Download Chapter Twelve: Macromolecular Transport Across the Nuclear

Chapter Twelve:
Macromolecular Transport
Across the Nuclear Envelope
Amberlea Elliott
October 21, 2003
Objective:
The cell regulates the entrance and exit of
all molecules within its nucleus to prevent
problems and allow for easy movement of
approved molecules.
The movement is done through many
highly specialized structures which all
work together to regulate transport.
The Nuclear Envelope
Present in all Eukaryotic cells
Two- membrane system
Separates the Nucleus from the cytoplasm
Consists of a water impermeable
phospholipid bilayer and various proteins
Perforated by many nuclear pores
The Nuclear Pore Complex
 Nuclear pores are found on
the surface of the nuclear
envelope
 These pores allow for the
transport of macromolecules
into and out of the nucleus
 The complex forms an
elaborate structure, each
pore size is around 12.5
million Dalton
 The complex is made up of
fifty (yeast) to 100
(vertebrates)
NUCLEOPORINS- different
associated proteins
Structure of the Nuclear Pore Complex
Octagonal membrane
Eight 100nm long filaments extending into
the nucleoplasm
The filaments are joined by the terminal
ring
These two parts combine to form the
NUCLEAR BASKET
NUCLEAR LAMINA- network of
intermediate filaments extending over the
inner surface of the nuclear envelope
CYTOPLASMIC FILAMENTS- extend from
the cytoplasmic side of the nuclear pore
complex into the cytosol
Transport
Ions, small metabolites, and globular
proteins up to 60 kDa, are able to diffuse
through water filled channels
Large proteins and ribonucleicprotein
complexes are too big to diffuse in and out
of the the nucleus…
They need a little help….
Selective Transport
System of transporting proteins and
RNPC’s bigger than 60 kDa
Works by assisting the molecule
A water soluble transporter will bind with
the molecule and interact with the
nucleoporins to allow for movement across
the nuclear envelope
Importins
 Transport proteins via
selective transport
into the nucleus
 The proteins it carries
must contain a
nuclear localization
signal
Exportins
 Transport proteins out
of the nucleus
 It also shuttles tRNA
and ribosomes
between the nucleus
and cytoplasm
 The molecules
contain a nuclear
export signal
NLS and NES
There are many types of both of these
signals that exist
each signal interacts with a specific
receptor protein (importins and exportins)
The receptor proteins binding the the
signal are termed KARYOPHARINS
Fusion of NLS
Mechanism of the Signals
Proteins carrying the signal moves through
the nuclear pores by binding to the
receptor
the importin or exportin diffuse through
channels by binding to different FGrepeats
once this cargo complex reaches its
destination; it will dissociate releasing the
protein and other components
In summary:
The highly regulated import and export of
macromolecules allows for protection from
harmful mistakes or problems that free
movement of any molecule could cause to
the cell within the nucleus.