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Peptides fail to deliver proteins to particular places
Post-translational modifications play important roles in regulating the activity of many proteins. For certain proteins, particularly those that are destined for the endomembrane system or for secretion, correct folding, glycosylation and protein cleavage are intimately connected in producing the active protein in the right cell compartment. It is this aspect of horse radish peroxidase (HRP) that has interested Kis et al., (Maynooth and Dublin, pp. 303-310). One of the HRP isozymes, isozyme C, is synthesized as a pre-protein with both N-terminal and C-terminal peptide extensions; it has been suggested that these peptides are involved in protein targeting. The authors used a synthetic copy of the isozyme C gene and transferred it to tobacco plants under the control of either a strong constitutive promoter (CaMV 35S) or a strong light-inducible promoter (RUBISCO small subunit). The gene itself was used in four versions: the protein coding sequence alone, and the protein sequence spliced to the sequences encoding either the C-terminal or the N-terminal peptide or both. The results were both clear and surprising. In the absence of either peptide, very little expression of the transgene was detected, either at protein or mRNA levels. Adding the C-terminal peptide had no significant effect; indeed, the mRNA was actually undetectable with this construct. There was certainly no evidence that the C-terminal peptide was involved in vacuolar targeting. Only when the N-terminal peptide was present were high levels of expression observed; the presence of the C-terminal peptide did not affect this. In respect of post-translational modification, the protein synthesised from constructs containing the N-terminal peptide appeared to be glycosylated and to be mostly located in the ER. About 20 % of activity was associated with the cell walls, a proportion too low, the authors suggest, to represent a specific secretion mechanism. Thus there remains much to do before we understand the processing and secretion of HRP isozyme C.
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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