• The biosynthetic pathway of a eukaryotic cell consists of a series of distinct membrane-bound organelles• Materials are carried between compartments by vesicle that bud from donor membranes and fuse with acceptor membranes
These electron micrographs show the membranes of these vesicles to be covered on their outer (cytosolic) surface by a distinct protein coat a protein coat formed from soluble proteins that assemble on the cytosolic surface of the donor membrane at sites where budding takes place Activation of a small G protein recruited to the site
Act as mechanical device that causes membrane to curve and form budding vesicle Provide a mechanism in selecting the components to be carried by vesicle cargo consisting of secretory, lysosomal, and membrane proteins to be transported The machinery required to target and dock the vesicle to the correct acceptor membrane
move materials from the ER “forward” to the ERGIC and Golgi complex. Contains a number of proteins that were first identified in mutant yeast cells that were unable to carry out transport from the ER to the Golgi complex select and concentrate certain components for transport in vesicles
enzymes that act at later stages in the biosynthetic pathway, such as the glycosyltransferases of the Golgi complex membrane proteins involved in the docking and fusion of the vesicle with the target compartment Membrane proteins that are able to bind soluble cargo Mutations in one of these cargo receptors has been linked to an inherited bleeding disorder
a small G protein called Sar1, which is recruited specifically to the ER membrane Sar1 plays a regulatory role
Sar1-GDP molecules have been recruited to the ER membrane by a protein called a GEF (guanine- exchange factor) that catalyzes the exchange of the bound GDP with a bound GTP each Sar1-GTP molecule has extended a finger-like helix along the membrane within the cytosolic leaflet. This event induces the curvature of the lipid bilayer at that site a dimer composed of two COPII polypeptides(Sec23 and Sec24) has been recruited by the bound Sar1- GTP.
Transmembrane cargo accumulates within the forming COPII vesicle as their cytosolic tails bind to the Sec24 polypeptide of the COPII coat the remaining COPII polypeptides (Sec13 and Sec31) have joined the complex to form an outer structural scaffold of the coat.
Two copies of the Sar1-Sec23-Sec24 complex (shown in red, magenta, and blue, respectively) that would form the inner layer of the COPII
move materials in a retrograde direction from the ERGIC and Golgi stack “backward” toward the ER and from trans Golgi cisternae “backward” to cis Golgi cisternae first identified in experiments in which cells were treated with molecules similar in structure to GTP Unlike GTP, cannot be hydrolyzed
accumulate in the presence of a nonhydrolyzable GTP analogue the coat contains a small GTP binding protein, called ARF1, whose bound GTP must be hydrolyzed before the coat can disassemble
proteins are maintained in an organelle by a combination of two mechanisms: Retention - may be based primarily on the physical properties of the protein ex. Soluble proteins that are part of large complexes or membrane proteins with short transmembrane domains are not likely to enter a transport vesicle. Retrieval of “escaped” molecules back to the compartment in which they normally reside
trans Golgi network Major sorting station Sorting and transport of lysosomal enzymes Lysosomal proteins are synthesized on membrane-bound ribosomes of the ER and carried to the Golgi complex along with other types of proteins
soluble lysosomal enzymes are specifically recognized by enzymes that catalyze the two-step addition of a phosphate group to certain mannose sugars of the N-linked carbohydrate chains Lysosomal enzymes are transported from the TGN in clathrin-coated vesicles Lysosomal enzymes are escorted from the TGN by a family of adaptor proteins called GGAs The outer ends of the GGA adaptors bind to clathrin molecules GGA adaptors bind to a sorting signal in the cytosolic tails of the mannose 6-phosphate receptors
MPRs in the TGN membrane and lysosomal enzymes within the TGN lumen become concentrated into clathrin-coated vesicles production of clathrin-coated vesicles begins with the recruitment to the membrane of a small GTP-binding protein
Lysosomal proteins are not the only materials that are exported from the TGN membrane proteins destined for the plasma membrane and secretory materials destined for export from the cell are also transported from the TGN, but the mechanisms are poorly understood
