The ordered regions of the yeast BLM10 or PA200 (human homologue), full-length protein encode 32 HEAT repeat (HR)-like modules [1], each comprising two helices joined by a turn, with adjacent repeats connected by a linker. Whereas a standard HEAT repeat is composed of ~50 residues, the BLM10 HEAT repeats are highly variable. The length of helices ranges from 8 to 35 residues, turns range from 2 to 87 residues, and linkers range from 1 to 88 residues, with the longest linker, between HR21 and HR22, containing additional secondary structures (two strands and three helices). BLM10_mid is the middle ordered region of the three in BLM10. BLM10 is found to surround the proteasome entry pore in the 1.2 MDa complex of proteasome and BLM10 to form a largely closed dome that is expected to restrict access of potential substrates. Thus Blm10 and PA200 are predominantly nuclear and stimulate the degradation of model peptides, although they do not appear to stimulate the degradation of proteins, recognise ubiquitin, or utilise ATP [2].

1: New HEAT-like repeat motifs in proteins regulating proteasome structure and function. Kajava AV, Gorbea C, Ortega J, Rechsteiner M, Steven AC; J Struct Biol. 2004;146:425-430. PMID:15099583

2: Structure of a Blm10 complex reveals common mechanisms for proteasome binding and gate opening. Sadre-Bazzaz K, Whitby FG, Robinson H, Formosa T, Hill CP; Mol Cell. 2010;37:728-735. PMID:20227375