Editing Cell nucleus (section)

===Nuclear transport===
{{Main|Nuclear transport}}

[[Image:RanGTPcycle.png|360px|right|thumb|[[Macromolecule]]s, such as [[RNA]] and [[protein]]s, are [[active transport|actively transported]] across the nuclear membrane in a process called the [[Ran (biology)|Ran]]-[[Guanosine triphosphate|GTP]] nuclear transport cycle.]]

The entry and exit of large molecules from the nucleus is tightly controlled by the nuclear pore complexes. Although small molecules can enter the nucleus without regulation,<ref name="Watson">{{cite book | vauthors = Watson JD, Baker TA, Bell SP, Gann A, Levine M, Losick R | title = Molecular Biology of the Gene | publisher = Peason Benjamin Cummings; CSHL Press. | year = 2004 | edition = 5th | chapter = Ch9–10 | isbn = 978-0-8053-9603-4 }}</ref> macromolecules such as RNA and proteins require association karyopherins called [[importin]]s to enter the nucleus and [[exportin]]s to exit. "Cargo" proteins that must be translocated from the cytoplasm to the nucleus contain short amino acid sequences known as [[nuclear localization signal]]s, which are bound by importins, while those transported from the nucleus to the cytoplasm carry [[nuclear export signal]]s bound by exportins. The ability of importins and exportins to transport their cargo is regulated by [[GTPase]]s, enzymes that [[Hydrolysis|hydrolyze]] the molecule guanosine triphosphate (GTP) to release energy. The key GTPase in nuclear transport is [[Ran (protein)|Ran]], which is bound to either GTP or GDP (guanosine diphosphate), depending on whether it is located in the nucleus or the cytoplasm. Whereas importins depend on RanGTP to dissociate from their cargo, exportins require RanGTP in order to bind to their cargo.<ref name="Pemberton" />

Nuclear import depends on the importin binding its cargo in the cytoplasm and carrying it through the nuclear pore into the nucleus. Inside the nucleus, RanGTP acts to separate the cargo from the importin, allowing the importin to exit the nucleus and be reused. Nuclear export is similar, as the exportin binds the cargo inside the nucleus in a process facilitated by RanGTP, exits through the nuclear pore, and separates from its cargo in the cytoplasm.<ref name="Cavazza_2015">{{cite journal | vauthors = Cavazza T, Vernos I | title = The RanGTP Pathway: From Nucleo-Cytoplasmic Transport to Spindle Assembly and Beyond | journal = Frontiers in Cell and Developmental Biology | volume = 3 | pages = 82 | date = 2015 | pmid = 26793706 | pmc = 4707252 | doi = 10.3389/fcell.2015.00082 | department = Review | doi-access = free }}</ref>

Specialized export proteins exist for translocation of mature mRNA and tRNA to the cytoplasm after post-transcriptional modification is complete. This quality-control mechanism is important due to these molecules' central role in protein translation. Mis-expression of a protein due to incomplete excision of exons or mis-incorporation of amino acids could have negative consequences for the cell; thus, incompletely modified RNA that reaches the cytoplasm is degraded rather than used in translation.<ref name="Lodish" />