This family consists of several thermopsin proteins from archaebacteria. Thermopsin is a thermostable acid protease which is capable of hydrolysing the following bonds: Leu-Val, Leu-Tyr, Phe-Phe, Phe-Tyr, and Tyr-Thr. The specificity of thermopsin is therefore similar to that of pepsin, that is, it prefers large hydrophobic residues at both sides of the scissile bond [1].

1: Enzymic properties of thermopsin. Fusek M, Lin XL, Tang J; J Biol Chem 1990;265:1496-1501. PMID:2104844