Molecular Biology Wk 6 Flashcards
What is protein sorting
Each compartment contains a unique set of proteins that have to be transferred selectively from the cytosol, where they are made, to the compartment where they are used. This transfer process, called protein sorting, depends on signals built into the amino acid sequence of the proteins.
What is the nucleus
The nucleus is generally the most prominent organelle in eukaryotic cells. It is surrounded by a double membrane, known as the nuclear envelope, and communicates with the cytosol via nuclear pores that perforate the envelope. The outer nuclear membrane is continuous with the membrane of the endoplasmic reticulum
What is the ER
The ER is the major site of synthesis of new membranes in the cell. Large areas of the ER have ribosomes attached to the cytosolic surface and are designated rough endoplasmic reticulum (rough ER). On the ribosomes are actively synthesizing proteins that are delivered into the ER membrane or into the ER interior, a space called the lumen.
The smooth endoplasmic reticulum (smooth ER) lacks ribosomes. It is scanty in most cells but is highly developed for performing particular functions in others: for example, it is the site of steroid hormone synthesis in some endocrine cells of the adrenal gland and the site where a variety of organic molecules, including alcohol, are detoxified in liver cells.
Look at GOODNOTES for table
Membrane-enclosed Organelles Evolved in Different Ways
Nuclear membranes and the ER may have evolved through invagination of the plasma membrane. This envelope is presumed to have eventually pinched off completely from the plasma membrane, ultimately producing a nuclear
compartment penetrated by channels called nuclear pores, which enable communication with the cytosol. Other portions of the invaginated membrane may have formed the ER, which would explain why the space between the inner and outer nuclear membranes is continuous with the ER lumen.
Mitochondria are thought to have originated when an aerobic prokaryote was engulfed by a larger pre- eukaryotic cell.
It is virtually certain that mitochondria originate from bacteria that were engulfed by an ancestral pre-eukaryotic cell and survived inside it, living in symbiosis with their host. Note that the double membrane of presentday mitochondria is thought to have been derived from the plasma membrane and outer membrane of the engulfed bacterium.
Protein Sorting
❖For some organelles, including mitochondria, peroxisomes, and the interior of the nucleus, proteins are delivered directly from the cytosol.
❖ For others, including the Golgi apparatus, lysosomes, endosomes, and the inner nuclear membrane, proteins and lipids are delivered indirectly via the ER, which is itself a major site of lipid and protein synthesis.
Proteins Are Transported into Organelles by Three Mechanisms
Nuclear pores (nucleus)
Protein translocators (ER,MITOCHONDRIA, CHLOROPLASTS AND PEROXISOMES)
Vesicular transport (Golgi,lysosomes, cell surface)
The fate of any protein molecule synthesized in the cytosol depends on its amino acid sequence, which can contain a sorting signal that directs the protein to the organelle in which it is required.
Signal Sequences Direct Proteins to the Correct Compartment
Signal sequences are both necessary and sufficient to direct to protein to a particular destination. This has been shown by experiments in which the sequence is either deleted or transferred from one protein to another by genetic engineering techniques
Proteins Enter the Nucleus Through Nuclear Pores
Nuclear envelope- defines nuclear compartment- formed from two concentric membranes
Inner nuclear membrane- contains proteins that act as binding sites for the chromosomes and provide anchorage for the nuclear lamina
Nuclear lamina- protein filaments that provide structural support for the nuclear envelope
Outer nuclear membrane- membrane similar composition as the ER membrane
Nuclear pores- form the gates which all molecules enter of leave the nucleus
Proteins Enter the Nucleus Through Nuclear Pores /cont./
A nuclear pore is a large, elaborate structure composed of a complex of about 30 different proteins .
The signal sequence that directs a protein from the cytosol into the nucleus, called a nuclear localization signal, typically consists of one or two short sequences containing several positively charged lysines or arginines
Many of the proteins that line the nuclear pore contain extensive, unstructured regions in which the polypeptide chains are largely disordered.
Proteins Enter the Nucleus Through Nuclear Pores /cont./
The nuclear localization signal on proteins destined for the nucleus is recognized by cytosolic proteins called nuclear import receptors.
❖Nuclear import receptors interact with the cytosolic fibrils that extend from the side of the pore.
❖After cargo delivery, the receptors return to the cytosol via nuclear pores for reuse.
❖Similar types of transport receptors, operating in the reverse direction, export mRNAs from the nucleus .
Proteins Enter the Nucleus Through Nuclear Pores /cont./
Energy supplied by GTP hydrolysis drives nuclear transport. A nuclear import receptor picks up a prospective nuclear protein in the cytosol and enters the nucleus. There it encounters a small monomeric GTPase called Ran, which carries a molecule of GTP. This Ran-GTP binds to the import receptor, causing it to release the nuclear protein. Having discharged its cargo in the nucleus, the receptor—still carrying Ran- GTP—is transported back through the pore to the cytosol. There, an accessory protein (not shown) triggers Ran to hydrolyze its bound GTP. Ran-GDP falls off the import receptor, which is then free to bind another protein destined for the nucleus.
A similar cycle operates to export mRNAs and ribosomal subunits from the nucleus into the cytosol, using nuclear export receptors that recognize nuclear export signals.
Ran (RAs-related Nuclear protein)
Proteins Unfold to Enter Mitochondria
Mitochondrial precursor proteins are unfolded during import. (A) A mitochondrion has an outer and inner membrane, both of which must be crossed for a mitochondrial precursor protein to enter the organelle. (B) To initiate transport, the mitochondrial signal sequence on a mitochondrial precursor protein is recognized by a receptor in the outer mitochondrial membrane. This receptor is associated with a protein translocator. The complex of receptor, precursor protein, and translocator diffuses laterally in the outer membrane until it encounters a second translocator in the inner membrane. The two translocators then transport the protein across both the outer and inner membranes, unfolding the protein in the process.
Chaperone proteins inside the organelles help to pull the protein across the two membranes and to fold it once it is inside. Phospholipids are transported to these organelles by lipid-carrying proteins that extract a phospholipid molecule from one membrane and deliver it into another.
Proteins Enter Peroxisomes from Both the Cytosol and the Endoplasmic Reticulum
These organelles are present in all eukaryotic cells, where they break down a variety of molecules, including toxins, alcohol, and fatty acids. Proteins do not need to unfold to enter the peroxisome
Although most peroxisomal proteins—including those embedded in the peroxisomal membrane—come from the cytosol, a few membrane proteins arrive via vesicles that bud from the ER membrane. The vesicles either fuse with preexisting peroxisomes or import peroxisomal proteins from the cytosol to grow into mature peroxisomes.
