Novel insights into cellular trafficking pathways
22 Nov 2016
Understanding cellular trafficking pathways involving cell surface protein “Clathrin”
Cell-fate determination, self and non-self discrimination and nutrient uptake from the cellular microenvironment are phenomena, which rely on a finely controlled display of membrane proteins on the cell surface. The cell surface localization of most membrane proteins is a balanced outcome of complex trafficking pathways. Most membrane proteins are taken up from the cell surface by a protein called clathrin, which assembles into a basket-shaped structure to bud-out a portion of the cell surface membrane in a membrane protein-laden, clathrin-coated pit. Clathrin does not directly interact with the membrane but requires specialized adaptors for its recruitment to and subsequent build-up of a clathrin-coat on the cell membrane. Clathrin adaptors recognize clathrin via short sequence motifs; referred to as clathrin boxes and adaptors display diversity both in the numbers and types of evolutionarily conserved clathrin-binding boxes. How this diversity relates to the process of clathrin assembly during the growth of a coated pit remains unclear. Using real-time, fluorescence microscopy-based assays, we directly reconstitute the process of adaptor-mediated clathrin assembly using five unique clathrin adaptors. Remarkably, we find that clathrin assembly selectively leads to the clustering of some but not all adaptors. Furthermore, our results indicate that adaptor clustering is determined not by the amount of clathrin recruited or the degree of clathrin clustered but instead by the rate of clathrin assembly thus emphasizing the need to invoke kinetics of protein interactions to better understand cellular trafficking pathways.