High impact information on AHA3

• Immunofluorescence studies with tissue sections of transgenic plants have revealed that c-Myc-tagged AHA3 is restricted to the plasma membrane of phloem companion cells, whereas other AHA isoproteins are more widely distributed in the plasma membrane of other cell types [1].
• The abundance of the proton pump in the plasma membrane of companion cells supports an apoplastic model for phloem loading in which the metabolic energy that drives sugar uptake is consumed by AHA3 at the companion cell plasma membrane [1].
• When compared with AHA3, AHA1 and AHA2 had an apparent higher turnover rate for ATP hydrolysis, exhibited a 10-fold higher apparent affinity for ATP, and a 3-fold higher sensitivity toward vanadate [2].
• In the 3'-untranslated sequence of AHA1 and AHA2 but not AHA3, there is a 65-bp region of 85% identity and a second shorter region of 16-bp identity harboring an unusual putative poly(A) addition signal (dTTTGAAGAAACAAGGC) [3].
• Companion cell protoplasts, which could be recognized by their nucleus, vacuole and chloroplasts, and by their expression of the phloem-specific marker genes SUC2 and AHA3, formed the basis for a cell-specific cDNA library and expressed sequence tag (EST) collection [4].

Biological context of AHA3

• The AHA3 mRNA start site was mapped and 464 bp of the putative upstream regulatory region sequenced [5].
• A translational fusion of AHA3 to the beta-glucuronidase (GUS) reporter gene was constructed and used to generate transgenic Nicotiana and Arabidopsis plants [5].
• In comparison with the 16 introns reported in AHA3, AHA2 is missing one intron in the 5'-untranslated region and a second intron in the C-terminal coding region [3].
• A causative role for AHA3 in male gametogenesis was proven by complementation with a normal transgenic gene and rescue of the mutant phenotype back to wild type [6].

Regulatory relationships of AHA3

• The overall pattern of SUC2-GUS expression correlated well with that of the Arabidopsis thaliana AHA3 plasma-membrane H(+)-ATPase which is also expressed in the phloem and most likely represents the primary pump generating the energy for secondary active transporters such as SUC2 [7].

References