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Since
protein synthesis occurs in the cytoplasm,
there must be a mechanism for
the
transport of proteins to the nucleus. In addition,
it is important that the transport of
proteins out of the nucleus is regulated. In order
for molecules larger than 40-60 kDa to
cross
the nuclear membrane, they must be actively
transported through the nuclear pore
complex
(NPC). To be transported through the
NPC, the proteins must contain a signal that
is
recognized by carrier proteins. Once carrier proteins
bind the cargo through interaction with
this
nuclear localization or nuclear export signal, the
complex can interact with the NPC. In many
pathways,
the directionality of nuclear transport involves
the cellular location and levels of Ran
in
different nucleotide states. The regulation of protein
import and export from the nucleus is
important
in many cellular processes.
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