Figure 3. C2 domains have a β-sandwich structure with loop regions adjacent to the “cationic patch”.

These structures have been crystallized with or without inositol headgroups and demonstrate PI-binding through interaction with positively-charged residues in the cationic patch region. PI-binding residues are depicted in blue. Dark blue residues represent positively charged lysines or argininges and light blue are other inositol-headgroup coordinating residues. Residues highlighted in magenta are PS-binding, residues highlighted in green are membrane-inserting, and residues highlighted in red are negative and calcium-coordinating. A. PKCα C2 (3GPE) coordinates PS with Asn¹⁸⁹, Arg²¹⁶, Arg²⁴⁹, and Thr²⁵¹ where the sulfate is coordinated in the structure (orange and red spheres). In this loop region, negatively charged residues coordinate 3 calcium inos (green spheres) which are crucial to its membrane localization. The PIP₂ headgroup is coordinated by Tyr¹⁹⁵, Lys¹⁹⁷, Lys²⁰⁹, Lys²²¹, Trp²⁴⁵, and Asn²⁵³ in the cationic patch region. B. KIBRA C2 (2ZOU) harbors a cationic patch region with Arg⁷⁵³, His⁷⁵⁵, Arg⁷¹⁴, Arg⁷¹⁶, Lys⁶⁹², and Arg⁶⁹⁶. In addition, Cys⁷⁷¹ participates in a disulfide bond between two monomers in the structure. While the molecular basis of WT KIBRA binding to monophosphorylated PIPs such as PI(3)P is unknown, mutation of Met⁷³⁴ or Ser⁷³⁵ alters the C2 domain affinity in such a way it increases the affinity of the C2 domains for PI(4)P, PI(5)P, and PI(4,5)P₂ (Duning et al. 2013). C. Rabphilin C2A bound to IP₃ (4NP9) coordinates the inositol headgroup in the cationic patch region using Tyr⁴²¹, Lys⁴²³, His⁴²⁵, Lys⁴³⁵, Arg⁴³⁷ and Asn⁴⁸¹.