MB 207 – Molecular Cell Biology. Intracellular compartments and protein sorting. Compartmentalization of cells. Bacterium consists of a single intracellular compartment surrounded by plasma membrane. Eukaryotic cell is subdidvided into functionally distinct, membrane-enclosed compartments.
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Intracellular compartments and protein sorting
Compartmentalization of cells
Major intracellular compartments of an animal cell
→ Vital biochemical processes take place in or on membrane surfaces.
→ Compartments increasing surface area as well as providing specialized aqueous spaces for reaction.
The evolution of internal membranes: Development of plastid
Hypothetical scheme for the evolution of the cell nucleus and ER
Topological relationships between compartments of the secretory and endocytic pathways in eucaryotic cell
A simplified ‘roadmap’ of protein traffic
Three main mechanisms of protein transport
Vesicular transport: Vesicular budding and fussion
Summarizes the routes by which protein are carried forward or diverted to other organelles
- Signal sequence: signal resides in a single discrete stretch of amino acid sequence, often cleaved, not part of final protein product
- Signal patch: 3-D arrangements of amino acids on the protein’s surface that forms when the protein folds up
Signal sequence and signal patch
→ Bidiectional traffic occurs continuously between cytosol and nucleus.
Nuclear pore complexes perforate the nuclear envelope
Possible paths for free diffusion through nuclear pore complex
Immunofluorescence micrographs showing T-antigen localization.
Nuclear import receptors bind nuclear localization signals and nucleoporins
Ran-GTP causes cargo binding of export receptor.
Ran-GTP causes cargo release in the nucleus and GTP-bound receptors return to cytosol.
Nuclear localization of activated T cells control gene expression
→ Lamins are special class of intermediate filament proteins that polymerize into a two-dimensional lattice.
The breakdown and reformation of the nuclear envelope during mitosis
NLS is not cleaved off after transport into nucleus.
Translocation into mitochondrial matrix depends on a signal sequence and protein translocators
Protein import by mitochondria
ATP hydrolysis and a H+ gradient are used to drive protein import into mitochondria
(1) Bound cytosolic hsp70 is released from the protein in a step that depends on ATP hydrolysis. After the initial insertion of the signal sequence and of adjacent portions of the polypeptide chain into TOM complex, the signal sequence interacts with a TIM complex. (2) The signal sequence is then translocated into the matrix in a process that requires
Two plausible models of how mitochondrial hsp70 could drive protein import
Protein import from the cytosol into the inner mitochondrial membrane or intermembrane space
Translocation of a precursor protein into thylakoid space of chloroplasts
RH2 + O2 → R + H2O2
A model for how new peroxisomes are produced
Free and membrane-bound ribosomes
The signal hypothesis: protein translocation across the ER membrane
The signal-recognition particle (SRP)
How ER signal sequences and SRP direct ribosomes to the ER membrane
Three ways in which protein translocation can be driven through structurally similar translocators
A model for how a soluble protein is translocated across ER membrane
How a single-pass transmembrane protein with a cleaved ER signal sequence is integrated into the ER membrane
Protein glycosylation in the rough ER
The role of N-linked glycosylation in ER protein folding
The export and degradation of misfolded ER proteins
The unfolded protein response in yeast
Phospholipid exchange proteins help to tranport phospholipids from the ER to mitochondria and peroxisomes