Identification of an IcsB-like Acyltransferase Family

Abstract

Shigella flexneri is a Gram-negative enteropathogen and one of the main causes of diarrheal diseases. Shigella uses its type three secretion system (T3SS) to secrete effector proteins into host cells, thus allowing their invasion. Through vacuole rupture, Shigella cells are transferred into the host cell cytosol whereby eliciting actin-based movement they disseminate into neighboring cells, a process known as cell-to-cell spread. IcsB is one of the secreted proteins that is crucial for Shigella to help escaping the vacuole formed during cell-to-cell spread. Recently, it was proven that IcsB has acyltransferase activity that is essential to vacuole escape. Surprisingly, using a bioinformatic approach, we were able to identify proteins that share homology with IcsB when we queried against the catalytic domain. Those proteins have high homology with IcsB that is limited in most cases to a central segment of their primary structure that corresponds to the catalytic domain. Furthermore, the three catalytic residues Histidine 145, Aspartate 195 and Cystine 306 are completely conserved in all homologous proteins. Interestingly, most of those homologs have not been studied yet. Thus, we hypothesized that these proteins might be forming an IcsB-like acyltransferase family. Based on previous findings that IcsB expression is toxic in budding yeast, we established a yeast cytotoxicity to test whether the putative IcsB homologs triggered the same phenotype. The results demonstrated that seven out of 11proteins were toxic to yeast. Mutating the catalytic histidine rescued the toxicity observed in their wildtype counterpart. The results in this thesis are the first step toward confirming the relation between IcsB and the homologous proteins we discovered. Additional experiments from our lab spurred by my data have confirmed that several of the toxic homologs are acyl transferase, while those that were not toxic are not. Future work needs to be conducted to determine whether the acylation activity of these proteins contributes to pathogenesis.