Organization and expression of the TUP gene in Daucus carota
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Differential screening of a somatic embryo cDNA library for genes expressed during embryogenesis has disclosed a small RNA which I have named Tiny Up-regulated Protein (TUP). This mRNA is 402 bp in length, is expressed during embryo formation, and reaches its highest accumulation in the flowers and roots of adult carrot plants. The mRNA is barely detectable in leaves. The accumulation of the TUP mRNA in seedlings is increased by auxin and by abscisic acid. The open reading frame encodes a tiny, uniquely structured protein of 6100 Daltons which has a pI of 5.3. Algorithms predicting protein structure suggest that this tiny, unique protein contains a leader sequence targeting it to the endoplasmic reticulum and potentially through the default secretory pathway. Database searches show homology to antibacterial and arabinogalactan proteins. Protein prediction algorithms also suggest that the gene product may be a GPI-anchored protein. Synthesized TUP protein did not affect the growth of bacteria or fungus, and the accumulation of the mRNA decreased in seedlings grown with a fungus. These data suggest that the TUP does not have a role in defense responses in plants. Antisera generated against the synthetic TUP protein reveal 5 detectable proteins in leaf organs but none in roots. Only one of the 5 proteins is detected in flowers and at a much lower abundance than in leaves. Attempts to immune-precipitate these proteins has been unsuccessful to date. These data outline a very unique, tiny, acidic protein which is most likely secreted in some form to the extracellular matrix in plants. This protein could be anchored to the membrane through a glucosylphosphatidylinositol tether, or it could be O-glycosylated at the abundant proline (hydroxyproline) amino acids and serve a function in wall formation.