15 HEAT motifs of Protein Phosphatase 2A PR65/A subunit.
The protein domain HEAT (Huntington, Elongation Factor 3, PR65/A, TOR) is found in the four diverse eukaryotic proteins for which the domain is named. Similarities between HEAT and Armadillo (ARM) repeat families are found in the conserved residues that form the hydrophobic domain core. In contrast to conserved ARM repeats, HEAT repeats vary more in length, amino acid sequence and 3D-structure. The HEAT family of repeats is divided into three classes based on similarities and includes IMB, AAA and ADB repeats. HEAT containing proteins include the Ran-GTP binding importin and exportin cargo transport proteins. A substantial number of HEAT repeats are found in proteins involved in translation including eIF4Gs, p97/DAP5, Paip-1, GCN1 and FRAP/mTOR.
The HEAT domain consists of repeats of two anti-parallel α-helices and two turns arranged about a common axis. These repeats are linked by flexible inter-unit loops. A signature motif of the HEAT repeats is the presence of conserved Asp and Arg residues at positions 19 and 25, respectively.