The centrosome is a site where tubulin-based microtubules form and reorganize. The animal centrosome is formed around a pair of centrioles and is located within a matrix called the pericentriolar matrix (PCM). It contains proteins that tether and activate g-tubulin, an active component in the nucleation of microtubules (Desai and Mitchison, 1997; Bettencourt-Dias, 2018; Goldspink et al., 2017; Tovey and Conduit, 2018).

The head-to-tail arrangement of a- and b-tubulin subunits in tubulin molecules gives them molecular polarity and allows them to grow as stiff, cylindrical polymers. This polarity is important for the assembly and growth of microtubules and other cellular components. In addition, the head-to-tail arrangement of tubulin dimers at the growing plus and minus ends of microtubules allows a molecule to switch between periods of growth and shrinkage (Desai and Mitchison, 2001; Xu et al., 2011).

TnpA solution dimers are radically different from other SRs, which are known to oligomerize via interactions between the helix E regions of their catalytic domains (Bond and Tovey, 2010; Mouw et al., 2008; Yang and Steitz, 1995). In contrast, TnpA solution dimers are arranged over their B and D helices and do not share contacts in smSRs (Figure 2B,C), which are almost entirely hydrophobic.

Rather than forming disulfide-linked dimers as is common for SRs, oxidized cysteines in TnpAIS1535 near the N-terminal end of helix D and Cys138 at the C-terminal end of helix D efficiently assemble a covalent dimer that locks the two dimeric subunits together within the catalytic domain core (Figure 7A,B). This is necessary for the cooperative binding of DNA to a TnpAIS1535 mutant that truncates its helix E region up to the b4 residue.