Structural Biochemistry/Membrane Traffic
Numerous methods such as biochemical and genetic approaches have been used to determine the process of protein secretion and endocytosis. These processes are pertinent as it explores many new issues that involve cell biology and human physiology.
The Golgi is made up of microtubules that yield dispersed mini-stacks that functions for protein secretion. The Golgi complex contains cellular compartments of stack, flattened cisternae. These cellular compartments define the cis, medial, trans of the Golgi network compartments. The cellular compartment, cis-Golgi SNARES shows less active trafficking than trans- Golgi SNARES. Unfortunately, protein localizations in and around the Golgi does not reveal transport direction of vesicles and whether or not they’re about to depart or arrive.
There are two identified class of COPI-coated vesicles. One class contains KDEL receptors which serve as retrograde carriers from the Golgi to the endoplasmic reticulum (ER). The other class carried Golgi-restricted GS28 SNARE protein together with anterograde cargo. An experiment was conducted that used Golgi tethers to isolate the different classes of Golgi-derived transport vesicles. COPI vesicles where generated in vitro and were found to be enriched in Golgin-84. CASP, a Golgin can bind to Golgin- 84 and localize to the cisternal membrane. CASP vesicle lacks members of a p24 family but is substituted with Golgi enzymes and mannosidase I and II. This substitution proves that these vesicles are retrograde carriers for transport. In contrast, p115 protein vesicles were enriched in p24 family members, cargo proteins and Ig receptor but not mannosidase I and II. This suggests that the latter vesicles are anterograde carriers from within the early Golgi. Vesicle markers have been used for vesicle isolation and characterization.
Other proteins on the vesicle, such as the ArfGAP, functions as a structural coat component. Another important protein is the COG (conserved oligomeric Golgi complex) which has been revealed that the loss of this subunit leads to hypoglycosylation of multiple classes of proteins due to mislocalization of certain glycostyltransferases.
Because the Golgi is structurally composed of the stack by lateral fusion, it is poised for homotypic fusion. It also undergoes fission reaction when microtubules depolymerize.
Coat proteins on membrane surface create stabilization and new generation of membrane curvature. A BAR domain takes on a banana shape that is best adapted to interact with acidic, curved lipid membranes. ArtGAP1 becomes concentrated at the edge of a promising bud and influences the amount of Art-GTP there. GTPase, specifically Dynamin plays a critical role in cellular membrane pinching.
Studies show a satisfying framework that helps explain vesicle and budding from the compartment. But more studies are still underway as detailed explanations of membrane traffic are still needed.
Proteins that get secreted undergo translation at the endoplasmic reticulum (ER) where they get folded, modified and later transported in vesicles to the Golgi. Once at the Golgi, proteins undergo modification once again before they are exported to either the cell surface, the endocytic compartment or back to the ER. SNARE proteins. Transport intermediates vesicles contain SNARE proteins that promote membrane fusion in target proteins. Tethering factors and SNARE proteins work together to facilitate the docking and fusion process of cell transportation. Tethering factors interact with SNARE and also facilitate in SNARE assembly. Multiple copies of SNARE are required in order to initiate bilayer fusion. These SNARE complexes are recycled after completion of membrane transfusion. SNARE proteins provide specificity for membrane traffic because they are not allowed to engage with other components outside of the cell.
Members of the Rab and Arf branches of the Ras GTPase superfamily are present in every step of intracellular membrane traffic. They regulate these steps by networking with one another through a variety of mechanisms that coordinate independent events of one stage together with other stages of the entire transport pathway. These mechanisms include many different variables:
- GEFs cascades
- GAPs cascades
- effectors that bind many GTPases
- positive feedback loops stemming from exchange factor-effector interactions.
When these mechanisms come together, an ordered series of transitions from one GTPase to the next can take place. Since each GTPase has its own unique group of effectors, the transitions that occur can help define differences in the functionality of the membrane compartments that they are associated with.
Dynamin is a considered model for large GTPases. It is responsible for endoctyosis, a process in which cells absorb molecules by engulfment. Specifically, it is involved in the division of newly formed vesicles from the membrane of one compartment to their fusion with another compartment-- at both the cell surface or Golgi body. Along with division of vesicles, Dynamin is also involved in the division of organelles, cytokinesis, and pathogen resistance (microbial). In mammals, there are 3 different types of genes:
1. Pfeffer, Suzanne. “Protein Unsolved Mysteries in Membrane Traffic. Annual Review of Biochemistry. Vol. 76: 629-645 (Volume publication date July 2007) DOI: 10.1146/annurev.biochem. 76.061705.130002.
2. Mizuno-Yamasaki, E., F. Rivera-Molina, and et al. "GTPase networks in membrane traffic.." Pub Med. N.p., 29 2012. Web. 7 Dec 2012. <http://www.ncbi.nlm.nih.gov/pubmed/22463690>.
3. "Dynamin." Wikipedia. Wikimedia Foundation, Inc. 2 Apr 2012. Web. 7 Dec 2012. <http://en.wikipedia.org/wiki/Dynamin>