Structure of the PTEN-like region of auxilin, a detector of clathrin-coated vesicle budding

Abstract

Auxilin, a J-domain containing protein, recruits the Hsc70 uncoating ATPase to newly budded clathrin-coated vesicles. The timing of auxilin arrival determines that uncoating will commence only after the clathrin lattice has fully assembled and after membrane fission is complete. Auxilin has a region resembling PTEN, a PI3P phosphatase. We have determined the crystal structure of this region of bovine auxilin 1; it indeed resembles PTEN closely. A change in the structure of the P loop accounts for the lack of phosphatase activity. Inclusion of phosphatidylinositol phosphates substantially enhances liposome binding by wild-type auxilin, but not by various mutants bearing changes in loops of the C2 domain. Nearly all these mutations also prevent recruitment of auxilin to newly budded coated vesicles. We propose a specific geometry for auxilin association with a membrane bilayer and discuss implications of this model for the mechanism by which auxilin detects separation of a vesicle from its parent membrane.

Figure 1. Domain organization of auxilin and description of mutants

(A) Domain organization of the neuron-specific auxilin 1, highlighting regions that interact with its most important effector molecules. (B) List of mutants used in this work for in vitro lipid binding studies of the PTEN-like region of auxilin and for in vivo studies by live-cell imaging.

Figure 2. Overall structure of the PTEN-like region of auxilin 1

(A) Ribbon diagram showing the structure of auxilin 1 (40-400). The phosphatase-like region is in blue; the C2 domain, in orange. The views in A are related by a 180° rotation about the vertical axis. Figure made with Chimera. (B) Representative example of the σA-weighed 2Fo-Fc electron density map, showing a region of the interface between the phosphatase-like and the C2-domain regions contoured at 1σ. (C) Sequence of bovine auxilin 1 highlighting the elements of secondary structure; the nomenclature follows the description of PTEN (Lee et al., 1999).

Figure 2. Overall structure of the PTEN-like region of auxilin 1

(A) Ribbon diagram showing the structure of auxilin 1 (40-400). The phosphatase-like region is in blue; the C2 domain, in orange. The views in A are related by a 180° rotation about the vertical axis. Figure made with Chimera. (B) Representative example of the σA-weighed 2Fo-Fc electron density map, showing a region of the interface between the phosphatase-like and the C2-domain regions contoured at 1σ. (C) Sequence of bovine auxilin 1 highlighting the elements of secondary structure; the nomenclature follows the description of PTEN (Lee et al., 1999).

Figure 3. Interaction in vitro of the auxilin 1 PTEN-like region with specific phosphoinositides

(A) Ribbon diagram of the PTEN-like region of auxilin showing the location of residues within the C2 region we have mutated to test their role in lipid binding. The model is oriented to show the proposed orientation of this part of auxilin with respect to the cell membrane. (B) The upper panel shows Coomassie Blue staining after SDS-PAGE fractionation of wild type PTEN-like region of auxilin 1 bound to liposomes containing either PC only or varying amounts (% w/w) of the indicated lipids. The bottom panel shows a bar plot comparing the average relative binding obtained from two independent experiments. To facilitate the comparison, the data were normalized to zero (binding to liposomes containing only PC) and 100% (binding to liposomes containing 15% PI4P). (C) Bar plot comparing the relative binding (average +/- std deviation) of wild-type and mutants forms of the PTEN-like region of auxilin to liposomes containing 85% and 15% PI4P. The results in each bar are from three independent experiments.

Figure 4. The PTEN-like region of auxilin is required for the major recruitment to endocytic clathrin-coated structures

(A) Live-cell imaging time series in TIRF illumination presented as kymograph views, with frames acquired every 1.6 s, from BSC1 monkey cells expressing a wild-type EGFP-auxilin 1 chimera (construct #1, Aux1 wt), an EGFP-auxilin1 mutant with three point mutations in its PTEN-like region (construct #3, Aux1 R301E/R307E/K311E), or a EGFP-auxilin 1 deletion mutant lacking the entire PTEN-like region (construct #0, ΔPTEN Aux1); a clathrin light chain LCa-cherry was co-expressed in each case. All constructs were expressed transiently for 20 h. Selected examples are presented with equally normalized fluorescence intensities. (B) The bar plot compares the distribution of auxilin recruitment bursts in cells expressing EGFP chimeras of wild-type auxilin with the distribution in cells expressing mutants within the PTEN-like region of EGFP-auxilin. As shown previously (Massol et al., 2006), auxilin bursts are seen only in coated vesicles just after they have pinched off from the plasma membrane and just before disassembly.