An image correlation analysis of the distribution of clathrin associated adaptor protein (AP-2) at the plasma membrane

Clathrin associated adaptor protein is involved in endocytosis at the plasma membrane (AP-2) and protein sorting at the Golgi membrane (AP-1). There is a great deal of information available on the structure, function and binding characteristics of AP-2, however, there is little quantitative data on the AP-2 distribution at the membrane. Image correlation spectroscopy is a technique which yields number counts from an autocorrelation analysis of intensity fluctuations within confocal microscopy images. Image correlation spectroscopy analysis of the indirect immunofluorescence from AP-2 at the plasma membrane of CV-1 cells shows that AP-2 is in a bimodal distribution consisting of large coated pit associated aggregates of approximately 60 AP-2 molecules, and smaller aggregates containing approximately 20 AP-2 molecules, which we propose are coated pit nucleation sites. Following hypertonic treatment 25% of the AP-2 molecules dissociate from the large AP-2 aggregates and form AP-2 dimers, leaving the remaining AP-2 as large aggregates with approximately 45 molecules. The smaller AP-2 aggregates completely dissociate forming AP-2 dimers. Dispersion of AP-2 with hypertonic treatment is not seen qualitatively because the number of large AP-2 aggregates is unchanged, the aggregates are just 25% smaller. Change in temperature from 37 degrees C to 4 degrees C has no affect on the number of AP-2 aggregates or the AP-2 distribution between the two populations. These data and estimates of the coated pit size suggest that coated pits cover approximately 0.9% of the cell membrane. Combination of image correlation spectroscopy analysis and measurements of the CV-1 cell surface area show that there are approximately 6x10(5) AP-2 molecules per CV-1 cell with approximately 2x10(5) AP-2 molecules within coated pit structures.