TIAM-1/GEF can shape somatosensory dendrites independently of its GEF activity by regulating F-actin localization

Dendritic arbors are crucial for nervous system assembly, but the intracellular mechanisms that govern their assembly remain incompletely understood. Here, we show that the dendrites of PVD neurons in Caenorhabditis elegans are patterned by distinct pathways downstream of the DMA-1 leucine-rich transmembrane (LRR-TM) receptor. DMA-1/LRR-TM interacts through a PDZ ligand motif with the guanine nucleotide exchange factor TIAM-1/GEF in a complex with act-4/Actin to pattern higher order 4° dendrite branches by localizing F-actin to the distal ends of developing dendrites. Surprisingly, TIAM-1/GEF appears to function independently of Rac1 guanine nucleotide exchange factor activity. A partially redundant pathway, dependent on HPO-30/Claudin, regulates formation of 2° and 3° branches, possibly by regulating membrane localization and trafficking of DMA-1/LRR-TM. Collectively, our experiments suggest that HPO-30/Claudin localizes the DMA-1/LRR-TM receptor on PVD dendrites, which in turn can control dendrite patterning by directly modulating F-actin dynamics through TIAM-1/GEF.

Esc1, a Nuclear Periphery Protein Required for Sir4-Based Plasmid Anchoring and Partitioning

ABSTRACT A targeted silencing screen was performed to identify yeast proteins that, when tethered to a telomere, suppress a telomeric silencing defect caused by truncation of Rap1. A previously uncharacterized protein, Esc1 (establishes silent chromatin), was recovered, in addition to well-characterized proteins Rap1, Sir1, and Rad7. Telomeric silencing was slightly decreased in Δesc1 mutants, but silencing of the HM loci was unaffected. On the other hand, targeted silencing by various tethered proteins was greatly weakened in Δesc1 mutants. Two-hybrid analysis revealed that Esc1 and Sir4 interact via a 34-amino-acid portion of Esc1 (residues 1440 to 1473) and a carboxyl-terminal domain of Sir4 known as PAD4 (residues 950 to 1262). When tethered to DNA, this Sir4 domain confers efficient partitioning to otherwise unstable plasmids and blocks the ability of bound DNA segments to rotate freely in vivo. Here, both phenomena were shown to require ESC1. Sir protein-mediated partitioning of a telomere-based plasmid also required ESC1. Fluorescence microscopy of cells expressing green fluorescent protein (GFP)-Esc1 showed that the protein localized to the nuclear periphery, a region of the nucleus known to be functionally important for silencing. GFP-Esc1 localization, however, was not entirely coincident with telomeres, the nucleolus, or nuclear pore complexes. Our data suggest that Esc1 is a component of a redundant pathway that functions to localize silencing complexes to the nuclear periphery.

Activation of Hypodermal Differentiation in theCaenorhabditis elegans Embryo by GATA Transcription Factors ELT-1 and ELT-3

ABSTRACT The Caenorhabditis elegans GATA transcription factor genes elt-1 and elt-3 are expressed in the embryonic hypodermis (also called the epidermis). elt-1 is expressed in precursor cells and is essential for the production of most hypodermal cells (22). elt-3 is expressed in all of the major hypodermal cells except the lateral seam cells, and expression is initiated immediately after the terminal division of precursor lineages (13). Although this expression pattern suggests a role for ELT-3 in hypodermal development, no functional studies have yet been performed. In the present paper, we show that either elt-3 or elt-1 is sufficient, when force expressed in early embryonic blastomeres, to activate a program of hypodermal differentiation even in blastomeres that are not hypodermal precursors in wild-type embryos. We have deleted the elt-3gene and shown that ELT-3 is not essential for either hypodermal cell differentiation or the viability of the organism. We showed that ELT-3 can activate hypodermal gene expression in the absence of ELT-1 and that, conversely, ELT-1 can activate hypodermal gene expression in the absence of ELT-3. Overall, the combined results of the mutant phenotypes, initial expression times, and our forced-expression experiments suggest that ELT-3 acts downstream of ELT-1 in a redundant pathway controlling hypodermal cell differentiation.