nucleo-cytoplasmic transport

PABPN1

required to enhance PAP activity, producing poly(A) tail

nuclear pore complex

• 125 megadaltons, 30x bigger than a ribosome

• composed of ~50 (in yeast) to ~100 (in vertebrates) different proteins

• molecules up to ~40-60 kDa can freely diffuse through

• cytoplasmic filaments extend into the cytoplasm

• nuclear basket extends into the nucleoplasm

• proximal filaments present on both sides of nuclear envelope

transport through the nuclear pore complex

FG nucleoporins are hydrophobic and possess disordered domains that impedes movement of large molecules but allowing small molecules to diffuse through

protein import through the nuclear pore complex

nuclear proteins synthesized in the cytoplasm and imported through NPCs contain a nuclear localization signal

nuclear localization signal

• recognized by transporter proteins

• rich in lysines and arginines

Ran

monomeric G protein that exists in GTP- and GDP-bound states mediated by GEFs (GDP → GTP) and GAPs (GTP → GDP)

nuclear transport receptors (importins)

proteins that bind to nuclear localization signal domains present on cargo proteins to facilitate transport through the pore by associating with FG repeats on the nucleoporins

mechanism for nuclear import of NLS-containing cargo proteins

• mediated by concentration gradients

• high concentration of import complex (importin + cargo) in cytoplasm

• high concentration of RanGTP/importin complex in nucleoplasm

  • RAN-GEF is tethered to chromosomes, maintaining gradient

exportin t

• functions to export tRNAs

• binds fully processed tRNAs and Ran-GTP and passes through NPCs

• the complex dissociates in the cytosol when it interacts with Ran-GAP

export of ribosomal subunits (rRNA)

requires Ran

export of mRNAs

• some specific mRNAs that associate with particular hnRNP proteins can be exported through associated with Ran

• most are exported in a Ran-independent process using an mRNA exporter

• only fully spliced mature mRNAs get exported

model of mRNP export through NPCs

• consists of two subunits (NXF1 and NXT1)

• together they bind RNA cooperatively with specific mRNP proteins including the SR proteins

• form a domain that interacts with FG repeats in nucleoporins

• since ribosomes associate with the mRNA while it’s being exported, the 5’ end must be exported first

Balbiani rings

insect polytene chromosomes that provide a system where transcription and mRNP export can be microscopically imaged, mRNP becomes croissant-shaped

mechanism of mRNP export

1. mRNA associated with nuclear proteins (PABPN1, REF, NXF1, NXT1, CBC) enters nuclear pore complex

2. cytoplasmic remodelling

   1. RNA helicase in NPC removes nuclear proteins

   2. cytoplasmic proteins (eIF4E, PABPC1) associate with mRNA once it’s in the cytoplasm

3. nuclear proteins return to nucleus through NPC