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 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.