Mitochondria are essential eukaryotic organelles often forming intricate networks. The overall network morphology is determined by mitochondrial fusion and fission. Among the multiple mechanisms that appear to regulate mitochondrial fission, the ER and actin have recently been shown to play an important role by mediating mitochondrial constriction and promoting the action of a key fission factor, the dynamin‐like protein Drp1. Here, we report that the cytoskeletal component septin 2 is involved in Drp1‐dependent mitochondrial fission in mammalian cells. Septin 2 localizes to a subset of mitochondrial constrictions and directly binds Drp1, as shown by immunoprecipitation of the endogenous proteins and by pulldown assays with recombinant proteins. Depletion of septin 2 reduces Drp1 recruitment to mitochondria and results in hyperfused mitochondria and delayed FCCP‐induced fission. Strikingly, septin depletion also affects mitochondrial morphology in Caenorhabditis elegans, strongly suggesting that the role of septins in mitochondrial dynamics is evolutionarily conserved.

We have identified two redundant GTPase activating proteins (GAPs) -RGA-3 and RGA-4 -that regulate Rho GTPase function at the plasma membrane in early Caenorhabditis elegans embryos. Knockdown of both RhoGAPs resulted in extensive membrane ruffling, furrowing and pronounced pseudo-cleavages. In addition, the non-muscle myosin NMY-2 and RHO-1 accumulated on the cortex at sites of ruffling. RGA-3 and RGA-4 are GAPs for RHO-1, but most probably not for CDC-42, because only RHO-1 was epistatic to the two GAPs, and the GAPs had no obvious influence on CDC-42 function. Furthermore, knockdown of either the RHO-1 effector, LET-502, or the exchange factor for RHO-1, ECT-2, alleviated the membrane-ruffling phenotype caused by simultaneous knockdown of both RGA-3 and RGA-4 [rga-3/4 (RNAi)]. GFP::PAR-6 and GFP::PAR-2 were localized at the anterior and posterior part of the early C. elegans embryo, respectively showing that rga-3/4 (RNAi) did not interfere with polarity establishment. Most importantly, upon simultaneous knockdown of RGA-3, RGA-4 and the third RhoGAP present in the early embryo, CYK-4, NMY-2 spread over the entire cortex and GFP::PAR-2 localization at the posterior cortex was greatly diminished. These results indicate that the functions of CYK-4 are temporally and spatially distinct from RGA-3 and RGA-4 (RGA-3/4). RGA-3/4 and CYK-4 also play different roles in controlling LET-502 activation in the germ line, because rga-3/4 (RNAi), but not cyk-4 (RNAi), aggravated the let-502(sb106) phenotype. We propose that RGA-3/4 and CYK-4 control with which effector molecules RHO-1 interacts at particular sites at the cortex in the zygote and in the germ line.

N-glycosylation of proteins is an essential process, and N-glucans serve as important beacons in protein folding and ER associated degradation. More importantly, N-glycosylation increases the structural repertoire of proteins because the addition of the N-glucan on proteins will serve as a base for further sugar additions in the Golgi apparatus, and hence complex three-dimensional structures can be build. N-glycosylation is mediated by the ER-resident OST complex, which is essential throughout eukaryotes. Partial knockdown of conserved OST complex members, such as C. elegans RIBO-1, led to an embryonic lethal phenotype. Although the ER morphology was not grossly altered in ribo-1(RNAi) oocytes and embryos, secretion of yolk and of the yolk receptor RME-2 was perturbed in those worms. Perhaps as a consequence of reduced arrival of N-glycosylated proteins at the plasma membrane, cytokinesis occurred less efficiently leading to multinuclear cells. Unexpectedly, we detected a chromosome segregation defect in ribo-1(RNAi) embryos suggesting an essential role of at least one N-glycosylated protein in metaphase-anaphase transition.

The mammalian serum- and glucocorticoid-inducible kinase SGK1 regulates the endocytosis of ion channels. Here we report that in C. elegans sgk-1 null mutants, GFP-tagged MIG-14/Wntless, the sorting receptor of Wnt, failed to localize to the basolateral membrane of intestinal cells; instead, it was mis-sorted to lysosomes. This effect can be explained in part by altered sphingolipid levels, because reducing glucosylceramide biosynthesis restored the localization of MIG-14::GFP. Membrane traffic was not perturbed in general, as no obvious morphological defects were detected for early endosomes, the Golgi apparatus, and the endoplasmic reticulum (ER) in sgk-1 null animals. The recycling of MIG-14/Wntless through the Golgi might be partially responsible for the observed phenotype because the subcellular distribution of two plasma membrane cargoes that do not recycle through the trans-Golgi network (TGN) was affected to a lesser degree. Consistently, knockdown of the ArfGEF gbf-1 altered the distribution of SGK-1 at the basolateral membrane of intestinal cells. In addition, we found that sgk-1(RNAi) induced unfolded protein response in the ER, suggesting at least an indirect role of SGK-1 early in the secretory pathway. We propose that SGK-1 function is required for lipid homeostasis and that it acts at different intracellular trafficking steps.

The Caenorhabditis elegans early embryo is highly polarized, requiring sequestration of cytoplasmic polarity factors at the plasma membrane. This compartmentalization aids asymmetric distribution of lipids and proteins, which is partially responsible for the fates of the daughter cells. Since most plasma membrane proteins are glycosylated, we determined the effect of attenuation of N-glycosylation on cell polarity. While polarity establishment was not perturbed, the size difference between the two cells formed in first cell division (AB and P1) was more variable in embryos with reduced N-glycosylation than in the mocktreated embryos. In addition, among other deficiencies, we observed spindle orientation defects in two-cell embryos. Moreover, cell-cell adhesion was specifically lost at the two-cell stage when N-glycosylation was reduced. This loss-of-adhesion phenotype was rescued by interfering with polarity establishment, indicating that polarity establishment enforces plasma membrane compartmentalization. Consistent with this idea, the decreased plasma membrane levels of the adhesion proteins E-cadherin and MAGI-1 in ribo-1(RNAi) embryos were restored in the absence of functional PAR-2. Our data suggest a general role for N-glycosylation in plasma membrane compartmentalization and cell polarity.