadic colon cancers. APC is known to function as a tum or suppressor through downregulation of beta -catenin as part of a high mol ecular weight complex known as the beta -catenin destruction complex. The m olecular composition of the intact complex and its site of action in the ce ll are still not well understood. Reports on the subcellular localization o f APC in various cell systems have differed significantly and have been con sistent with an association with a cytosolic complex, with microtubules, wi th the nucleus, or with the cortical actin cytoskeleton. To better understa nd the role of APC and the destruction complex in colorectal cancer, we hav e begun to characterize and isolate these complexes from confluent polarize d human colon epithelial cell monolayers and other epithelial cell types. S ubcellular fractionation and immunofluorescence microscopy reveal that a pr edominant fraction of APC associates tightly with the apical plasma membran e in a variety of epithelial cell types. This apical membrane association i s not dependent on the mutational status of either APC or beta -catenin. An additional pool of APC is cytosolic and fractionates into two distinct hig h molecular weight complexes, 20S and 605 in size. Only the 20S fraction co ntains an appreciable portion of the cellular axin and small but detectable amounts of glycogen synthase kinase 3 beta and beta -catenin. Therefore, i t is likely to correspond to the previously characterized beta -catenin des truction complex. Dishevelled is almost entirely cytosolic, but does not si gnificantly cofractionate with the 20S complex. The disproportionate amount of APC in the apical membrane and the lack of other destruction complex co mponents in the 60S fraction of APC raise questions about whether these poo ls of APC take part in the degradation of beta -catenin, or alternatively, whether they could be involved in other functions of the protein that still must be determined.