-separates the contents of the nucleus from the cytoplasm
-two phospholipid bilayer membranes
-underlying nuclear lamina and nuclear pore complexes
-outer membrane continuous with the E.R.
Nuclear Pore Complexes
-allow selective traffic of proteins and RNAs through the nuclear envelope
-critical role in regulating gene expression, RNA processing and ribosome assembly
-eight fold symmetry
-30 times the size of a ribosome, needed because ribosomes are made in the nucleus and need to be transported to the cytoplasm
-composed of 30-50 nucleoporins
-very high transport rate
Structure of Nuclear Pore (pic)
Traffic through Nuclear Pore
-small hydrophobic molecules pass freely across pore
-two mechanisms for polar molecule passing
-passive-molecules less than 20-40 kDa
-energy dependent- all larger molecules (most proteins and RNA)
NLS
-nuclear localization signals
-specific AA that are recognized by transport receptors and direct the transport of proteins from cytoplasm into nucleus
Nuclear Transport Receptors
-proteins that recognize NLS and mediate transport across nuclear envelope
Karyopherins
-family of nuclear transport receptors
-importins are karyopherins that direct nuclear import
System for Protein Import into Nucleus
-specific importin binds to NLS (nuclear localization signal) of a protein in cytoplasm to be transported
-importin then crosses nuclear pore while carrying cargo protein
-importin/protein complex is acted on by active Ran complex that splits off protein and actively transports importin complex back out of nucleus (costs GTP)
Transport of Proteins from Nucleus
-use NES (nuclear export signals-specific AA sequences that target proteins for export from the nucleus)
-NES interact with exportins (also members of the karyopherin family)