Study of Hip1r: insights from a Dictyostelium discoideum clathrin adaptor
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The transport of material across the plasma membrane and between subcellular compartments is a major trafficking event by which cells communicate, regulate developmental processes and maintain homeostasis. Clathrin-coated vesicles select and traffic proteins to specific compartments in eukaryotic cells. Recently a large number of proteins have been identified that serve as clathrin adaptors and accessory proteins. Information regarding the interaction between individual clathrin adaptors and accessory proteins during coated pit formation is currently lacking. Here we investigated the intracellular role of one clathrin adaptor, Dictyostelium discoideum Hip1r, and identified a functional relationship between Hip1r and a second clathrin adaptor, epsin. Hip1r is phosphorylated and localizes to punctae on the plasma membrane that also contain epsin. Moreover, expression of the NH₂-terminal ENTH domain of epsin alone was sufficient to restore both the phosphorylation and the restricted localization of Hip1r to the plasma membrane. Analysis of the individual Hip1r domains demonstrated the phosphorylation event is also dependent upon the expression of the central coiled-coil region of the Hip1r. During development, Hip1r null cells form mature fruiting bodies that yield defective spores. While the mutant spores contain both cellulose and at least one protein secreted from prespore vesicles, spore coats lack the organized fibrils typical of wild type spores. Moreover, Hip1r spores are round, rather than ovoid, and exhibit decreased viability. Domain analysis of Hip1r in conjunction with investigation of phenotypes associated with a Hip1r/epsin double mutant reveal a requirement for full length Hip1r in the production of robust spores. Results from this study suggest that the Hip1r protein functions with epsin during cellular events in both growing and developing Dictyostelium cells and reveals a previously unidentified interaction between two clathrin adaptors.