transport vesicles
Membrane traffic occurs by vesicular transport between successive compartments of the secretory pathway.
Coat complexes function to collect cargo from donor membranes and deform them to generate transport vesicles with a diameter of 60-80 nm.
Sorting of Proteins to Transport Vesicles
The first step in transport involves the selection of passengers. Proteins destined for transport and the vesicle-targeting machinery are actively recruited to specific sites on the limiting membrane of the organelle where transport intermediates bud off.
This process takes place with the aid of cytosolic complexes of coat protein that directly interact with transmembrane proteins or receptors for luminal molecules.
Along with proteinaceous regulatory factors, the lipid composition of the membranes has an important role in the recruitment of coat proteins and the budding of transport vesicles.
The inclusion of proteins in transport intermediates involves the interpretation of hierarchical sorting signals in the molecules to be transported.
These represent biochemically different recognition principles, including specific amino acid determinants, saturated fatty acyl moieties, and carbohydrates that can be recognized by lectin-like receptors.
Furthermore, dynamic lipid domains formed by cholesterol and glycosphingolipids are instrumental in the sorting of certain membrane-associated proteins.
Together, these different processes generate an extensive integrated network of sorting events, the outcome of which is a fine-tuned dynamic equilibrium in the distribution of cellular constituents.
References
Palmer KJ, Stephens DJ. Biogenesis of ER-to-Golgi transport carriers: complex roles of COPII in ER export. Trends Cell Biol. 2004 Feb;14(2):57-61. PMID: #15106609#
Olkkonen VM, Ikonen E. Genetic defects of intracellular-membrane transport. N Engl J Med. 2000 Oct 12;343(15):1095-104. PMID: #11027745#