The adenomatous polpyposis coli (APC) protein is mutated in most colorectal tumours. Nearly all APC mutations are truncations, and many of these terminate in the mutation cluster region located halfway through the protein. In cancer cells expressing mutant APC, beta-catenin is stabilized and translocates into the nucleus to act as a transcriptional co-activator of T-cell factor. During normal development, APC also promotes the destabilization of beta-catenin and Drosophila Armadillo. It does so by binding to the Axin complex which earmarks beta-catenin/Armadillo for degradation by the proteasome pathway. APC has a regulatory role in this process, which is poorly understood. Here we show that APC contains highly conserved nuclear export signals 3' adjacent to the mutation cluster region that enable it to exit from the nucleus. This ability is lost in APC mutant cancer cells, and we provide evidence that beta-catenin accumulates in the nucleus as a result. Thus, the ability of APC to exit from the nucleus appears to be critical for its tumour suppressor function.

The Wnt signalling pathway is pivotal in normal and malignant development. A key effector is Armadillo (Arm)/beta-catenin, which functions with TCF to transcribe Wnt target-genes. Here, we report the discovery of pygopus (pygo), whose mutant phenotypes specifically mimic loss-of-Wingless (Wg) signalling. pygo is required for dTCF-mediated transcription, but not for Wg-induced stabilization of Arm. Pygo is a nuclear protein that is found in a complex with Arm in vivo. Humans possess two Pygo proteins, both of which are required for TCF-mediated transcription in colorectal cancer cells. The presence of a PHD domain implicates Pygo proteins in a chromatin-related function, and we propose that they mediate chromatin access to TCF or Arm/beta-catenin.

Destruction of mitotic cyclins by ubiquitindependent proteolysis is required for cells to complete mitosis and enter interphase of the next cell cycle. In clam eggs, this process is catalyzed by a cyclin-selective ubiquitin carrier protein, E2-C, and the cyclosome͞anaphase promoting complex (APC), a 20S particle containing cyclin-selective ubiquitin ligase activity. Here we report cloning a human homolog of E2-C, UbcH10, which shares 61% amino acid identity with clam E2-C and can substitute for clam E2-C in vitro. Dominant-negative clam E2-C and human UbcH10 proteins, created by altering the catalytic cysteine to serine, inhibit the in vitro ubiquitination and destruction of cyclin B in clam oocyte extracts. When transfected into mammalian cells, mutant UbcH10 inhibits the destruction of both cyclin A and B, arrests cells in M phase, and inhibits the onset of anaphase, presumably by blocking the ubiquitin-dependent proteolysis of proteins responsible for sister chromatid separation. Thus, E2-C͞UbcH10-mediated ubiquitination is involved in both cdc2 inactivation and sister chromatid separation, processes that are normally coordinated during exit from mitosis.