Translational regulation of δ-tubulin through its 5'-untranslated region

δ-tubulin - a member of tubulin superfamily, is found in a subset of eukaryotes including human where it has a role in centriole maturation. The mutation in the gene results in a disorganized microtubule triplet arrangement leading to formation of defective centriole. Since centriole maturation is a periodic event, it will be interesting to see if δ-tubulin is also regulated in a cell cycle dependent manner.

In this regard we show that the abundance of δ-tubulin mRNA remains unchanged throughout the cell cycle. However, the protein level varies periodically with a significantly higher expression in S-phase, implying regulation at the level of translation. Sequence analysis establishes the presence of a 90-base long conserved region, including a consensus motif of nine residues in the 5´-untranslated region (5´-UTR) of δ-tubulin transcript. The deletion analysis of the conserved region using luciferase reporter assay system confirms its strong inhibitory effect on translation. Interestingly, microtubule associated protein 4 (MAP4) is found to interact specifically with the 90-base long conserved region in the 5´-UTR and possibly responsible, at least partially, for the translation inhibitory activity of the UTR. Remarkably, MAP4 interacts with δ-tubulin in a periodic manner at protein level also.

The results reported here show that δ-tubulin protein expression is regulated at posttranscriptional level and strongly suggest the role of MAP4 in modulation of both abundance and function of δ-tubulin.