Describe the types of coated vesicles and their functions

Coated vesicles are specialized membrane-bound carriers used for transporting proteins and lipids between different compartments inside the cell. They are called "coated" because their outer membrane is covered by specific protein coats that help in vesicle formation, cargo selection and targeting.

There are three main types of coated vesicles: clathrin-coated, COPI-coated and COPII-coated vesicles. Each type is associated with a particular direction and function of transport.

1. Clathrin-Coated Vesicles

Clathrin-coated vesicles are among the most extensively studied vesicles. These are mainly involved in transport between the trans-Golgi network, endosomes and the plasma membrane.

Mechanism:

Clathrin forms a unique structure made of three-legged units known as triskelions, which assemble into a lattice-like cage around the budding vesicle. This coat provides mechanical support and helps in vesicle formation.

Clathrin works together with adaptor proteins, such as AP1 and AP2, which link the clathrin coat to the membrane and assist in selecting specific cargo molecules.

Functions:
  • Endocytosis: Clathrin-coated vesicles internalize molecules from the plasma membrane into the cell. This includes receptor-mediated uptake of important substances like LDL (low-density lipoprotein) and transferrin.
  • Trans-Golgi to endosome transport: These vesicles also transport lysosomal enzymes from the trans-Golgi network to endosomes, especially those tagged with mannose-6-phosphate, which directs them toward lysosomal destinations.

2. COPI-Coated Vesicles

COPI-coated vesicles are mainly involved in retrograde transport i.e., from the Golgi apparatus back to the ER or between Golgi cisternae (from trans to cis side).

Mechanism:

The COPI coat is assembled from coatomer complexes, which are recruited by a small GTP-binding protein called ARF1 (ADP-ribosylation factor 1). This process helps the vesicle to bud and select cargo proteins meant for recycling.

Functions:
  • Transport of escaped ER-resident proteins back to the ER (retrieval pathway).
  • Recycling of Golgi enzymes and maintaining Golgi structure and membrane balance.

3. COPII-Coated Vesicles

COPII-coated vesicles are involved in anterograde transport, meaning movement of materials from the ER to the Golgi apparatus.

Mechanism:

COPII coat proteins include Sec23, Sec24, Sec13 and Sec31, which are assembled on the ER membrane with the help of Sar1, a GTPase that initiates coat formation and vesicle budding.

Functions:
  • Transport of newly synthesized secretory proteins, membrane proteins and lipids from the ER to the cis-Golgi.
  • Sorting of properly folded proteins while preventing misfolded or unassembled ones from moving forward.






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