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.

The adenomatous polyposis coli (APC) protein is inactivated in most colorectal tumours. APC loss is an early event in tumorigenesis, and causes an increase of nuclear b-catenin and its transcriptional activity. This is thought to be the driving force for tumour progression. APC shuttles in and out of the nucleus, but the functional signi®cance of this has been controversial. Here, we show that APC truncations are nuclear in colorectal cancer cells and adenocarcinomas, and this correlates with loss of centrally located nuclear export signals. These signals confer ef®cient nuclear export as measured directly by¯uorescence loss in photobleaching (FLIP), and they are critical for the function of APC in reducing the transcriptional activity of b-catenin in complementation assays of APC mutant colorectal cancer cells. Importantly, targeting a functional APC construct to the nucleus causes a striking nuclear accumulation of b-catenin without changing its transcriptional activity. Our evidence indicates that the rate of nuclear export of APC, rather than its nuclear import or steady-state levels, determines the transcriptional activity of b-catenin.

The NAP motif of activity-dependent neuroprotective protein (ADNP) enhanced memory scores in patients suffering from mild cognitive impairment and protected activities of daily living in schizophrenia patients, while fortifying microtubule (MT)-dependent axonal transport, in mice and flies. The question is how does NAP fortify MTs? Our sequence analysis identified the MT end-binding protein (EB1)-interacting motif SxIP (SIP, Ser-Ile-Pro) in ADNP/NAP and showed specific SxIP binding sites in all members of the EB protein family (EB1-3). Others found that EB1 enhancement of neurite outgrowth is attenuated by EB2, while EB3 interacts with postsynaptic density protein 95 (PSD-95) to modulate dendritic plasticity. Here, NAP increased PSD-95 expression in dendritic spines, which was inhibited by EB3 silencing. EB1 or EB3, but not EB2 silencing inhibited NAP-mediated cell protection, which reflected NAP binding specificity. NAPVSKIPQ (SxIP=SKIP), but not NAPVAAAAQ mimicked NAP activity. ADNP, essential for neuronal differentiation and brain formation in mouse, a member of the SWI/SNF chromatin remodeling complex and a major protein mutated in autism and deregulated in schizophrenia in men, showed similar EB interactions, which were enhanced by NAP treatment. The newly identified shared MT target of NAP/ADNP is directly implicated in synaptic plasticity, explaining the breadth and efficiency of neuroprotective/neurotrophic capacities.

Adenomatous polyposis coli (APC) is mutated in most colorectal cancers. APC downregulates nuclear β‐catenin, which is thought to be critical for its tumour suppressor function. However, APC may have additional and separate functions at the cell periphery. Here, we examine polarized MDCK and WIF‐B hepatoma cells and find that APC is associated with their lateral plasma membranes. This depends on the actin cytoskeleton but not on microtubules, and drug wash‐out experiments suggest that APC is delivered continuously to the plasma membrane by a dynamic actin‐dependent process. In polarized MDCK cells, APC also clusters at microtubule tips in their basal‐most regions. Microtubule depolymerization causes APC to relocalize from these tips to the plasma membrane, indicating two distinct peripheral APC pools that are in equilibrium with each other in these cells. Truncations of APC such as those found in APC mutant cancer cells can neither associate with the plasma membrane nor with microtubule tips. The ability of APC to reach the cell periphery may thus contribute to its tumour suppressor function in the intestinal epithelium.