Ubiquitin found in the archaebacterium Thermoplasma acidophilum.

Systematic N-terminal sequencing of the low molecular weight proteins from Thermoplasma acidophilum separated by two-dimensional poly-acrylamide gel electrophoresis led to the discovery of a polypeptide with an apparent M(r) of 4.5 kDa identical as its first 18 amino acid residues to human ubiquitin. The occurrence of ubiquitin and proteasomes in an archaebacterium strongly suggests that ATP-ubiquitin-dependent proteolysis is a cellular function that developed early in evolution.