Regulatory role of PopN and its interacting partners on type III secretion of Pseudomonas aeruginosa

Department of Molecular Genetics and Microbiology, P. O. Box 100266, University of Florida, Gainesville, FL 32610-0266; Department of Microbiology, Institute of Life Sciences, Nankai University, Tianjin, P.R. China; Korea Research Institute of Bioscience and Biotechnology, Taejon 305-600, Republic of Korea

^* To whom correspondence should be addressed. Email: sjin{at}mgm.ufl.edu.

	Abstract

Type III secretion system (T3SS) of Pseudomonas aeruginosa plays a significant role in pathogenesis. We have previously identified type III secretion factor (TSF) that is required for effective secretion of the type III effector molecules, in addition to the low calcium signal. The TSF includes many low affinity high capacity calcium binding proteins such as serum albumin and casein. Search for the TSF binding targets on the bacterial outer membrane identified PopN, a component of the T3SS that is readily detectable on the bacterial cell surface. The PopN specifically interacts with Pcr1, and both popN and pcr1 mutants display constitutive type III secretion phenotype, suggesting the two proteins form a complex that functions as T3SS repressor. Further analysis of the popN operon genes identified protein-protein interactions between Pcr1 and Pcr4, Pcr4 and Pcr3 as well as PopN and Pcr2 in the presence of PscB. Unlike popN and pcr1 mutants, pcr3 and pcr4 mutants are totally defective in the type III secretion while pcr2 mutant displays reduced type III secretion. Interestingly, PopN, Pcr1, Pcr2 and Pcr4 are all secreted in a type III secretion machinery-dependent manner while Pcr3 is not. These findings imply important regulatory roles these components play in the controlling of type III secretion.