Histology Wk 4 Flashcards
Name some membranous and non membranous protein complexes
Membranous- mitochondria
Non membranous- ribosomes and proteasomes
What are ribosomes
About 20 x 30 nm in size
They assemble proteins from a.a on molecules of tRNA in a sequence specified by mRNA
What are the two subunits of ribosomes bound to a strand of mRNA
The core of the small ribosomal subunit is highly folded rRNA chain associated with more than 30 unique proteins
The core of the large subunit has three other rRNA molecules and nearly 50 other basic proteins
What do the rRNA molecules do in the ribosomal subunits
Provide structural support
Catalyse the formation of the peptide bonds
The more peripheral proteins of the ribosome seem to function primarily to stabilise the catalytic RNA core
What are polyribosomes or polysomes
During protein synthesis many ribosomes typically bind the same strand of mRNA to from larger complexes
Why are polyribosomes intensely basophilic
Numerous phosphate groups of the constituent RNA molecules that act as poly anions
What are free polyribosomes
Synthesise cytosolic and cytoskeletal proteins and proteins for the import into the nucleus, mitochondria and peroxisomes
What are ER bound polyribosomes
Stored in lysosomes or eventually secreted from the cell are made on polysomes attached to the membranes of ER.
The proteins produced by these ribosomes are segregated during translation into the interior of the ER’s membrane cisternae
In both pathways misfolded proteins are conjugated to ubiquitin and targeted to proteasomal degradation
What is the endoplasmic reticulum
Extends from the surface of the nucleus throughout most of the cytoplasm and encloses a series of inter communicating channels called cisternae
What is the RER
Prominent in cells specialised for protein synthesis
What is the function of the RER
Production of membrane associated proteins of many membranous organelles and proteins to be secreted by exocytosis
Includes the initial glycosylation of glycoproteins, certain postranslational modifications of newly formed polypeptides and the assemble of multi chain proteins
What are the functions of the ER
Synthesis- provides a place for chemical reactions
A. Smooth ER is the site of lipid synthesis and carbohydrate metabolism
B. Rough ER synthesises proteins for secretion, incorporation into the plasma, membrane and as enzymes with lysosomes
Transport- moves molecules through the cisternae space from one part of the cell to another
Storage- stores newly synthesised molecules
Detoxification- smooth ER detoxifies drugs and alcohol
Explain the movement of polypeptides into the RER
The newly translated amino terminus of a protein to be incorpo- rated into membranes or sequestered into vesicles contains 15-40 amino acids that include a specific sequence of hydrophobic residues comprising the signal sequence or signal peptide. This sequence is bound by a signal-recognition particle (SRP), which then recognizes and binds a receptor on the ER. Another recep- tor in the ER membrane binds a structural protein of the large
ribosomal subunit, more firmly attaching the ribosome to the ER.
The hydrophobic signal peptide is translocated through a protein pore (translocon) in the ER membrane, and the SRP is freed for reuse. The signal peptide is removed from the growing protein by a peptidase and translocation of the growing polypeptide contin- ues until it is completely segregated into the ER cisterna.
What is the smooth endoplamsic reticulum (SER)
Regions of ER that lack bound polyribosomes make up the smooth endoplasmic reticulum (SER), which is continuous with RER but frequently less abundant
What are the functions of the SER
Enzymes in the SER perform synthesis of phospholipids and steroids, major constituents of cellular membranes
How are these lipids synthesised
These lipids are then transferred from the SER to other membranes by lateral diffusion into adjacent membranes, by phospholipid transfer proteins, or by vesicles which detach from the SER for movement along the cytoskeleton and fusion with other membranous organelles. In cells that secrete steroid hormones (eg, cells of the adrenal cor- tex), SER occupies a large portion of the cytoplasm
How does detoxification occur
including those of the cytochrome P450 family, allow detoxification of potentially harmful exogenous molecules such as alcohol, barbiturates, and other drugs.
In liver cells, these enzymes also process endogenous molecules such as the components of bile.
How is the SER involved in the sequestrian and controlled release of ca2+
which is part of the rapid response of cells to various stimuli.
This function is par- ticularly well developed in striated muscle cells, where the SER has an important role in the contraction process and assumes a specialized form called the sarcoplasmic reticulum
What are eryhtroblasts
Cells that make few or no proteins for secretion have very little RER, with essentially all polyribosomes free in the cytoplasm
What is an eosinophilic leukocyte
Cells that synthesize, segregate, and store various proteins in specific secretory granules or vesicles always have RER, a Golgi apparatus, and a supply of granules containing the proteins ready to be secreted.
What is a plasma cell
Cells with extensive RER and a well-developed Golgi apparatus show few secretory granules because the proteins undergo exocytosis immediately after Golgi processing is complete. Many cells, especially those of epithelia, are polarized, meaning that the distribution of RER and secretory vesicles is different in various regions or poles of the cell
What are epithelial cells
specialized for secretion have distinct polarity, with RER abundant at their basal ends and mature secretory gran- ules at the apical poles undergoing exocytosis into an enclosed extracellular compartment, the lumen of a gland.
Functions of the Golgi in the RER
• New proteins are translocated into ER cisternae
• Preassembled mannose-rich oligosaccharides are added to specific asparagine residues (N-linked)
• Proteins are folded, guided by chaperones, with strict quality control
• Disulfide bonds are formed between specific cysteine residues
What is the cis Golgi network
Vesicle movement from RER and forward through the CGN is
promoted by the coat protein COP-II
• Similarly, COP-I controls retrograde vesicle movements
• Mannose-6-phophate is added to future lysosomal enzymes
• N-linked oligosaccharides are trimmed and other sugars added
What is the medial Golgi cisternae
New glycosylation occurs on –OH groups of some lipids
and serine and threonine residues (O-linked)
• N-linked oligosaccharides on proteins are modified further
• Glycoproteins and glycolipids are sorted into specific vesicles
What is trans Golgi network
Sialic acid is added as the terminal sugar to certain
oligosaccharides
• Sulfation of tyrosine residues and some sugars occurs
• Specific vesicles with different destinations are separated and
sorted
