In animal cells, the endoplasmic reticulum (ER) forms a lacelike network throughout the cytoplasm; in addition, a distinct domain of the ER is used to surround the nuclear material to form the nuclear envelope. ER networks can be produced in cell-free systems, and their formation requires the activity of a pair of ER proteins, Rtn4a and DP1/NogoA. Audhya et al. further examined the process of ER network formation in vitro and studied ER dynamics within living cells. In embryos of the nematode Caenorhabditis elegans, depletion of the homologs of Rtn4a and DP1/NogoA--RET-1 and YOP-1--produced cellular defects in the peripheral ER network. Furthermore, a member of the small guanosine triphosphatase Rab family, RAB-5, was required for ER network formation in vitro, and in nematode embryos, reduction of RAB-5 also caused peripheral ER defects. RAB-5, RET-1, and YOP-1 were also important in the kinetic control of nuclear envelope disassembly at the beginning of mitosis, as depleting them resulted in the generation of daughter cells with atypical double nuclei. Previously, Rab5 has been shown to be important in the regulation of membrane trafficking during endocytosis. Its effects on ER morphology appear to be independent of these functions, because other perturbations that directly affect endocytosis did not lead to similar defects in ER morphology or to nuclear envelope breakdown during mitosis. -- SMH
J. Cell Biol. 178, 43 (2007).
