General Secretory Pathway from marine Antarctic Pseudoalteromonas haloplanktis TAC125

In Gram-negative bacteria Type II secretion system (T2SS) is one of six protein secretion machineries that permit the export of proteins from within the bacterial cell to the extra-cellular milieu and/or into target host cells. The ability to secrete proteins in the extra-cellular medium is a key aspect of the physiology of Gram-negative bacteria. Indeed, secreted proteins provide the means, among others, by which microorganisms interact and modify the surrounding environment. This is a crucial aspect for microorganisms capable to survive in hostile extreme habitats, such as Antarctica. The sequencing and the annotation of marine Antarctic Pseudoalteromonas haloplanktis TAC125 (P. haloplanktis TAC125) genome paved the way to investigate on the molecular mechanisms of Type II dependent protein secretion in this bacterium. The present work reports a detailed in silico description and a functional analysis of P. haloplanktis TAC125 T2SS pathway, which features were compared with other already characterized T2SS machineries. The cold adapted Type II secretion cluster resulted to be characterized by the notably presence of a downstream and physically associated gene (called gspXX) which was found only in two other Alteromonadales T2SS gene clusters. A transcriptional analysis of some P. haloplanktis TAC125 T2SS genes demonstrated that the psychrophilic cluster was constitutively transcribed during the cell growth and that gspXX gene is co-transcribed with another gene of the cluster. These data support the inclusion of gspXX into the cold-adapted Type II secretion system cluster and point towards the existence of a novel and previously not described component of the T2SS machinery. This evidence opens new questions concerning the role played by GspXX protein in the architecture/function of the P. haloplanktis TAC125 T2SS machinery.