Complex sentence: Lactacystin modifies the amino-terminal threonine of specific proteasome subunits, and this discovery helped to establish the proteasome as a mechanistically novel amino-terminal threonine protease.
Split into simpler sentences: Lactacystin modifies the amino-terminal threonine of specific proteasome subunits. This discovery helped to establish the proteasome as a mechanistically novel class of protease: an amino-terminal threonine protease.