Ascaffold protein required to hold membrane proteins inside microvilli is surprisingly dynamic, Garbett and Bretscher reveal.
Microvilli are actin-rich membrane protrusions on the apical surface of epithelial cells. Microvillar assembly requires both ezrin, a protein that links actin to the plasma membrane, and EBP50, a scaffold protein that binds tightly to ezrin and couples it to a variety of membrane proteins like podocalyxin that interact with EBP50’s PDZ domains. Individual microvilli last for 10–15 minutes before disassembling, so Garbett and Bretscher decided to compare the dynamics of different microvillar proteins.
Photobleaching experiments revealed that ezrin turned over fairly slowly in microvilli and that podocalyxin was even more stably associated with the apical protrusions. But EBP50 was much more dynamic within microvilli, turning over rapidly despite its tight association with ezrin and its role in retaining podocalyxin in the microvillar membrane. Even more surprisingly, mutating EBP50’s PDZ domains to inhibit their interactions with membrane proteins stabilized EBP50 inside microvilli instead of accelerating the protein's turnover still further.
The researchers recapitulated their findings in vitro. EBP50 attached firmly to ezrin-coated beads, but its grip was loosened in the presence of epithelial cell extract. An EBP50 mutant with nonfunctional PDZ domains maintained its hold on ezrin, however.
EBP50’s scaffolding function may therefore be self-limiting, its association with ezrin weakening as it retains increasing numbers of membrane proteins in microvilli. The authors now want to identify the components of the epithelial cell extract that destabilize EBP50’s interaction with ezrin.