Secretory Pathway I Flashcards
What is the Endoplasmic reticulum and what are it 3 domains?
The endoplasmic reticulum (ER) is a network of membrane-enclosed tubules and sacs (cisternae) that extends from the nuclear membrane throughout the cytoplasm.
The membrane is continuous and is the largest organelle of most eukaryotic cells.
In mammalian cells most proteins are transferred into the ER while they are being translated on membrane-bound ribosomes.
Others are synthesized on free ribosomes and released into the cytosol.
What type of proteins are translated by free ribosme vs er bound ribosome?
Proteins destined for the cytosol, nucleus, mitochondria, chloroplasts, or peroxisomes are translated in the cytosol.
Proteins destined for secretion, membrane insertion (plasma, peroxisome, or nuclear membranes), Golgi, endosomes, or lysosomes (collectively termed the secretory pathway) are translated on ribosomes bound to the ER. A small subset of ER pathway proteins start out being translated in the cytosol and translocate to the ER.
What are the two ways proteins get into the ER?
How is the ribosome targeted to the ER in Co-translational translation?
What are the 5 steps of Co-translational translocation?
GTP is bound to the SRP and its receptor. Together the SRP and SRP receptor complex form an active GTPase. Once the SRP binds the receptor the active GTPase hydrolyses GTP to GDP + Pi.
How does the post translational translocation process work?
This is a rare mechanism for ER based translation, but similar to the process by which proteins enter organelles such as the mitochondria.
what are the 3 types of integral proteins?
c’ terminal on cytosol side, n’ terminal on cytosol side, and multipass (with c’ on cytosol side)
Transmembrane portions of proteins can be either internal signal sequences or alpha helices.
T or F. Is the topology of the secretory pathway always maintained?
The lumen of the ER is topologically equivalent to the exterior of the cell. This means that if a transmembrane protein has its carboxy terminus in the cytosol and its amino in the lumen while it is newly fashioned in the ER, it will always have the carboxy terminus in the cytosol.
What causes a protein to be embeded in the membrane and which way does it exit the translocon?
An internal stop-transfer sequence and it exits laterally.
An internal stop-transfer sequence (alpha helix) closes the translocon, blocking further translocation of the polypeptide across the ER membrane. The protein exits laterally and becomes anchored in the ER membrane. The remaining C-terminal portion of the peptide is synthesized in the cytosol.
Which part of the protein anchors it into the membrane?
The signal sequence acts as a transmembrane α helix that exits the translocon and anchors the protein in the ER membrane.
The amino terminus has properties (thought to be charged amino acids) that prevent it from being inserted into the lumen(or which drag it back out to the cytosol).
What determines domain topology of the integral protein?
The structure of the internal signal sequence, as well as the charged character of the amino acids proximate to the signal sequence determines whether the amino or carboxy end of the sequence winds up in the lumen or cytosol. Either can happen. In this case, the signal sequence was a stop transfer sequence, as it prevented the transfer of the peptide chain after it entered the translocon.
How are multi-pass transmembrane proteins formed?
Proteins that span the membrane multiple times are inserted by an alternating series of internal signal sequences and transmembrane stop-transfer sequences.
Note how the SRP binds to the internal sequence to bring the ribosome to the translocon. Through a combination of stop transfer and signal sequences, the multi-pass transmembrane proteins are synthesized into the membrane of the ER.
What are 4 important aspects of the processing of proteins in the ER?
What are the functions of the Golgi Apparatus?
What are the 4 regions of the Golgi Apparatus?
The Golgi has 4 regions: cis Golgi network—receives molecules from the ER Golgi Intermediate Compartment (ERGIC)
medial and trans Golgi stacks—most modifications are done here
trans Golgi network—the sorting and distribution center
Stacks of membranes (called cisternae) between the ER and the apical plasmamembrane
What part of the golgi does vesicles fuse with?
ERGIC
Proteins in the lumen are packaged into budding transport vesicles, then released to the lumen of the recipient following vesicle fusion. Membrane proteins and lipids are transported in a similar way; their topological orientation is maintained.
What signals are present to trigger exprot from the ER to the Golgi?
Lumenal proteins are often exported bound to transmembrane proteins.
Note: there may be a default pathway wherein unmarked proteins are simply moved to the Golgi
What are the sequences that trigger the retrieval of proteins back to the ER from the ERGIC or Golgi?
KDEL or KKXX at the C terminus
Note that there may be other such signals that remain undetermined. Certain proteins may be retrieved because they bind to or interact with proteins that contain the retrieval sequences.
What is the process of how proteins move through the Golgi Apparatus?
T or F. Cisternae are stable structures?
TRUUUUU
Describe the proces in which N-linked oligosaccharides are altered?
O‐linked glycosylation, in which sugar residues are one at a time to serine or threonine residues, also occurs in the Golgi. This is usually started with the addition of an NAG residue.
What is the sinal patch sequence for N-linked manose phosphorylation and where does it target to?
—-non-contiguous AA residues and to the lysosomes
This mannose 6 phosphate residue is recognized by a receptor in the trans Golgi Network that sorts these proteins for transport to endosomes and lysosomes.
Please note how the signal patch differs from other signal peptides.
What molecule are spyngomylein and glycolipids synthesized from and where does the synth take place?
—-ceramide and the golgi apparatus
Topologically, sphingomyelin and glycolipids are usually found on the plasma membranes outer surface.
Sphingomyelin is synthesized by transfer of a phosphorylcholine group from phosphatidylcholine to ceramide.
Addition of carbohydrates to ceramide yields a variety of different glycolipids. Thefirst glucose is added to the cytosolic side of ceramide
What are the 3 routes to the cell surface from the Golgi?
—-Constituative secretion (direct transport), Regulated secretory pathway, recyclying endosomes indirectly reach plasma membrane
Regulated pathways include release of hormones from endocrine cells and neurotransmitters from nerve cells.
