Protein Processing And Targetting In Cells Flashcards
Proteins for the mitochondrial matrix
- protein with signal kept unfolded by chaperones
- signal binds to receptor
- protein fed through pore into outer membrane
- moves through channel in adjacent inner membrane
- targeting signal then cleaved
Pyruvate dehydrogenase deficiency
Arg –> Pro substitution
Receptors can’t recognise targeting sequences as well so there is a reduced uptake into the mitochondria
Results in build up of lactic acid and neurological problems
Secretion from cells
- constitutive secretion
- regulated secretion
Endocrine cells - secrete hormones
Exocrine cells - secrete digestive juices
Neurocrine cells - secrete neurotransmitters
Insertion into the ER membrane
Required for delivery of membrane proteins destined for the plasma membrane or internal membrane of secretory pathway
Stop Transfer Anchor Sequence
A hydrophobic sequences exists the protein which anchors the protein in the membrane and prevents further transfer into the ER lumen
Functions of endoplasmic reticulum
- insertion of proteins into membranes
- specific proteolytic cleavage
- glycosylation
- formation of disulphide bonds
- ensures proper folding of proteins
- assembly of multi subunit proteins
- hydroxyl atom of selected Lys and Pro residues
Glycosylation
- ensures correct protein folding
- increases protein stability
- facilitates interactions with other molecules
- deficiencies in N-linked glycosylation lead to severe inherited human disease: Congenital Disorders of Glycosylation (CDG)
Protein Disulphide Isomerase
PDI
- protein passes through the ER which catalyses disulphide bond formation
KDEL
KDEL carrying protein has a higher affinity
Allows it to bind and then be released back into the ER
Folding problems
ER chaperone proteins attempt to correct misfolding
BiP - binds to exposed amino acid sequences that would normally be buried in the interior of a folded protein
Calnexin and Calreticulin - binds to oliosaccharides on incompletely folded proteins
IF IT CANNOT BE CORRECTED
protein returned to cytosol for degradation
Or may accumulate to toxic levels in the ER and result in disease
(e.g. Alzheimer’s, CF, diabetes mellitus)
Collagen
most abundant protein in the body (25-35%)
connective tissue - tendons, ligaments, etc.
Secreted protein produced by fibroblast cells in the connective tissue
Assembly of collagen fibres has to occur outside of the cell otherwise it would be catastrophic
Collagen fibres
300nm rod-shaped protein
3 polypeptides (alpha helixes)
Glycine in every third position (Gly-X-Y)n - allows chains to come together - stabilised by H-bonds (prolyl hydroxylase - allows increased H-bonding)
Triple helix - RIGHT HANDED - non-extensible, non-compressible and has a high tensile strength
Distribution of collagen types
Type I - skin, bone (90% of all body collagen)
Type II - cartilage, intervertebral discs
Type III - foetal skin, cardiovascular system
Type IV - basement membranes
Type V - placenta, (skin)
Formation of a collagen fibre
The conversion of procollagen to tropocollagen is done extracellularly
Removal of the N and C terminal properties outside of cell
Lateral association of collagen molecules followed by covalent cross-linking
Electron light and electron dense bands alternate
Then the aggregation of fibrils
Insulin
Proinsulin is one polypeptide chain has three disulphide bonds (in the ER) which is cleaved to form two polypeptide chains which has three disulphide bonds to form mature insulin (occurs post Golgi)
Requires three enzymes
Proteolytic processing
One polypeptide chain can yield several different enzymes depending on the way it is processed - cleaved and rejoined in different sequences
Common in the secretory pathway as hydrolytic enzymes would be destructive if they were active in the cell
To ER
Signal sequence at N-terminus Unfolded during transfer Signal recognition protein/receptor Signal cleaved by signal peptidase Requires energy - hydrolysis of GTP by SRP
To nucleus
NLS - nuclear localising signal BASIC on surface of folded protein Folded during transfer Importin recognises NLS Signal is retained Requires energy - hydrolysis of GTP
To mitochondria
Amphipathic signal for initial targeting to matrix located on the N-terminus
Held partially unfolded by chaperones during transfer
Mitochondrial-import simulating factor and Tom&Tim channel complexes
Signal is cleaved
Requires energy - ATP hydrolysis
To Lysosomes
Post translational addition of mannose-6-phosphate
Signal patch to distinguish lysosomal proteins from other mannose-labelled proteins
Folded during transfer (delivered via vesicle)
M-6-P receptor in trans-Golgi
Phosphate removed by phosphatase
Requires energy for phosphotransferase (not directly)
For retention in ER
KDEL signal (Lys-Asp-Glu-Leu) on the C-terminus
Folded during transfer (delivered via vesicle)
KDEL receptor in cis-Golgi
Signal is retained
Doesn’t require energy - binding and release is dependent on pH
What is required for protein sorting?
a signal (address), intrinsic to the protein shown as a pre___ protein
a receptor that recognises the signal and directs the protein to the correct membrane
translocation machinery
energy to transfer the protein to its new place
