Intracellular Citrus Huanglongbing Bacterium, ‘Candidatus Liberibacter asiaticus’ Encodes Two Novel Autotransporters

• Published in: PloS one Vol. 8; no. 7; p. e68921
• Main Authors: Hao, Guixia, Boyle, Michael, Zhou, Lijuan, Duan, Yongping, Rudel, Thomas
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 11.07.2013; Public Library of Science (PLoS)
• Subjects: Agriculture; Alphaproteobacteria - genetics; Alphaproteobacteria - metabolism; Amino acids; antigens; Bacteria; Bacterial Outer Membrane Proteins - genetics; Bacterial Outer Membrane Proteins - metabolism; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Biofilms; bioinformatics; C-Terminus; Candidatus Liberibacter asiaticus; Cell surface; Citrus; Citrus - microbiology; Computational Biology - methods; E coli; Endopeptidase K; Escherichia coli; Escherichia coli - genetics; Escherichia coli - metabolism; gene expression; Gene silencing; genome; Genomes; Green fluorescent protein; Green Fluorescent Proteins - genetics; Green Fluorescent Proteins - metabolism; greening disease; Haemophilus; Horticulture; Intracellular; Laboratories; Leaves; Localization; Mitochondria; Mitochondria - metabolism; Mitochondria - microbiology; Monoclonal antibodies; Nicotiana - microbiology; Passengers; Pathogenesis; Pathogens; peptidase K; Peptides; Plant bacterial diseases; Plant Diseases - microbiology; Plant Leaves - microbiology; Plants (botany); Poles; Protein Structure, Tertiary; Proteinase; Proteins; Shigella; signal peptide; Tobacco; type V secretion system; Virulence; Virulence Factors - genetics; Virulence Factors - metabolism; Florida; United States > US
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0068921

Proteins secreted by the type V secretion system (T5SS), known as autotransporters, are large extracellular virulence proteins localized to the bacterial poles. In this study, we characterized two novel autotransporter proteins of ‘ Candidatus Liberibacter asiaticus’ (Las), and redesignated them as LasA I and LasA II in lieu of the previous names Hyv I and Hyv II . As a phloem-limited, intracellular bacterial pathogen, Las has a significantly reduced genome and causes huanglongbing (HLB), a devastating disease of citrus worldwide. Bioinformatic analyses revealed that LasA I and LasA II share the structural features of an autotransporter family containing large repeats of a passenger domain and a unique C-terminal translocator domain. When fused to the GFP gene and expressed in E. coli , the LasA I C-terminus and the full length LasA II were localized to the bacterial poles, similar to other members of autotransporter family. Despite the absence of a typical signal peptide, LasA I was found to localize at the cell surface by immuno-dot blot using a monoclonal antibody against the partial LasA I protein. Its surface localization was also confirmed by the removal of the LasA I antigen using a proteinase K treatment of the intact bacterial cells. When co-inoculated with a P19 gene silencing suppressor and transiently expressed in tobacco leaves, the GFP-LasA I translocator targeted to the mitochondria. This is the first report that Las encodes novel autotransporters that target to mitochondria when expressed in the plants. These findings may lead to a better understanding of the pathogenesis of this intracellular bacterium.