Tubulohelical membrane arrays: From the initial observation to the elucidation of nanophysical properties and cellular function

Lipids undergo self-assembly to form ordered nonlamellar, nanoperiodic arrays both in vitro and in vivo. While engineering of such membrane arrays for technical devices is envisaged, we know little about their cellular function. Do they represent building blocks of an inherent cellular nanotechnology? Prospects for answering this question could be improved if the nanophysical properties of the membrane arrays could be studied in the context of specific cellular functions. Therefore, we draw attention to exceptional complex membrane arrays found in the renal epithelial cell line PtK2 that could provide perfect conditions for both biophysical and cell functional studies. The so-called tubulohelical membrane arrays (TUHMAs) combine nanoperiodicity of lipid membranes with that of helix-like proteinaceous core structures. Strikingly, they show several characteristics of dynamic, microtubule-associated single organelles. Our initial data indicate that TUHMA formation occurs in the depth of the cytoplasm under participation of cytoplasmic nucleoporins. Once matured, they may fuse with the nuclear membrane in polarized positions, either perpendicularly or in parallel to the nucleus. As a starting point for the initiation of functional studies we found a connection between TUHMAs and primary cilia, indicated by immunolabeling patterns of detyrosynated tubulin and cytoplasmic nucleoporins. We discuss these observations in the context of the ciliary cycle and of the specific requirement of ciliated renal epithelial cells for oriented cell division. Finally, we raise the question of whether putative nanooptical properties of TUHMAs could serve for communicating orientation between dividing cells

Nuclear Distributions of NUP62 and NUP214 Suggest Architectural Diversity and Spatial Patterning among Nuclear Pore Complexes

The shape of nuclei in many adherent cultured cells approximates an oblate ellipsoid, with contralateral flattened surfaces facing the culture plate or the medium. Observations of cultured cell nuclei from orthogonal perspectives revealed that nucleoporin p62 (NUP62) and nucleoporin 214 (NUP214) are differentially distributed between nuclear pore complexes on the flattened surfaces and peripheral rim of the nucleus. High resolution stimulated emission depletion (STED) immunofluorescence microscopy resolved individual NPCs, and suggested both heterogeneity and microheterogeneity in NUP62 and NUP214 immunolabeling among in NPC populations. Similar to nuclear domains and interphase chromosome territories, architectural diversity and spatial patterning of NPCs may be an intrinsic property of the nucleus that is linked to the functions and organization of underlying chromatin

Phosphorylation of nucleoporins: Signal transduction-mediated regulation of their interaction with nuclear transport receptors

The nuclear pore complex (NPC) is composed of ∼30 unique proteins, collectively referred to as nucleoporins or Nups. While metazoan Nups are known to be phosphorylated during mitosis to cause disassembly of the NPC, what is less clear is whether Nups are phosphorylated and regulated by extracellular stimuli in interphase cells. Our multi-step phosphoproteomic approach revealed a number of physiologically relevant extracellular signal-regulated kinase (ERK) targets, including Nups containing FG repeats (FG Nups) that provide binding sites for nuclear transport receptors (NTRs) during the NPC passage. The phosphorylation of FG Nups by ERK does not affect the overall architecture of the NPC but directly inhibits their interactions with NTRs and regulates the permeability barrier properties of the NPC. Such regulation at the levels of transport machinery is expected to have a broad impact on cellular physiology through the spatiotemporal control of signaling events. Until recently, many studies have focused on cellular signaling-mediated phosphorylation of individual cargo proteins, such as transcription factors. An understanding of the effects of signaling pathways on nucleocytoplasmic transport machinery is only beginning to emerge

Temporal control of nuclear envelope assembly by phosphorylation of lamin B receptor

The metazoan nuclear envelope breaks down in early mitosis and does not reform until late anaphase. Phosphorylation of lamin B receptor by Cdk1 not only prevents premature nuclear envelope assembly, but also facilitates complete dissociation of the nuclear envelope from the chromatin during nuclear envelope breakdown