Seminal role of deletion of amino acid residues in H1-S2 and S-loop regions in eukaryotic β-tubulin investigated from docking and dynamics perspective.

Tubulin is the fundamental unit of microtubules. It is reported to effect different functions like cell division, chromosomal segregation, motility and intracellular transportation. α- and β-tubulin associate laterally and longitudinally to form protofilaments. Both the subunits are structurally identical to each other except for the deletions reported in H1-S2 and S loop regions in eukaryotic β-tubulin. These deletions mimic the ancestral tubulin protein named Latest Common FtsZ-Tubulin Ancestor (LCFTA) with a shorter S-loop region resulting in weak dimerization. However, in eukaryotic beta tubulin, the significance of this shorter region remains elusive till date. The main objective of this study was to model variants of beta tubulin (βmut1, βmut2 and βmut3) with inserts that lengthened the loop, and to compare them with the native α- and β-subunits to understand their biological significance. Further, one more mutant was modeled with the intention of understanding the counter effect of additional deletion of amino acid residues from both H1-S2 and S-loop regions; this mutant was designated as βmut4. Our study confirms that the insertion of amino acid residues considerably increases the protein-protein interactions in βmut1-βmut1, βmut2-βmut2 and βmut3-βmut3 compared to their native β-subunit. Similarly, the binding affinity of GTP also increases in βmut2 and βmut3 as compared to the wild type. However, these deletions result in decreased protein-protein and ligand interactions in wild beta tubulin and βmut4, as compared to βmut1, βmut2,and βmut3. Therefore, we conclude here that residual inserts in the H1-S2 and S loop sub segments bring about conformational changes in regions critically involved in lateral interactions and in the nucleotide binding site, thus altering the binding affinities between the dimers and the ligands. Regarding the biological importance of such deletions in wild beta tubulin, these deletions result in flexible M-loop leading to weak protein-protein interaction. This could be an adaptive feature playing a crucial role in protofilament dissociation during GTP hydrolysis, because of weak dimerization.