This domain is part of the protein Nicastrin, a component of gamma secretase present in Homo sapiens. Gamma-secretase is thought to contribute to Alzheimer's disease development by generating beta-amyloid peptides. This domain is the known as the small lobe which forms the 'lid'. The lid is an extended surface loop that covers the hydrophilic pocket that is thought to be responsible for substrate recruitment. On substrate binding, the large lobe is thought to rotate relative to the small lobe [1].

1: An atomic structure of human gamma-secretase. Bai XC, Yan C, Yang G, Lu P, Ma D, Sun L, Zhou R, Scheres SHW, Shi Y; Nature. 2015;525:212-217. PMID:26280335

2: Crystal structure of the gamma-secretase component nicastrin. Xie T, Yan C, Zhou R, Zhao Y, Sun L, Yang G, Lu P, Ma D, Shi Y; Proc Natl Acad Sci U S A. 2014;111:13349-13354. PMID:25197054