This is the C2 domain-like domain, in greek key form, of the PTEN protein, phosphatidyl-inositol triphosphate phosphatase, and it is the C-terminus. This domain may well include a CBR3 loop which means it plays a central role in membrane binding. This domain associates across an extensive interface with the N-terminal phosphatase domain DSPc (Pfam:PF00782) suggesting that the C2 domain productively positions the catalytic part of the protein onto the membrane [1].

1: Crystal structure of the PTEN tumor suppressor: implications for its phosphoinositide phosphatase activity and membrane association. Lee JO, Yang H, Georgescu MM, Di Cristofano A, Maehama T, Shi Y, Dixon JE, Pandolfi P, Pavletich NP; Cell. 1999;99:323-334. PMID:10555148