This is a helical domain found in NLRC4 , Nucleotide-binding and oligomerization domain-like receptor (NLR) proteins. Structural and functional studies indicate that the helical domain HD2 repressively contacted a conserved and functionally important alpha-helix of the NBD (nucleotide binding domain) in Swiss:Q3UP24. Furthermore, the HD2 domain was shown to cap the N-terminal side of the LRR (leucine-rich repeat) domain via extensive interactions [1]. Other family members carrying this domain include baculoviral IAP repeat-containing protein 1 (Birc1) also known as neuronal apoptosis inhibitory protein (Naip).

1: Crystal structure of NLRC4 reveals its autoinhibition mechanism. Hu Z, Yan C, Liu P, Huang Z, Ma R, Zhang C, Wang R, Zhang Y, Martinon F, Miao D, Deng H, Wang J, Chang J, Chai J; Science. 2013;341:172-175. PMID:23765277