No abstract

Circumferential actin belts underlying the adherens junctions of columnar epithelial cell monolayers control intercellular surface tension and cell shape to maintain tissue integrity. Yes-associated protein (YAP) and its paralog TAZ are proliferation-activating transcriptional coactivators that shuttle between the nucleus and cytoplasm. Previous studies suggest the importance of stress fibers in the actin cytoskeleton for regulation of YAP nuclear localization; however, the role of the circumferential actin belt on YAP localization remains unclarified. By manipulating actin tension, we demonstrate that circumferential actin belt tension suppresses YAP/TAZ nuclear localization. This suppression requires Merlin, an F-actin binding protein associated with adherens junctions. Merlin physically interacts with YAP/TAZ, and nuclear export sequences of Merlin are required for suppression. Together, with the observation that the association between E-cadherin and Merlin was diminished by tension in circumferential actin belts, our results suggest that released Merlin undergoes nucleocytoplasmic shutting and mediates export of YAP/TAZ from the nucleus.

SummaryThe establishment of epithelial polarity is tightly linked to the dramatic reorganization of microtubules (MTs) from a radial array to a vertical alignment of non-centrosomal MT bundles along the lateral membrane, and a meshwork under the apical and basal membranes. However, little is known about the underlying molecular mechanism of this polarity-dependent MT remodeling. The evolutionarily conserved cell polarity-regulating kinase PAR-1 (known as MARK in mammals), whose activity is essential for maintaining the dynamic state of MTs, has indispensable roles in promoting this process. Here, we identify a novel PAR-1-binding protein, which we call microtubule crosslinking factor 1 (MTCL1), that crosslinks MTs through its N-terminal MT-binding region and subsequent coiledcoil motifs. MTCL1 colocalized with the apicobasal MT bundles in epithelial cells, and its knockdown impaired the development of these MT bundles and the epithelial-cell-specific columnar shape. Rescue experiments revealed that the N-terminal MT-binding region was indispensable for restoring these defects of the knockdown cells. MT regrowth assays indicated that MTCL1 was not required for the initial radial growth of MTs from the apical centrosome but was essential for the accumulation of non-centrosomal MTs to the sublateral regions. Interestingly, MTCL1 recruited a subpopulation of PAR-1b (known as MARK2 in mammals) to the apicobasal MT bundles, and its interaction with PAR-1b was required for MTCL1-dependent development of the apicobasal MT bundles. These results suggest that MTCL1 mediates the epithelial-cell-specific reorganization of non-centrosomal MTs through its MT-crosslinking activity, and cooperates with PAR-1b to maintain the correct temporal balance between dynamic and stable MTs within the apicobasal MT bundles.

The Spalt ( sal ) gene family is conserved from Drosophila to humans. Mutations of human SALL1 cause Townes-Brocks syndrome, with features of ear, limb, anal, renal and heart anomalies. Sall1 , a murine homolog of SALL1 , is essential for kidney formation, and both Sall1 and SALL1 localize to heterochromatin in the nucleus. Here, we present a molecular mechanism for the heterochromatin localization of Sall1. Mutation analyses revealed that the 7th− − − − 10th C-terminal double zinc finger motifs were required for the localization. A recombinant protein of the most C-terminal double zinc finger (9th− − − − 10th) bound to specific A/T-rich sequences. Furthermore, Sall1 associated with A/T-rich sequences of the major satellite DNA in heterochromatin. Thus Sall1 may bind to A / Trich sequences of the major satellite DNA via its C-terminal double zinc fingers, thereby mediating its localization to heterochromatin.