Cell Biology

... from active transport to vesicles

nuclear membrane

The nuclear membrane encloses the nucleus in eukaryotes. The membrane is penetrated by nuclear pore complexes.

The nuclear membrane (nuclear envelope, nucleolemma) isolates the interior of the nucleus from the cytoplasm. The membrane has two layers enclosing a distinct inner lumen (em). The nuclear membrane is penetrated by large nuclear pore complexes, which selectively transport molecules into or out of the nucleus. The luminal space within the double-layered nuclear membrane is continuous at points with the endoplasmic reticulum, a membrane-enclosed organelle (continuous with the outer nuclear membrane) to which are attached ribosomes (em2) executing translation of genetic coding into polypeptides and proteins.

Type V intermediate filaments are lamin proteins that have a nuclear signal sequence and form a filamentous support called the nuclear lamina just beneath the inner nuclear membrane. The lamina a net-like meshwork array that lines the inner nuclear membrane and governs the shape of the nucleus. Of the three nuclear lamin proteins, two are alternatively spliced products encoded by a single gene, while the third lamin is encoded by a separate gene. Nuclear lamins form a fibrous network that supports the nuclear membrane.

Lamins have a very long rod domain and carry a nuclear transport signal, they are located in the nucleus just beneath the nuclear envelope so they are vital to the re-assembly of the nuclear envelope after cell division. Lamins are phosphorylated at the end of prophase and this causes them to disassemble simultaneous with dissolution of the nuclear envelope. After cell division, they are dephosphorylated just before the nuclei of the daughter cells form and lamin filaments reassemble around each set of chromosomes. They are continuous except for a break at the sites of nuclear pore complexes. Lamins were probably the first intermediate filaments to evolve. Controlled by cdc2 kinase, lamin filaments disassemble prior to or early in mitosis then reassemble after cell division. Phosphorylation of serine residues in the N-terminal domain of lamin A by cdc2 kinase induces the disassembly of intact filaments and prevents reassembly.

Nuclear lamins help attach the chromosomes to the nuclear membrane and provide anchorage points for the nuclear pores. It is believed that nuclear lamins are the evolutionary ancestor of cytoplasmic intermediate filaments, which evolved through duplication and translocation of the gene product to the cytoplasm.

Џ beautiful Flash 8 animation - Inner Life of the Cell, which shows nuclear membrane with nuclear pore complexes, and Interpretation: Inner Life of the Cell Џ

• A • adhesion • C • cell membranescellular adhesion moleculescellular signal transductioncentrioleschemotaxischloroplastcilia & flagellacommunicationconcentration gradientscytokine receptorscytoplasmcytoskeleton • E • energy transducersendoplasmic reticulumendosomesexosome • F • flagella & cilia • G • Golgi apparatusGPCRs • H • hormones • I • ion channels • L • lysosome • M • meiosismicrotubulesmitosismitochondrion • N • Nitric Oxideneurotransmissionneuronal interconnectionsnuclear membranenuclear pore • P • pinocytosisproteasomepumps • R • receptor proteinsreceptor-mediated endocytosis • S • second messengerssignaling gradientssignal transductionspindlestructure • T • transporttwo-component systems • V • vacuolevesicle

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