Term
| Describe the differences between and signal sequences and signal patches. |
|
Definition
| Signal patches are far more difficult to analyze than signal sequences, so less is known about their structure. Because they often result from a complex three-dimensional protein-folding pattern, they cannot be easily transferred experimentally from one protein to another. Some of these signal sequences are removed from the finished protein by specialized signal peptidases once the sorting process has been completed. The other type consists of a specific three-dimensional arrangement of atoms on the protein's surface that forms when the protein folds up. Signal sequences are used to direct proteins from the cytosol into the ER, mitochondria, chloroplasts, and peroxisomes, and they are also used to transport proteins from the nucleus to the cytosol and from the Golgi apparatus to the ER. |
|
|
Term
| Describe the structure and function of a nuclear pore complex. |
|
Definition
A nuclear pore complex seems to have four structural building blocks: column subunits, which form the bulk of the pore wall; annular subunits, which extend “spokes” toward the center of the pore; lumenal subunits, which contain transmembrane proteins that anchor the complex to the nuclear membrane; and ring subunits, which form the cytosolic and nuclear faces of the complex. The nuclear pore complex (NPC), perhaps the largest protein complex in the cell, is responsible for the protected exchange of components between the nucleus and cytoplasm and for preventing the transport of material not destined to cross the nuclear envelope.
|
|
|
Term
| How do nuclear localization signals work? What type of signal are they? |
|
Definition
| The selectivity of this nuclear import process resides in nuclear localization signals (NLSs), which are present only in nuclear proteins. The signals characterized this far can be located almost anywhere in the amino acid sequence and are thought to form loops or patches on the protein surface. Many function even when linked as short peptides to lysine side chains on the surface of a cytosolic protein, suggesting that the precise location of the signal within the amino acid sequence of a nuclear protein is not important |
|
|
Term
| Describe nuclear import receptors. |
|
Definition
To initiate nuclear import, most nuclear localization signals must be recognized by nuclear import receptors, which are encoded by a family of related genes. Each family member encodes a receptor protein that is specialized for the transport of a group of nuclear proteins sharing structurally similar nuclear localization signals. Nuclear import receptors do not always bind to nuclear proteins directly. |
|
|
