Chapter Twelve:Macromolecular Transport

Across the Nuclear Envelope

Amberlea ElliottOctober 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 cellsTwo- membrane systemSeparates the Nucleus from the cytoplasmConsists of a water impermeable

phospholipid bilayer and various proteinsPerforated 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 membraneEight 100nm long filaments extending into

the nucleoplasmThe filaments are joined by the terminal

ringThese 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 moleculeA water soluble transporter will bind with

the molecule and interact with the nucleoporins to allow for movement across the nuclear envelope

Importins Exportins

Transport proteins via selective transport into the nucleus

The proteins it carries must contain a nuclear localization signal

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 FG-repeats

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.