Скачать или смотреть Type 4 secretion system: iii- Substrate transfer

Type 4 secretion systems (T4SS) are secretion nanomachines operating in bacteria to transport proteins and DNAs. They are typically composed of 12 proteins termed "VirB1-11 and VirD4". They are most commonly used to transport plasmid and genomic DNAs from one bacterium to another in a process termed "conjugation". Conjugation is the most common means by which antibiotic resistance genes spread among bacterial populations.

During conjugation, the DNA is transfered from a donor cell to a recipient cell via a protein-ssDNA conjugate. Indeed prior to DNA transfer, a large cytosolic complex needs to be assembled around the plasmid to be transferred. The region of the plasmid DNA where the complex assembles is called "Origin of Transfer or OriT". The complex is called "the relaxosome" and is composed of a major protein, called "relaxase" and some accessory proteins. In the movie shown here, the relaxase is called TraI and the accessory proteins are called TraM, TraY, and IHF. The first part of the movie shows TraI, TraM, TraY and IHF assembling at OriT. Once the relaxosome is assembled, it will connect with the T4SS, shown here in the same way as in the two previous movies.

Following contact with a recipient cell, the relaxosome is activated. This means that the relaxase will react with the DNA as a sub-site within OriT called "nic". The relaxase will nick the "nic" site on one strand, the strand to be transfered, and the 5'phosphate resulting from the nicking reaction will react covalently with the relaxase: thus, it is a covalent complex of relaxase and ssDNA that will get transported through the T4SS. Activation of the relaxosome also result in a ssDNA bubble onto which a second molecule of TraI (the relaxase) wil bind, this time through its helicase domains (in green and blue). This second TraI will be used to pump the relaxase-ssDNA through the T4SS into the recipient cell. Recently, we have shown that the relaxase is transported through the system as an unfolded peptide. Thus, transfer of the relaxase-ssDNA conjugate starts with the protein unfolding and transport, presumably through the VirB4 ATPase, and is followed by the transport of the ssDNA, presumably through the VirD4 ATPase. In the movie, we hypothesize a handover of the relaxase-ssDNA substrate from VirB4 to VirD4 once relaxase transport is completed.

Professor Gabriel Waksman, FMedSci, FRS

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Produced for Professor Gabriel Waksman by Said Sannuga

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