SprA and SprT Are Essential Components of the Type IX Secretion System Required for Ca Acquisition, Cellulose Degradation, and Cell Motility.

The type IX secretion system (T9SS) is a novel protein secretion system, which is found in and confined to the phylum . T9SS is involved in the secretion of virulence factors, cell surface adhesins, and complex biopolymer degrading enzymes to the cell surface or extracellular medium. is a widely distributed bacterium, which is able to efficiently digest cellulose and rapidly glide along the solid surfaces. has a full set of orthologs of T9SS components. However, the functions of most homologous proteins have not been verified. In . , CHU_0029 and CHU_2709 are similar in sequence to T9SS components SprA and SprT, respectively. In this study, the single deletion mutants of () and () were obtained using a complex medium with the addition of Ca and Mg. Single deletion of or resulted in defects in cellulose utilization and gliding motility. Moreover, the Δ and Δ mutants showed growth defects in Ca- and Mg-deficient media. The results of ICP-MS test showed that both the whole cell and intracellular concentrations of Ca were dramatically reduced in the Δ and Δ mutants, indicating that SprA and SprT are both important for the assimilation of trace amount of Ca. While the assimilation of Mg was not obviously influenced in the Δ and Δ mutants. Through proteomics analysis of the cell surface proteins of the wild type and mutants, we found that the Δ and Δ mutants were defective in secretion of the majority of T9SS substrates. Together, these results indicate that SprA and SprT are both essential components of . T9SS, which is required for protein secretion, Ca acquisition, cellulose degradation, and gliding motility in Our study shed more light on the functions of SprA and SprT in T9SS, and further proved the link between the T9SS and Ca uptake system.