1.1 - Nucleus and Nuclear Import Flashcards
structure of outer nucleus membrane (relating to endoplasmic reticulum)
continuous with ER - space between inner and outer nuclear membranes directly connected with lumen of ER
functionality of outer nucleus membrane (relating to endoplasmic reticulum membranes)
functionally similar - differ slightly in protein composition
how may nuclear envelope and ER evolve?
through invagination of plasma membrane
inner nuclear membrane role (nuclear lamina)
carries specific proteins such as membrane proteins that organise the nuclear lamina
nuclear lamina
fibrous network that provides structural support to the nucleus
what would happen without nuclear lamina?
nuclear membrane will fragment
why is protein import into the nucleus required
transcription and replication require enzymes
nuclear pore complexes (2)
- stud nuclear membrane
- sole channels through which polar molecules and macromolecules can pass through the nuclear envelope
nuclear pore composition
large multi-protein complexes composed of about 30 different proteins
what does negatively stained electron microscopy of purified nuclear pores reveal?
structure with eight-fold symmetry organised around large central channel
nuclear pore complex structure (3)
- protein fibrils protrude from both sides of complex (form basket-like structure on nuclear side)
- many nuclear core proteins contain extensive unstructured regions that form a jumbled meshwork that fills centre of channel
- proteins over 5kD blocked from passively diffusing through
Nuclear localisation signals (NLS) (2)
- protein tag that identified proteins destined for nucleus
- tag typically consists of 1/2 short sequences containing positively charged lysine residue
importins
carry proteins into nucleus
exportins
carry proteins out of nucleus
nuclear transport receptors
(importins/exportins)
escort large molecules through the nuclear pore
energy supplied for nuclear transport
GTP hydrolysis
how does RAN-GTP binding lead to cargo release (3)
- nuclear transport receptors made up of repeated a-helices that stack into large arches/snail-shaped coils
- cargo and RAN-GTP bind different arches
- RAN-GTP covers loop which is important for NLS binding
how does GTP hydrolysis drive nuclear transport (4)
- protein binds to receptor
- RAN-GTP binds to receptor
- protein delivered to nucleus
- RAN hydrolyses its bound GTP - RAN-GDP dissociates from receptor
nuclear export (4)
- nuclear export signal
- recognised by exportin (similar to importins)
- in this case, RAN-GTPase binds to exportins with cargo to promote export through pore
- GTP/GDP swithc on entering cytosol GTPase
RAN-GAP
cytoplasmic
RAN-GEF
nuclear
nucleolus (2)
- most prominent structure of the nucleus (electron dense)
- site of ribosome biogenesis where transcribed and processed rRNA are combined with proteins to form ribosomal subunits
ribosome (2)
- translate mRNA -> proteins in the cytoplasm (rough ER/ cytosol
- very large/complex structure (2/3 RNA and 1/3 protein)
ribosomal subunits (2)
- small subunit
- large subunit
ribosomal small subunit
platform where tRNAs matched to codons of mRNA
ribosomal large subunit
catalyses formation of peptide bonds linking amino acids in polypeptide chain
when do ribosomal subunits come together
come together on an mRNA for translation
why are rRNA genes highly transcribed?
cells require lots of ribosomes
how are rRNA coding genes organised in higher eukaryotes?
large tandem arrays
ribosomal RNAs in higher eukaryotes (4)
- 55
- 5.8S
- 18S
- 28S
processing of rRNAs
5.8S, 18S and 28S rRNAs are transcribed as single 45S pre-rRNA the is subsequently cleaved to give rise to 3 mature rRNAs
where does rRNA processing occur?
nucleuolus
what are ribosomes composed of?
rRNAs and proteins that interact with them
how do proteins interact with pre-RNA?
proteins imported into the nucleus and assembled on the pre-RNA in the nucleolus
what happens as rRNAs mature?
additional ribosomal proteins are added to the pre-ribosomal particles
what happens to mature ribosomes
exported from the nucleus via nuclear pores to yield active ribosomal subunits
