In studying the modulation of multicatalytic proteinase (MCP), we have focused on three main aspects: (1) modulation of the activity of the MCP complex by lipids, showing that cardiolipin, sulfatides and gangliosides are potent activators of the enzymatic activity of the complex; (2) modulation by interconversion of MCP subunits, showing that casein kinase II is able to phosphorylate the C8 (this subunit is also be main in vivo phosphorylated subunit) and C9 subunits of the complex in vitro and that a 26-kD subunit is phosphorylated in vitro by protein kinase C, and (3) modulation by proteolytic processing, extending our previous observation of proteolytic processing of the C2 COOH terminus, the presence of enzymatic activities in different subcellular fractions able to convert the intact C2 (32 kD) subunit to a 28-kD polypeptide by removal of at least the last 9-13 amino acids of the C2 polypeptide. The data presented illustrate that the MCP complex is probably under tight and multifactorial control in vivo.