Secretory pathway Flashcards
Golgi compartments mammalian cells
- ERGIC
- cis-Golgi
- medial Golgi
- trans Golgi
- TGN (trans Golgi network)
Each comportment is characterized by a distinct set of membrane-bound enzymes.
No resident luminal proteins known!
Structure of the Golgi apparatus in different Eukaryotes
- S. cerevisiae no general stack-like structure
- plants and invertebrates stacks that are scattered throughout the cytoplasm
- mammalian stacks are concentrated at MTOC (Microtubule organizing center) and laterally connected by tubular bridges
Protein packing into Golgi-bound structures
Two modes:
I. sorting of proteins into vesicles or vesicular-tubular carriers destined for the Golgi in ER-exit sites (soluble factors via interaction with membrane receptors)
II. proteins enter exit sites and are packed into carriers by chance (bulk flow)
The COPII coat in mammalian cells can have different curvatures
Allows the formation of small vesicles (50 nm) and of large membrane carriers (300 … 500 nm) with patches that have almost planar membrane surfaces which could carry pro collagen or chylomicrons.
In the ER, more general factors (TANGO1) and substrate specific ones (TALI for Lipid cargos) have been described. Many details (role of outer coat layer, or ERGIC of other factors like MIA13) are unknown.
Models for ER to Golgi transport in vertebratae
a) Transport via stationary intermediate compartment (IC)
b) Transcport carrier form transient IC to cis-Golgi
c) Transport carrier form transient ICs, fuse and thus form new cis-Golgi
d) Transport via transient tubules between ER and cis-Golgi
The process of back-transport is not included in the schemes.
ER-Golgi intermediate compartment (ERGIC) in mammalia
ERGIC here is understood as a “maturating” VTC en route between ER and Golgi
Maintenance of ER protein composition
Two systems:
- Retention of proteins
- Retrieval of proteins
RETENTION
- due to direct or indirect association with cytoplasmic or nucleoplasmic elements (ribosomes, lamins etc) disfavoring entry into exit sites; indirect association could be driven by Ca2+ dependent interactions
- due to TMD with weak hydrophobicity
RETRIEVAL - needs sorting signals
- KDELcooh for luminal proteins (recognized by KDEL-receptor family)
- others (recognized by Erv41/46)
- signal in TM of proteins (recognized by Rer1)
- KKXXcooh for membrane proteins and
- XXRR - for Type II membrane proteins (both recognized directly by COPI-coat)
Transport of Golgi-derived vesicles
RAB6-marked vesicles have been implicated in Golgi-to-ER, but also in intra-Golgi, and Golgi-derived exocytic vesicle trafficking
Foreward and recycling transport are balanced
Early hypothesis: BFA somehow blocks forward transport
-> recycling transport outweighed
-> Golgi disappears
As expected, dominant-negative versions of SAR1 that block COPII assembly rapidly disrupt Golgi structure and function.
But — brefeldin A blocks ARF-GEF = crucial element for back-transport!!!
New hypothesis: BFA = impaired COPI assembly
-> pure retrieval of factors needed for COPII coat assembly
-> block of ER to Golgi transport
OR
-> no COPI on Golgi
-> no repression of SNARE interaction
-> unregulated back fusion of Golgi to ER
Golgi matrix maintains the structure of the Golgi
Involved are large tethering proteins (Giantins, GMs and GRASPs) which bind transmembrane enzymes typical for the respective Golgi stack; binding is modulated by RABs and their regulators.
Factors involved in structural maintenance are also tethering factors involved in the intra-Golgi transport
- COPI vesicle
- Giantin
- p115
- GM130
- GRASP65
- Rab1
Maintenance of proteins in the Golgi
- motifs in cytoplasmic tail (esp. for TGN localization)
- motifs in membrane anchors
- “kin recognition” - specific affinity of Golgi-membrane proteins for each other (forming domains that are too large to be included in transport vesicles)
PQC in the Golgi
- targets obviously only membrane proteins
- defect proteins are Ubylated and than either directed to the lysosome or extracted and degraded by the proteasome in the cytoplasm
Sorting of proteins in the TGN
-> PM
- apical
- basolateral
- …
-> secretory granules
- different versions in one cell possible
-> endo-/lysosomal System
- early endosome
- recycling endosome
- late endosome (to lysosome/vacuole)
- ..
- storage vacuole
- lytic vacuole
also sorting of lipids (e.g. sphingolipids or sterols)
Sorting in a mammalian TGN
The TGN is composed of tubules emanating from the last two trans-Golgi cisternae. Only tubules deriving from the last cisterns are Cathrin coated. The ER makes close contact with the last two trans-Golgi cisternae, through which lipid exchange can occur. Proteins arrive from earlier Golgi compartments and from the endocytic pathway. The exit routes include those towards the apical PM, the basslateral PM, recycling endosomes, early endosomes, late endosomes and specialized compartments such as secretory granules in secretory cells. These are the main destinations, and for each of them more than one type of carrier might be involved; for example, basslateral transmembrane and soluble trafficking proteins can use different carriers. Golgi-resident proteins (for example, glycosylating enzymes) recycle back to the Golgi stack, as secretion consumes the last trans- Golgi cistarnae.
