We have examined the in vitro behavior of clathrin-coated vesicles that have been stripped of their surface coats such that the majority of the clathrin is removed but substantial amounts of clathrin assembly proteins (AP) remain membrane-associated. Aggregation of these stripped coated vesicles (s-CV) is observed when they are placed under conditions that approximate the pH and ionic strength of the cell interior (pH 7.2, ∼100 mM salt). This s-CV aggregation reaction is rapid (t1/2 ≤ 0.5 min), independent of temperature within a range of 4-37°C, and unaffected by ATP, guanosine-5′-Ο-(3-thiophosphate), and in particular EGTA, distinguishing it from Ca2+-dependent membrane aggregation reactions. The process is driven by the action of membrane-associated AP molecules since partial proteolysis results in a full loss of activity and since aggregation is abolished by pretreatment of the s-CVs with a monoclonal antibody that reacts with the α subunit of AP-2. However, vesicle aggregation is not inhibited by PPPi, indicating that the previously characterized polyphosphate-sensitive AP-2 self-association is not responsible for the reaction. The vesicle aggregation reaction can be reconstituted: liposomes of phospholipid composition approximating that found on the cytoplasmic surfaces of the plasma membrane and of coated vesicles (70% L-α-phosphatidylethanolamine (type I-A), 15% L-α-phosphatidyl-L-serine, and 15% L-α-phosphatidylinositol) aggregated after addition of AP-2, but not of AP-1, AP-3 (AP180), or pure clathrin triskelions. Aggregation of liposomes is abolished by limited proteolysis of AP-2 with trypsin. In addition, a highly purified AP-2a preparation devoid of β causes liposome aggregation, whereas pure β subunit does not, consistent with results obtained in the s-CV assay which also indicate the involvement of the α subunit. Using a fluorescence energy transfer assay we show that AP-2 does not cause fusion of liposomes under physiological solution conditions. However, since the fusion of membranes necessarily requires the close opposition of the two participating bilayers, the AP-2-dependent vesicle aggregation events that we have identified may represent an initial step in the formation and fusion of endosomes that occur subsequent to endocytosis and clathrin uncoating in vivo.
|Original language||English (US)|
|Number of pages||10|
|Journal||Journal of Cell Biology|
|State||Published - Nov 1992|
ASJC Scopus subject areas
- Cell Biology