cell bio
Two sites of protein synthesis in eukaryotic cells:
cytosol and rough ER..... BUT All organelles contain proteins and different compartments have distinctly protein composition
Summary of secretory pathway p.1
-Although all cytosolic ribosomes are functionally equivalent, membrane-attached and membraneunattached ribosomes synthesize different classes of proteins, depending on a signal sequence in the protein itself. -Newly made secretory proteins are localized to the lumen of the rough ER. -All mammalian cells continuously secrete certain proteins, such as those in the extracellular matrix. -Certain cell types store proteins such as hormones and digestive enzymes in secretory vesicles, awaiting a neural or hormonal signal that triggers an elevation in cytosolic Ca2+ and then protein secretion.
Let's begin at the beginning...
-Bases are read in triplets by ribosomal subunits. -Proteins are "translated" by ribosomes according to genetic information carried by mRNA - All cellular compartments contain proteins...
Protein Synthesis
Both free and bound ribosomes produce proteins by translating the genetic code of mRNA into a string of amino acids. Ribosomes require tRNA to produce proteins. tRNA brings the amino acids that correspond with the genetic code in mRNA to the ribosome. The tRNA molecule binds to the ribosome at three places. There is a site for the portion of the tRNA that binds to the next amino acid and a site for the growing amino acid chain. The third area is the exit site which releases the tRNA after the protein is completed. Bound ribosomes send most proteins into the endoplasmic reticulum, while most of the proteins produced by the free ribosomes head toward the Golgi apparatus. In both cases, the proteins are eventually modified and transported to their destination.
The eukaryotic cell: What organelles are important for ATP (energy) generation
Cytoplasm, Mitochondrion
Organelles in protein trafficking
ER, Golgi, the nuclear envelope and (partially) the lysosome and part of the endomembrane system. The inside (lumen) of the endomembrane system is topologically equivalent to the extracellular space.
Organelle functions: Protein synthesis and trafficking
Endoplasmatic reticulum (rough), Ribosome, Golgi body
Destruction of organelles and other material
Lysosome
Terminal degradation of internalised material occurs in lysosomes
Lysosomes can breakdown old organelles, e.g. mitochondria or endocytosed nutrient vesicles or - as the phagolysosome in macrophages - bacteria.
The eukaryotic cell: What organelles are important for information storage and copying?
Nucleus, Envelope, Nucleolus, Chromatin, Nuclear pore
The eukaryotic cell: What part of the cell is important for for the transport of (mostly) small molecules
Plasma membrane
Summary of secretory pathway p.2
Proteins destined to be secreted move through the secretory pathway in the following order: rough ER → ER-to-Golgi transport vesicles → Golgi cisternae → secretory or transport vesicles → cell surface (exocytosis) (see Figure 17-13). -Small transport vesicles bud off from the ER and fuse to form the cis-Golgi reticulum. By cisternal migration, cis-Golgi vesicles with their luminal protein cargo move through the Golgi complex to the trans-Golgi reticulum. -Proteins are retrieved from the cis-Golgi to the ER and also from later Golgi cisternae to earlier ones by small retrograde transport vesicles. -Plasma-membrane glycoproteins follow the same maturation pathway as continuously secreted proteins. -Both secreted and integral membrane proteins undergo various modifications as they mature in the secretory pathway.
RAB and Snare
Rab and SNAREs co-operate in vesicle targeting Both "postcodes" must match • Rab mediates initial targeting • SNAREs trigger membrane fusion
Membrane flow moves newly made proteins from the rough ER
Rer moves proteins to lysosomes or secretory vescles
Translation organelles
Ribosome, rER, smooth ER
Why are ribosomes present at two sites and two sites only?
The place where the ribosome is located is one of the differences between free and bound ribosomes. Free ribosomes are located in the cytoplasm of the cell. They are not attached to any structure, but they may group together with other ribosomes to form polysomes. In the cytoplasm, ribosomes are free floating. They can move all around the cell. Bound ribosomes are located on the surface of the endoplasmic reticulum. The endoplasmic reticulum that contains ribosomes is described as the rough endoplasmic reticulum because of the bumpy surface. Bound ribosomes can not move to other areas of the cell. They are attached to the cytosolic side of the endoplasmic reticulum.
What are the key steps for protein reaching their destination?
Vesicles (balloons) must be filled with content appropriate to destination (= cargo" internal "molecular postcode") • Vesicle must be sent to the correct destination - Colour of balloons / "Molecular postcode" on the outside of vesicles • Vesicles must exchange content by "fusion", but it must be prevented that: - Vesicles also may scramble content of their membranes - Vesicles bud off from compartments uncontrolled
SNARE complexes
pull membranes together to facilitate fusion Form 4-helix bundle from 1 v-SNARE and 3 t-SNAREs Example of SNAREs interacting before fusion of a storage vesicle in brain cells with the plasma membrane 16/11/2015