requires specific interactions between different membranes Selective fusion is one of the factors that ensures a directed flow through the membranous compartments of the cell
1. Movement of the vesicle toward the specific target compartment2. Tethering vesicles to the target compartment3. Docking vesicles to the target compartment4. Fusion between vesicle and target membranes
fusion of a secretory vesicle or secretory granule with the plasma membrane and subsequent discharge of its contents continual basis in most cells, as proteins and other materials are delivered to both the plasma membrane and extracellular space membrane fusion produces an opening through which the contents of the vesicle or granule are released into the extracellular space
an animal cell’s digestive organelles contains at least 50 different hydrolytic enzyme sproduced in the rough ER and targeted to these organelles The enzymes of a lysosome share an important property: allhave their optimal activity at an acid pH and thus are acid hydrolases. Optimum pH is 4.6 breakdown of materials brought into the cell from the extracellular environment
Lysosomes also play a key role in organelle turnover, that is, the regulated destruction of the cell’s own organelles and their replacement Process is called autophagy Once the digestive process in the autophagolysosome has been completed, the organelle is termed a residual body
90% in a plant cell Solutes and macromolecules are temporarily stored May also store host of toxic compounds tonoplast,contains a number of active transport systems that pump ions into the vacuolar compartment to a concentration much higher than that in the cytoplasm or the extracellular fluid Osmosis sites of intracellular digestion have some of the same acid hydrolases found in lysosomes
Endocytosis - is primarily a process by which the cell internalizes cell-surface receptors and bound extracellular ligands bulk-phase endocytosis - is the nonspecific uptake of extracellular fluids removes portions of the plasma membrane and may function primarily in the recycling of membrane between the cellsurface and interior compartments
in contrast, brings about the uptake of specific extracellular macromolecules (ligands) following their binding to receptors on the external surface of the plasma membrane provides a means for the selective and efficient uptake of macromolecules that may be present at relatively low concentrations in the extracellula fluid
Cell eating In most animals, phagocytosis is a protective mechanism rather than a mode of feeding Not all bacteria ingested by phagocytic cells are destroyed. some species hijack the phagocytic machinery to promote their own survival in the body
Have 2 subcompartments in which imported proteins take place: boundary membrane and the internal matrix Proteins destined for a peroxisome possess a peroxisomal targeting signal, either a PTS for a peroxisomal matrix protein or an mPTS for a peroxisomal membrane protein PTS receptors bind to peroxisome-destined proteins in the cytosol and shuttle them to the surface of the peroxisome
The PTS receptor apparently accompanies the peroxisomal protein through the boundary membrane into the matrix and then recycles back to the cytosol to escort another protein Unlike mitochondria and chloroplasts, whose imported proteins must assume an unfolded state, peroxisomes are somehow able to import peroxisomal matrix proteins in their native, folded conformation, even those that consist of several subunits. The mechanism by which peroxisomes are able to accomplish this daunting assignment remains a matter of speculation.
have four subcompartments into which proteins can be delivered an outer mitochondrial membrane (OMM), inner mitochondrial membrane (IMM), intermembrane space, and matrix
Chloroplasts have six subcompartments into which proteins can be delivered: an inner and outer envelope membrane and intervening intermembrane space, as well as the stroma, thylakoid membrane, and thylakoid lumen Chloroplast and mitochondrial import mechanisms exhibit many similarities, although their translocation machineries have evolved independently
1. the vast majority of chloroplast proteins are imported from the cytosol, 2. the outer and inner envelope membranes contain distinct translocation complexes (Toc and Tic complexes, respectively) that work together during import, 3. chaperones aid in the unfolding of the polypeptides in the cytosol and folding of the proteins in the chloroplast, and 4. most proteins destined for the chloroplast are synthesized with a removable N-terminal sequence (termed the transit peptide).