TolA

Structure

TolA is located in the inner membrane and comprises of three domains: the N-terminal domain I (TolAI), from residues 1-47 including a 20-residue hydrophobic membrane spanning region which anchors the protein to the cytoplasmic membrane^[1]; domain II (TolAII), from residues 48-301, which forms a rigid helix connecting the domains either side of it; and the C-terminal domain III (TolAIII) from residues 302-421, which may be involved in the function of TolA by interacting with the periplasmic or outer membrane proteins, due to the tethering to domain II^[2]. For additional details see Tol.

C-terminal Domain

The C-terminal domain of TolA is directly involved with the N-terminal of both Colicin and the phage minor coat gene 3 protein^[3].

Comparison with TonB

TolA and TonB have been shown to have a common evolutionary origin: although the two proteins can undergo domain swapping, this is difficult to achieve. Through sedimentation experiments, it has been shown that TonB is able to remain as a monomer and form a TolA-like fold, but leaves an exposed β-ribbon which would need to undergo extensive conformational changes. TolA does have the potential to dimerise, but it is highly unlike that it will do so. Nevertheless, the structural similarities between these two proteins clearly indicates an evolutionary relationship, even if the functional properties have divulged^[4].

Function

Although the exact function of TolA is not yet known, it has been shown that unlike mutations in the proteins of the TonB system to which the TolQRA proteins show many similarities, mutations in the TolQRA proteins affect the outer membrane integrity. TolA could be involved structurally by bringing the inner and outer membranes together and forming a bridge or link between them^[2].

TolA plays an important role in the import mechanisms for the uptake of bacteriotoxins (see Colicin) and the DNA of filamentous bacteriophages^[3], and has also been shown to be involved in the bacterial sensitivity to these groups^[1], in particular the alpha helix of domain II, the deletion of which causes increased cellular sensitivity to deoxycholate (a detergent)^[5].

TolA and Colicin Uptake

Studies have shown that TolAIII plays an important role in the uptake mechanisms of colicin and DNA by being directly involved with the N-terminal of both colicin and the phage minor coat gene 3 protein^[3].

TolA interacts with different group A colicins in different ways^[5]:

• Colicin E1 requires different structural features of TolA to be transported than the other group A colicins, including Colicin E3, Colicin N and Colicin A. Colicin E3 requires the C-terminus of TolA, which it finds with ease even when the terminus is attached very close to the membrane-spanning domain, by means of utilising a different outer membrane receptor complex to that of the other colicins
• The other colicins require additional receptors in order to bind initially to the outer membrane
• Colicin E1 also does not require as high of TolA levels than the other colicins in order for translocation to occur