Karyopherins are
protein
Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...
s involved in
transporting molecules between the
cytoplasm
In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...
and the
nucleus
Nucleus ( : nuclei) is a Latin word for the seed inside a fruit. It most often refers to:
*Atomic nucleus, the very dense central region of an atom
* Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA
Nucl ...
of a
eukaryotic cell
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacter ...
. The inside of the nucleus is called the karyoplasm (or nucleoplasm). Generally, karyopherin-mediated transport occurs through
nuclear pore
A nuclear pore is a part of a large complex of proteins, known as a nuclear pore complex that spans the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are approximately 1,000 nuclear pore complexe ...
s which acts as a gateway into and out of the nucleus. Most proteins require karyopherins to traverse the nuclear pore.
Karyopherins can act as ''
importin
Importin is a type of karyopherin that transports protein molecules from the cell's cytoplasm to the nucleus. It does so by binding to specific recognition sequences, called nuclear localization sequences (NLS).
Importin has two subunits, impo ...
s'' (i.e. helping proteins get into the nucleus) or ''exportins'' (i.e. helping proteins get out of the nucleus). They belong to the nuclear pore complex family in the
transporter classification database (TCDB). Energy for transport is derived from the
Ran
Ran, RaN and ran may refer to:
Arts and entertainment
* ''Ran'' (film), a 1985 film directed by Akira Kurosawa
* "Ran" (song), a 2013 Japanese song by Luna Sea
* '' Ran Online'', a 2004 MMORPG (massively multiplayer online role playing game)
* ...
gradient.
Upon stress, several karyopherins stop shuttling between the nucleus and the cytoplasm and are sequestered in
stress granules, cytoplasmic aggregates of
ribonucleoprotein
Nucleoproteins are proteins conjugated with nucleic acids (either DNA or RNA). Typical nucleoproteins include ribosomes, nucleosomes and viral nucleocapsid proteins.
Structures
Nucleoproteins tend to be positively charged, facilitating int ...
complexes.
Importin beta
Importin beta
Importin is a type of karyopherin that transports protein molecules from the cell's cytoplasm to the nucleus. It does so by binding to specific recognition sequences, called nuclear localization sequences (NLS).
Importin has two subunits, import ...
is a variety of karyopherin that facilitates the transport of cargo proteins into the nucleus. First, it is binding
importin alpha
Importin is a type of karyopherin that transports protein molecules from the cell's cytoplasm to the nucleus. It does so by binding to specific recognition sequences, called nuclear localization sequences (NLS).
Importin has two subunits, import ...
– another type of karyopherin that binds the
cargo protein
Cargo consists of bulk goods conveyed by water, air, or land. In economics, freight is cargo that is transported at a freight rate for commercial gain. ''Cargo'' was originally a shipload but now covers all types of freight, including trans ...
in the cytoplasm—before the cargo protein is imported into the nucleus through the
nuclear pore
A nuclear pore is a part of a large complex of proteins, known as a nuclear pore complex that spans the nuclear envelope, which is the double membrane surrounding the eukaryotic cell nucleus. There are approximately 1,000 nuclear pore complexe ...
using energy derived from the
Ran gradient. Once inside the nucleus, the cargo dissociates from the karyopherins.
Importin beta can also carry proteins into the nucleus without the aid of the
importin alpha
Importin is a type of karyopherin that transports protein molecules from the cell's cytoplasm to the nucleus. It does so by binding to specific recognition sequences, called nuclear localization sequences (NLS).
Importin has two subunits, import ...
adapter protein.
[Poon, I. K. H. and D. A. Jans (2005). "Regulation of nuclear transport: Central role in development and transformation?" Traffic 6(3): 173-186.]
Human genes in the karyopherin family
*
KPNA1
Importin subunit alpha-5 is a protein that in humans is encoded by the ''KPNA1'' gene.
Interactions
Importin subunit alpha-5 has been shown to interact with KPNB1 and UBR5
E3 ubiquitin-protein ligase UBR5 is an enzyme that in humans is encoded by ...
*
KPNA2
Importin subunit alpha-1 is a protein that in humans is encoded by the ''KPNA2'' gene.
The import of proteins into the nucleus is a process that involves at least 2 steps. The first is an energy-independent docking of the protein to the nuclear ...
*
KPNA3
Importin subunit alpha-4 also known as karyopherin subunit alpha-3 is a protein that in humans is encoded by the ''KPNA3'' gene.
The transport of molecules between the nucleus and the cytoplasm in eukaryotic cells is mediated by the nuclear pore ...
*
KPNA4
Importin subunit alpha-3, also known as karyopherin subunit alpha-4, is a protein that in humans is encoded by the ''KPNA4'' gene.
Function
The nuclear import of karyophilic proteins is directed by short amino acid sequences termed nuclear loca ...
*
KPNA5
Importin subunit alpha-6 is a protein that in humans is encoded by the ''KPNA5'' gene.
The transport of molecules between the nucleus and the cytoplasm in eukaryotic cells is mediated by the nuclear pore complex (NPC) which consists of 60-100 prot ...
*
KPNA6
Importin subunit alpha-7 is a protein that in humans is encoded by the ''KPNA6'' gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ...
*
KPNB1
Importin subunit beta-1 is a protein that in humans is encoded by the ''KPNB1'' gene.
Function
Nucleocytoplasmic transport, a signal- and energy-dependent process, takes place through nuclear pore complexes embedded in the nuclear envelope. ...
*
CRM1
Exportin 1 (XPO1), also known as chromosomal region maintenance 1 (CRM1), is a eukaryotic protein that mediates the nuclear export of various proteins and RNAs.
History
XPO1 (CRM1) originally was identified in the fission yeast ''Schizosaccharom ...
Additional images
Image:RanGTPcycle.png, The Ran-GTP cycle
References
External links
*
Illustrations at berkeley.eduKaryopherin animationsKaryopherin illustrations3D electron microscopy structures of exportin from the EM Data Bank(EMDB)
Transport proteins
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