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A structural and enzymological study of auxin conjugate hydrolases from Medicago truncatula

We have previously isolated and characterized the auxin conjugate hydrolase family from the model legume Medicago truncatula (barrel clover). In the process of cloning the cDNA for the genes MtIAR31, -32, -33, -34, and -36, we also cloned a truncated version of MtIAR33 (delta-MTIAR33) which expresses an alternative hydrolase from a secondary AUG start site. We expressed this truncated protein in vitro, compared its enzymatic activity to the full-length protein, and found the activity and substrate recognition of the two differed substantially. The full-length MtIAR33 had high activity against IAA-Aspartate (~1160 pmol auxin released/min/ml) with low activity against IBA-Alanine, while delta-MtIAR33 lost almost all activity against IAA-Aspartate, but gained activity against IBA-Alanine (~140 pmol auxin released/min/ml). These major differences in activity suggested a possible biological importance, so we designed expression constructs containing MtIAR31, -32, -34, and -36 truncated at the homologous internal AUG start site found in delta-MtIAR33. All the truncated proteins demonstrate altered characters from the full-length. Structurally, the hydrolase active site appears to be in the 'head' region of the protein. The gain of enzymatic function found by cleaving off the 5'-end domain of the protein, suggests that this head region may be serving as an inhibitory domain in the full-length version of the enzymes. We are currently investigating whether these Medicago hydrolases act as monomers or dimers. In root inhibition experiments employing IBA-Alanine, IAA-Aspartate, IAA-Leucine, and IAA-Alanine, all are hydrolyzed in vivo and induce a biological effect. These results would support the hypothesis that all four substrates are recognized by the hydrolases in vivo.

Medicago truncatula; barrel clover; Fabaceae; auxin conjugate hydrolase; indole-3-acetic acid; IAA; indole-3-butyric acid; IBA

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