compartmentalisation of cells Flashcards
different types of protein targeting
- gated transport
- (nucleus→ cytoplasm)
- proteins imported through membranes
- plastids/mitochondria
- vesicular transport
- packaging
function of nuclear pore
allow transport in and out
where are nuclear pores formed
- pores are formed at junction of inner and outer membranes of nuclear envelope
- consists of multiple copies of around 30 different nucleoporins
functions of nuclear pores
- gating material into nucleus
- histones made in cytoplasm → need transporting to nucleus
- ribosomal subunits made in nucleolus need exporting
what size molecules can diffuse freely
5000 D = diffusible
60,000 = cannot enter by diffusion
how do molecules know to actively transport
- with appropriate signal, pore can increase in size to allow proteins to move into pore
- nuclear localisation signals = sequence of amino acids
- polar, charged = lys, arg, pro
where are most proteins made
most proteins are made in endoplasmic reticulum
why must ribosomes be associated closely with ER membrane
membrane proteins undergo cotranslational translocation or post-translocational translocation
how are membrane proteins synthesised
- proteins made in RER
- modifications in ER
- signal
- ribosome associated
- signal and ribosome site on translocator
- fed through pore
role of chaperones in membrane protein synthesis
folds assisted by chaperones (rish in ER) as well as protein disulfide isomerases
how are membrane proteins inserted into ER (single pass type I) ?
- signal sequence stimulates transfer
- signal is recognised by sec61 (translator) = opens
- newly translated protein is fed through translocator
- before going straught through, stop transfer sequence in protein (hydrophobic) allows protein to remain anchored in membrane
- rest of protein (C-terminus) is exposed to cytoplasm
how are membrane proteins inserted into ER (single pass type II) ?
- single pass type II
- C terminus = outside of cell
- N terminus = cytoplasm
- signal sequence acts as TM domain rather than stop codon
function of chaperones
chaperones ensure proper folding occurs e.g BIP stay attached on proteins until they fold or until folding correct structure
function of glycosilation in protein folding
allows effective quality control of protein folding
what happens if proteins are not correctly folded
- proteins are reverse translocated via eversion of same machinery
- moved out of ER lumen into cytosol and broken down
how can protein misfolding leading to disease
e.g CFTRdelta508
- deletion of this amino acids causes misfolding of CFTR that is retained in ER
- excess misfolding protein → unfolded protein response
what is the response to excess misfolding protein
- close down translation = no protein synthesis
- upregulate (via TF) synthesis of chaperones
- activation of apoptytic pathways if severe
types of protein targeting
- gated transport
- (nucleus→ cytoplasm)
- proteins imported through membranes
- plastids/mitochondria
- vesicular transport
- packaging
structure and function of nuclear pore
s
- pores are formed at junction of inner and outer membranes of nuclear envelope
- consists of multiple copies of aorund 30 different nucleoporins
f
- gating material into nucleus
- histones made in cytoplasm → need transporting to nucleus
- ribosomal subunits made in nucleolus need exporting
how is material transported through nucleus
diffusion (5000kD)
active transport = adjust pore size
what are nuclear localisation signals in transport
- sequence of amino acids
- polar, charged = lys, arg, pro
- mutation of amino acids = disrupts translocation of proteins
how do proteins get into mitochondrial matrix
- N terminal signal sequence recognised by TOM complex
- protein translocates through TOM and TIM23 into matrix
- signal is cleaved off
structure of signal sequence peptide
- amphipathic alpha helices structure
- polar and non polar residues = one side hydrophobic/one side hydrophilic
hydrophobic residues bind in hydrophobic groove
- polar and non polar residues = one side hydrophobic/one side hydrophilic
function of signal sequence peptide
is able to direct protein translocation through the membrane
how are proteins translocated into bacterial membrane
- similar mechanism to import into mitochondria due to origin
- mitochondria and bacteria have porins in outer membranes = exchange of metab and ions
- TOM complex translocates in polypeptide chains which combine with chaperones allowing protein to assemble
how are proteins translocated into chloroplasts
- receptor protein of TOC complex
- through TOC complex
- through TIC complex
- cleavage of signal sequence
- enters thylakoid space
