The Coiled Coil Region (Amino Acids 129–250) of the Tumor Suppressor Protein Adenomatous Polyposis Coli (APC)

The APC (adenomatous polyposis coli) tumor suppressor protein has many different intracellular functions including a nuclear export activity. Only little is known about the molecular architecture of the 2843-amino acid APC protein. Guided by secondary structure predictions we identified a fragment close to the N-terminal end, termed APC-(129–250), as a soluble and protease-resistant domain. We solved the crystal structure of APC-(129–250), which is monomeric and consists of three α-helices forming two separate antiparallel coiled coils. APC-(129–250) includes the nuclear export signal NES-(165–174) at the C-terminal end of the first helix. Surprisingly, the conserved hydrophobic amino acids of NES-(165–174) are buried in one of the coiled coils and are thus not accessible for interaction with other proteins. We demonstrate the direct interaction of APC-(129–250) with the nuclear export factor chromosome maintenance region 1 (Crm-1). This interaction is enhanced by the small GTPase Ran in its activated GTP-bound form and also by a double mutation in APC-(129–250), which deletes two amino acids forming two of the major interhelical interactions within the coiled coil. These observations hint to a regulatory mechanism of the APC nuclear export activity by NES masking.

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