Tumor-Associated NH2-Terminal Fragments Are the Most Stable Part of the Adenomatous Polyposis Coli Protein and Can Be Regulated by Interactions with COOH-Terminal Domains

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Abstract
Truncation mutations in the adenomatous polyposis coli (APC) gene are responsible for familial and sporadic colorectal cancer. APC is a large, multifunctional protein involved in cell migration, proliferation, and differentiation. Dominant effects that have been attributed to the NH2-terminal fragments of APC expressed in tumors may result from loss of functions due to lack of COOH-terminal regions or gain of functions due to fewer regulatory interactions. Resolving this issue and determining how structural changes contribute to the multiple functions of the APC protein requires knowledge about the structural organization of the APC molecule. To this end, we used limited proteolysis to distinguish regions of the molecule with limited structure from those that form well-folded domains. We discovered that the NH2-terminal region of APC was most resistant to proteolytic degradation, whereas middle and COOH-terminal regions were significantly more sensitive. Binding of APC to microtubules protected COOH-terminal regions of APC against proteolysis, consistent with the idea that this region of the molecule becomes ordered when bound to microtubules. Furthermore, interactions between the NH2- and COOH-terminal domains of APC were identified in vitro and in vivo, suggesting that NH2-terminal fragments of APC may be regulated by interactions with COOH-terminal domains. Indeed, expressing COOH-terminal APC fragments in tumor cells resulted in changes in the protein interactions of endogenous NH2-terminal fragments in these cells. Thus, the dominant function of NH2-terminal APC fragments found in tumor cells could be explained by loss of this regulation in tumors where COOH-terminal domains are missing.