Bacteria have developed numerous systems to secrete proteins or DNA in order to modify their immediate surroundings or to obtain an advantage in a competitive and hostile environment. Since Gram-negative bacteria possess two membranes, the inner (cytoplasmic) membrane and the outer membrane, transport machines for protein secretion have the challenging task of circumventing two barriers to reach the exterior. A rather simple transport apparatus, the Type I secretion machinery, composed of only three proteins residing in the inner and outer membrane of Gram-negative bacteria achieve this objective in a single step. The Type I secretion pathway although also present in Gram-positive bacteria, has been analysed in greatest detail in Gram-negative bacteria. Almost all Type I transport substrates are polypeptides, varying from the small Escherichia coli peptide colicin V, (10 kDa) to the large Pseudomonas fluorescens cell adhesion protein LapA of 900 kDa. While these two examples reflect the range of the size of Type I transport substrates, the best characterized are the RTX toxins and the lipases. Type I secretion is also involved in export of non-proteinaceous substrates like cyclic β-glucans or polysaccharides.

from Jenewein et al in Bacterial Secreted Proteins

Further reading:

1. Bacterial Secreted Proteins: Secretory Mechanisms and Role in Pathogenesis
2. Pseudomonas: Genomics and Molecular Biology
3. Microbial Toxins: Current Research and Future Trends

Labels: bacteria, protein, protein secretion, proteins, Pseudomonas, toxin