helix-loop-helix (HLH)

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Introduction

A conserved domain of 40-50 amino acid residues found in sequence specific DNA-binding proteins. It is formed of 2 amphipathic helices joined by a variable length linker region that could form a loop.

The HLH mediates protein dimerization. Most proteins with a HLH have an extra basic region of about 15 amino acid residues, adjacent to the HLH domain that specifically binds to DNA.

HLH proteins lacking the basic domain function as negative regulators since they form heterodimers, but fail to bind DNA.