Where is APC going?

Adenomatous polyposis coli (APC) protein has been thought to function as a tumor suppressor through its involvement in the Wnt/β-catenin signaling pathway. However, its connections to the cytoskeleton and microtubules in particular are becoming apparent, and the discovery of these new functions for APC is leading to a reevaluation of its role not only in tumorigenesis, but also in normal physiology

Requirement of ZO-1 for the formation of belt-like adherens junctions during epithelial cell polarization

The molecular mechanisms of how primordial adherens junctions (AJs) evolve into spatially separated belt-like AJs and tight junctions (TJs) during epithelial polarization are not well understood. Previously, we reported the establishment of ZO-1/ZO-2–deficient cultured epithelial cells (1[ko]/2[kd] cells), which lacked TJs completely. In the present study, we found that the formation of belt-like AJs was significantly delayed in 1(ko)/2(kd) cells during epithelial polarization. The activation of Rac1 upon primordial AJ formation is severely impaired in 1(ko)/2(kd) cells. Our data indicate that ZO-1 plays crucial roles not only in TJ formation, but also in the conversion from “fibroblastic” AJs to belt-like “polarized epithelial” AJs through Rac1 activation. Furthermore, to examine whether ZO-1 itself mediate belt-like AJ and TJ formation, respectively, we performed a mutational analysis of ZO-1. The requirement for ZO-1 differs between belt-like AJ and TJ formation. We propose that ZO-1 is directly involved in the establishment of two distinct junctional domains, belt-like AJs and TJs, during epithelial polarization

The dynamic behavior of the APC-binding protein EB1 on the distal ends of microtubules

AbstractAdenomatous polyposis coli protein (APC) is a well-characterized tumor suppressor protein [1–3]. We previously showed that APC tagged with green fluorescent protein (GFP) in Xenopus A6 epithelial cells moves along a subset of microtubules and accumulates at their growing plus ends in cell extensions [4]. EB1, which was identified as an APC-binding protein by yeast two-hybrid analysis [5], was also reported to be associated with microtubules [6–8]. To examine the interaction between APC and EB1 within cells, we compared the dynamic behavior of EB1–GFP with that of APC–GFP in A6 transfectants. Time-lapse microscopy of live cells at interphase revealed that EB1–GFP was concentrated at all of the growing microtubule ends throughout the cytoplasm and abruptly disappeared from the ends when microtubules began to shorten. Therefore, EB1 appeared to be co-localized and interact with APC on the growing ends of a subset of microtubules. When APC–GFP was overexpressed, endogenous EB1 was recruited to APC–GFP, which accumulated in large amounts on microtubules. On the other hand, when microtubules were disassembled by nocodazole, EB1 was not co-localized with APC–GFP, which was concentrated along the basal plasma membrane. During mitosis, APC appeared to be dissociated from microtubules, whereas EB1–GFP continued to concentrate at microtubule growing ends. These findings showed that the APC–EB1 interaction is regulated within cells and is allowed near the ends of microtubules only under restricted conditions

Tricellulin constitutes a novel barrier at tricellular contacts of epithelial cells

For epithelia to function as barriers, the intercellular space must be sealed. Sealing two adjacent cells at bicellular tight junctions (bTJs) is well described with the discovery of the claudins. Yet, there are still barrier weak points at tricellular contacts, where three cells join together. In this study, we identify tricellulin, the first integral membrane protein that is concentrated at the vertically oriented TJ strands of tricellular contacts. When tricellulin expression was suppressed with RNA interference, the epithelial barrier was compromised, and tricellular contacts and bTJs were disorganized. These findings indicate the critical function of tricellulin for formation of the epithelial barrier