Notes on Golgi Apparatus
Introduction to Golgi Apparatus
- The Golgi apparatus consists of flattened sacs known as cisternae.
- Unlike the endoplasmic reticulum (ER), the cisternae of the Golgi are not physically connected.
- Main Functions:
- Receiving products from the ER
- Sorting them into different groups
- Tagging for transportation (like a shipping address)
- Transportation to the destination
Structure and Directionality
- The Golgi apparatus has two main ends:
- Cis End: Receiving side, accepts products from the ER.
- Trans End: Shipping side, dispatches products to other organelles or the plasma membrane.
- Proteins can sometimes move backwards (retrograde) to previous cisternae or back to the ER.
Models of Transport
1. Vesicular Transport Model
- Proteins move in vesicles from one cisterna to the next.
- Vesicles are produced in the ER, fuse together to form a tubular cluster, and then enter the Golgi.
- Movement occurs as follows:
- From cis to trans in vesicular forms.
- Proteins may return to the previous cisterna if they are not matured properly.
2. Cisternal Maturation Model
- The entire cisterna moves from the cis phase to the trans phase, carrying its contents with it.
- Proteins undergo maturation (sorting and tagging) as the cisternae transit through the Golgi.
- Backward movement in vesicular forms is still possible.
Movement of Vesicles
From ER to Golgi (Anterograde Transport)
- Mediated by COPII vesicles which are coated with COPII proteins.
- Two types of cargo:
- Soluble proteins (cargo means transported proteins).
- Membrane-bound cargo (integrated into the vesicle membrane).
- Interaction with SNARE proteins (v-SNARE and t-SNARE) facilitates docking and fusion with the Golgi.
From Golgi to ER (Retrograde Transport)
- Mediated by COPI vesicles which are coated with COPI proteins.
- Proteins may return for several reasons:
- Mis-sorted proteins.
- Proteins needed back in the ER.
- Similar docking mechanism involving SNARE proteins.
SNARE Proteins
- V-SNARE: Found on vesicle surface.
- T-SNARE: Found on target membrane (e.g., Golgi).
- Rab GTPases: Assist in the tethering process and docking of vesicles to membranes.
Tethering and Fusion Process
- Tethering: Interaction between Rab GTPases on vesicle and effector proteins on target membrane.
- Docking: Interaction between v-SNARE and t-SNARE proteins.
- Fusion: Membranes merge, releasing cargo into the target compartment.
Types of Coat Proteins
- COPII: For anterograde transport from ER to Golgi.
- COPI: For retrograde transport from Golgi to ER.
- Clathrin: Used in transport from trans Golgi to the plasma membrane or endosomes and during endocytosis.
Conclusion
- Understanding the Golgi apparatus is essential for grasping cellular transport processes.
- Encouragement for questions to clarify concepts discussed.
Thank you for your time and understanding!