Tyrosine-Phosphorylated Bacterial Proteins

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Abstract
Gram-negative bacteria build and anchor different extracellular organelles, such as flagella and conjugative pili, using specialized supramolecular structures. These molecular engines transport the building blocks of flagella and pili across both the inner and outer membranes and polymerize the external filamentous structures by adding new monomers from the inside. In their evolution, bacteria found it convenient to duplicate the ancestral cluster of genes and use one set for further specialization as secretion apparatuses to translocate proteins or protein complexes into host cells (Fig. 1). In other terms, we may consider the type III and type IV secretion systems as spin-offs of flagella and conjugative pili, respectively. The two secretion systems have been extensively reviewed (4)(10); a list of the bacteria known to contain them is printed in Table 1. Here we will just mention that although both systems are functionally equivalent and are used to translocate proteins into mammalian or plant cells, a number of properties differentiate them: (a) Type III apparatus and flagella (4) are encoded by 15–35 genes, at least 8 of which are well conserved in most systems. Type IV system and conjugative pili (10) are encoded by 11–31 genes, at least 6 of which are well conserved in most of them. No homology is present between the genes present in the two systems; however, both encode proteins with predicted ATPase activity. (b) The type III system secretes monomeric proteins with no apparent cleavable sec-dependent signal sequence. The type IV system may secrete assembled multimeric proteins such as pertussis toxin that are composed of different monomers, each having a typical sec-dependent signal peptide. This suggests that in some cases the proteins may enter the export machinery after being exported across the inner membrane by the general secretion system. Type IV secretion may, however, also export across both the inner and outer membranes nucleoproteins containing proteins and DNA (as in the case of the Ti plasmid T-DNA of Agrobacterium tumefaciens). In the case of the CagA protein reported in references 5–7, it is not known whether this is exported as a monomer or a more complex structure; however, the absence of a typical sec-dependent signal sequence in this protein suggests that the type IV system assists the translocation of CagA across both bacterial membranes and also across the host cell membrane. Finally, both type III and type IV secretion systems are encoded by genes that are clustered, often present in pathogenicity islands with a GC content different from the rest of the chromosome and that most likely have been acquired by horizontal transfer.

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