Crystal structure of the ARF-GAP domain and ankyrin repeats of PYK2-associated protein beta

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Abstract
ADP ribosylation factors (ARFs), which are members of the Ras superfamily of GTP‐binding proteins, are critical components of vesicular trafficking pathways in eukaryotes. Like Ras, ARFs are active in their GTP‐bound form, and their duration of activity is controlled by GTPase‐activating proteins (GAPs), which assist ARFs in hydrolyzing GTP to GDP. PAPβ, a protein that binds to and is phosphorylated by the non‐receptor tyrosine kinase PYK2, contains several modular signaling domains including a pleckstrin homology domain, an SH3 domain, ankyrin repeats and an ARF‐GAP domain. Sequences of ARF‐GAP domains show no recognizable similarity to those of other GAPs, and contain a characteristic Cys‐X2‐Cys‐X16–17‐Cys‐X2‐Cys motif. The crystal structure of the PAPβ ARF‐GAP domain and the C‐terminal ankyrin repeats has been determined at 2.1 Å resolution. The ARF‐GAP domain comprises a central three‐stranded β‐sheet flanked by five α‐helices, with a Zn2+ ion coordinated by the four cysteines of the cysteine‐rich motif. Four ankyrin repeats are also present, the first two of which form an extensive interface with the ARF‐GAP domain. An invariant arginine and several nearby hydrophobic residues are solvent exposed and are predicted to be the site of interaction with ARFs. Site‐directed mutagenesis of these residues confirms their importance in ARF‐GAP activity.