cd00083: HLH (this model, PSSM-Id:238036 is obsolete and has been replaced by 381392)
Helix-loop-helix domain, found in specific DNA- binding proteins that act as transcription factors; 60-100 amino acids long. A DNA-binding basic region is followed by two alpha-helices separated by a variable loop region; HLH forms homo- and heterodimers, dimerization creates a parallel, left-handed, four helix bundle; the basic region N-terminal to the first amphipathic helix mediates high-affinity DNA-binding; there are several groups of HLH proteins: those (E12/E47) which bind specific hexanucleotide sequences such as E-box (5-CANNTG-3) or StRE 5-ATCACCCCAC-3), those lacking the basic domain (Emc, Id) function as negative regulators since they fail to bind DNA, those (hairy, E(spl), deadpan) which repress transcription although they can bind specific hexanucleotide sequences such as N-box (5-CACGc/aG-3), those which have a COE domain (Collier/Olf-1/EBF) which is involved in both in dimerization and in DNA binding, and those which bind pentanucleotides ACGTG or GCGTG and have a PAS domain which allows the dimerization between PAS proteins, the binding of small molecules (e.g., dioxin), and interactions with non-PAS proteins.
Jiyao W et al.(2023). "The conserved domain database in 2023.",