Dynamic and reversible restructuring of the ER induced by PDMP in cultured cells

In many cells, the endoplasmic reticulum (ER) contains segregated smooth and rough domains, but the mechanism of this segregation is unclear. Here, we used a HeLa cell line, inducibly expressing a GFP fusion protein [GFP-b(5)tail] anchored to the ER membrane, as a tool to investigate factors influencing ER organisation. Induction of GFP-b(5)tail expression caused proliferation of the ER, but its normal branching polygonal meshwork architecture was maintained. Experiments designed to test the effects of drugs that alter ceramide levels revealed that treatment of these cells with Phenyl-2-decanoyl-amino-3-morpholino-1-propanol-hydrocholride (PDMP) generated patches of segregated smooth ER, organised as a random tubular network, which rapidly dispersed after removal of the drug. The effect of PDMP was independent of its activity as sphingolipid synthesis inhibitor, but could be partially reversed by a membrane-permeant Ca(2+) chelator. Although the smooth ER patches maintained connectivity with the remaining ER, they appeared to represent distinct domains differing in protein and lipid composition from the remaining ER. PDMP did not cause detachment of membrane-bound ribosomes, indicating that smooth ER patch generation was due to a reorganisation of pre-existing ribosome-free areas. Our results demonstrate a dynamic relationship between smooth and rough ER and have implications for the mechanisms regulating ER architecture.

N-myristoylation determines dual targeting of mammalian NADH-cytochrome b5 reductase to ER and mitochondrial outer membranes by a mechanism of kinetic partitioning

Mammalian NADH-cytochrome b(5) reductase (b5R) is an N-myristoylated protein that is dually targeted to ER and mitochondrial outer membranes. The N-linked myristate is not required for anchorage to membranes because a stretch of hydrophobic amino acids close to the NH₂ terminus guarantees a tight interaction of the protein with the phospholipid bilayer. Instead, the fatty acid is required for targeting of b5R to mitochondria because a nonmyristoylated mutant is exclusively localized to the ER. Here, we have investigated the mechanism by which N-linked myristate affects b5R targeting. We find that myristoylation interferes with interaction of the nascent chain with signal recognition particle, so that a portion of the nascent chains escapes from cotranslational integration into the ER and can be post-translationally targeted to the mitochondrial outer membrane. Thus, competition between two cotranslational events, binding of signal recognition particle and modification by N-myristoylation, determines the site of translation and the localization of b5R.

Trafficking of tail-anchored proteins: transport from the endoplasmic reticulum to the plasma membrane and sorting between surface domains in polarised epithelial cells

Tail-anchored (TA) proteins, which are defined by an N-terminal cytosolic region and a C-terminal transmembrane domain (TMD), provide useful models for studying the role of the TMD in sorting within the exo-endocytic system. Previous work has shown that a short TMD is required to keep ER-resident TA proteins from escaping to downstream compartments of the secretory pathway. To investigate the role of the TMD in TA protein sorting, we used model constructs, which consisted of GFP linked at its C-terminus to the tail region of cytochrome b(5) with TMDs of differing length or hydrophobicity. Expression of these constructs in CV-1 cells demonstrated that the feature determining exit from the ER is hydrophobicity and that if exit occurs, at least a part of the protein reaches the cell surface. To investigate which pathway to the surface is followed by plasma-membrane-directed TA constructs, we expressed the TA constructs in polarised Madin Darby Canine Kidney (MDCK) cells. The constructs with 22 and 25 residue TMDs were localised basolaterally, but addition at the C-terminus of a 20-residue peptide containing an N-glycosylation site resulted in glycosylation-dependent relocation of∼50% of the protein to the apical surface. This result suggests that TA proteins may reach the basolateral surface without a signal or that our constructs contain a weak basolateral determinant that is recessive to the apical information carried by the glycan. To assess the effect of the TMDs of endogenous TA proteins, GFP was linked to the tails of syntaxin 3 and 4, which localise to the apical and basolateral surface, respectively, of MDCK cells. The two GFP fusion proteins showed a different surface distribution, which is consistent with a role for the two syntaxin TMDs in polarised sorting.

Expression of KIF3C kinesin during neural development and in vitro neuronal differentiation

KIF3A, KIF3B and KIF3C are kinesin-related motor subunits of the KIF3 family that associate to form the kinesin-II motor complex in which KIF3C and KIF3B are alternative partners of KIF3A. We have analysed the expression of Kif3 mRNAs during prenatal murine development. Kif3c transcripts are detectable from embryonic day 12.5 and persist throughout development both in the CNS and in some peripheral ganglia. Comparison of the expression patterns of the Kif3 genes revealed that Kif3c and Kif3a mRNAs colocalize in the CNS, while only Kif3a is also present outside the CNS. In contrast, Kif3b is detectable in several non-neural tissues. We have also performed immunocytochemical analyses of the developing rat brain and have found the presence of the KIF3C protein in selected brain regions and in several fibre systems. Using neuroblastoma cells as an in vitro model for neuronal differentiation, we found that retinoic acid stimulated the expression of the three Kif3 and the kinesin-associated protein genes, although with different time courses. The selective expression of Kif3c in the nervous system during embryonic development and its up-regulation during neuroblastoma differentiation suggest a role for this motor during maturation of neuronal cells.