Functional Analysis of the Tubulin-Folding Cofactor C in Arabidopsis thaliana

Abstract The biogenesis of microtubules comprises several steps, including the correct folding of α- and β-tubulin and heterodimer formation. In vitro studies and the genetic analysis in yeast revealed that, after translation, α- and β-tubulin are processed by several chaperonins [1, 2] and microtubule-folding cofactors (TFCs) to produce assembly-competent α-/β-tubulin heterodimers [3–11]. One of the TFCs, TFC-C, does not exist in yeast, and a potential function of TFC-C is thus based only on the biochemical analysis. In this study and in a very recently published study by Steinborn and coworkers [12], the analysis of the Arabidopsis porcino ( por ) mutant has shown that TFC-C is important for microtubule function in vivo. The predicted POR protein shares weak amino acid similarity with the human TFC-C (hTFC-C). Our finding that hTFC-C under the control of the ubiquitously expressed 35S promoter can rescue the por mutant phenotype shows that the POR gene encodes the Arabidopsis ortholog of hTFC-C . The analysis of plants carrying a GFP:POR fusion construct showed that POR protein is localized in the cytoplasm and is not associated with microtubules. While, in por mutants, microtubule density was indistinguishable from wild-type, their organization was affected.