Molecular Dissection of Novel Trafficking and Processing of the Toxoplasma gondii Rhoptry Metalloprotease Toxolysin‐1

Abstract

Toxoplasma gondii utilizes specialized secretory organelles called rhoptries to invade and hijack its host cell. Many rhoptry proteins are proteolytically processed at a highly conserved SΦXE site to remove organellar targeting sequences that may also affect protein activity. We have studied the trafficking and biogenesis of a secreted rhoptry metalloprotease with homology to insulysin that we named toxolysin-1 (TLN1). Through genetic ablation and molecular dissection of TLN1, we have identified the smallest rhoptry targeting domain yet reported and expanded the consensus sequence of the rhoptry pro-domain cleavage site. In addition to removal of its pro-domain, TLN1 undergoes a C-terminal cleavage event that occurs at a processing site not previously seen in Toxoplasma rhoptry proteins. While pro-domain cleavage occurs in the nascent rhoptries, processing of the C-terminal region precedes commitment to rhoptry targeting, suggesting that it is mediated by a different maturase, and we have identified residues critical for proteolysis. We have additionally shown that both pieces of TLN1 associate in a detergent-resistant complex, formation of which is necessary for trafficking of the C-terminal portion to the rhoptries. Together, these studies reveal novel processing and trafficking events that are present in the protein constituents of this unusual secretory organelle.