BIOL 200 Lecture Notes - Polytene Chromosome, Karyopherin, Helicase

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6 Apr 2012

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Naveen Sooknanan McGill Fall 2011
Nucleo-cytoplasmic Transport:
So far, all the mechanisms we have seen on DNA and RNA have taken place in the nucleus.
Protein synthesis, however, takes place in the cytoplasm. So, these final mRNAs must somehow
be transferred from the nucleus to the cytoplasm and then associate with a bunch of new proteins
associated with translation.
Nuclear pores are complexes which resemble baskets and are located all over the nuclear
membrane, which has a bilayer membrane
They make the nucleus look like a golf ball structure because of the perforations
Nuclear pore complexes are the sites of nucleo-cytoplasmic transport
o This process is a two way road; some proteins synthesized in the cytoplasm need
to be brought into the nucleus, while RNAs such as
mRNA need to be brought out of the nucleus
The nuclear pore complex is a highly organized structure made
of many protein subunits
o They contain nucleoporins with Fg repeat proteins, which
are zones of increased hydrophobicity
Much of the transport through the nuclear membrane is mediated by these
Fg repeats
The NPC itself is huge, over 125 megadaltons in size (30 times bigger than a ribosome)
o It is composed of many proteins; 50 in yeast and 100 in humans
Molecules up to 60 kDa can diffuse freely through the NPC, but larger complexes require
multimolecular transporters (RNPs) to be actively transported
It is by the interaction between various transporter proteins and the hydrophobic regions of the
Fg repeats within the NPC that molecules get in and out of the nucleus
Nuclear proteins, such as splicing factors, transcription factors, etc. must be brought back
into the nucleus through the NPC
All of these proteins contain a nuclear localization signal (NLS) which allows them to be
recognised by specific transporter protein
o NLS’s function has been seen by adding an artificially
labelled NLS onto pyruvate kinase, which is normally a
cytoplasmic protein
All the proteins immediately go to the nucleus, as can
be seen by the concentrated fluorescence
o NLS proteins are rich in lyseines and arginines, and with practice, one can almost
guess which proteins belong in the nucleus
There are two proteins which are heavily involved in the nuclear import mechanism: Ran and
Importin α and β
The first step in nuclear importing is the importin α binding to the NLS of the cargo
protein (the protein to be imported)
Importin β binds to the Fg nucleoporins and the interaction of importins β and α allow the
cargo protein to be imported into the nucleus
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