Introduction to Molecular and Cell Biology, Biol. 220
Lecture 23: Nuclear Transport
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Introduction to Molecular and Cell Biology, Biol. 220 Lecture 23: Nuclear Transport |
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Nucleus
The nucleus has a double membrane with pores that are about 30x bigger than a ribosome.
Molecules are transported out of and into the Nucleus based on intrinsic signals.
mRNA are transported only after splicing is completed.
Model for passage of mRNPs through the nuclear pore complex (Fig. 11.31).
Fig. 11.31 Model for passage
of mRNPs through the nuclear pore complex. After the coiled mRNP moves
through the terminal ring, it uncoils as it passes through the central plug
of the NPC, with the 5' end leading the way and becoming associated with ribosomes
in the cytoplasm.
Proposed mechanism for hnRNP protein mediated export of mRNA from the nucleus
(Fig. 11.34).
Fig. 11.34 Proposed mechanism
for mRNA export from the nucleus. (a) The 5' end of the fully processed
mRNA-hnRNP protein comples associated with cap-binding comples (CBC) which passes
through the nuclear pore complex (NPC). (b) Only proteins containing nuclear
export signals (NES) transport mRNA from nucleus. (c) Hydrolysis of GTP dissociates
hnRNPs from mRNA and allows re-entry of hnRNPs back into the nucleus.
Proposed mechanism for the transport of "cargo" proteins containing a leucine-righ nuclear-export signal (NES) from the nucleus to the cytosol (Fig. 11.33).
Fig. 11.33 Proposed mechanism
for the transport of "cargo" proteins from the nucleus to the cytosol.
Exportin 1 binds to the NES of the cargo protein to be transported and to Ran-GTP.
Hydrolysis of GTP in cytoplasm releases protein and Exportin 1 and Ran-GDP return
to nucleus where Ran is regenerated to Ran-GTP.
Proposed mechanism for the transport of "cargo" proteins containing nuclear-localization signal (NLS) from the cytoplasm to the nucleus (Fig. 11.37).
Fig. 11.37 Proposed mechanism
for the transport of "cargo" proteins containg a nuclear-localization signal
(NLS) from the cytoplasm to the nucleus.
The transport of mRNA from the nucleus to the cytoplasm can be regulated (Fig. 11.38).
Fig. 11.38 Regulation of mRNA
transport from the nucleus to the cytoplasm by the HIV Rev protein. The
HIV transcripts can be spliced into several alternative forms. The Rev
protein selectively improves transport of un-spliced and singly-spliced mRNA
from the nucleus by binding at the Rev response element (RRE) in the HIV mRNA.
| Created 2004 by CA Rinehart• email CA Rinehart | Index • CourseInfo LogIn • Syllabus • References • Other Resources |