INHIBITION OF PROTEASOME ACTIVITIES AND SUBUNIT-SPECIFIC AMINO-TERMINAL THREONINE MODIFICATION BY LACTACYSTIN

Lactacystin is a Streptomyces metabolite that inhibits cell cycle prog ression and induces neurite outgrowth in a murine neuroblastoma cell l ine. Tritium-labeled lactacystin was used to identify the 20S proteaso me as its specific cellular target. Three distinct peptidase activitie s of this enzyme complex (trypsin-like, chymotrypsin-like, and peptidy lglutamyl-peptide hydrolyzing activities) were inhibited by lactacysti n, the first two irreversibly and all at different rates. None of five other proteases were inhibited, and the ability of lactacystin analog s to inhibit cell cycle progression and induce neurite outgrowth corre lated with their ability to inhibit the proteasome. Lactacystin appear s to modify covalently the highly conserved amino-terminal threonine o f the mammalian proteasome subunit X (also called MB1), a close homolo g of the LMP7 proteasome subunit encoded by the major histocompatibili ty complex. This threonine residue may therefore have a catalytic role , and subunit X/MB1 may be a core component of an amino-terminal-threo nine protease activity of the proteasome.