Tomato ribonuclease LX with the functional endoplasmic reticulum retention motif HDEF is expressed during programmed cell death processes, including xylem differentiation, germination, and senescence.

Abstract	We have studied the subcellular localization of the acid S-like ribonuclease (RNase) LX in tomato (Lycopersicon esculentum Mill.) cells using a combination of biochemical and immunological methods. It was found that the enzyme, unexpectedly excluded from highly purified vacuoles, accumulates in the endoplasmic reticulum. The evidence that RNase LX is a resident of the endoplasmic reticulum (ER) is supported by an independent approach showing that the C-terminal peptide HDEF of RNase LX acts as an alternative ER retention signal in plants. For functional testing, the cellular distribution of chimeric protein constructs based on a marker protein, Brazil nut (Bertholletia excelsa) 2S albumin, was analyzed immunochemically in transgenic tobacco (Nicotiana tabacum) plants. Here, we report that the peptide motif is necessary and sufficient to accumulate 2S albumin constructs of both vacuolar and extracellular final destinations in the ER. We have shown immunochemically that RNase LX is specifically expressed during endosperm mobilization and leaf and flower senescence. Using immunofluorescence, RNase LX protein was detected in immature tracheary elements, suggesting a function in xylem differentiation. These results support a physiological function of RNase LX in selective cell death processes that are also thought to involve programmed cell death. It is assumed that RNase LX accumulates in an ER-derived compartment and is released by membrane disruption into the cytoplasma of those cells that are intended to undergo autolysis. These processes are accompanied by degradation of cellular components supporting a metabolic recycling function of the intracellular RNase LX.
Authors	K Lehmann, B Hause, D Altmann, M Köck
Journal	Plant physiology (Plant Physiol) Vol. 127 Issue 2 Pg. 436-49 (Oct 2001) ISSN: 0032-0889 [Print] United States
PMID	11598219 (Publication Type: Journal Article, Research Support, Non-U.S. Gov't)
Chemical References	Plant Proteins Protein Sorting Signals Recombinant Fusion Proteins Endoribonucleases Ribonucleases S-like RNase protein, plant ribonuclease LX
Topics	Amino Acid Motifs Apoptosis (genetics) Biological Transport Cell Differentiation Cell Fractionation Cells, Cultured Cellular Senescence Endoplasmic Reticulum (enzymology, genetics) Endoribonucleases (chemistry, genetics, metabolism) Gene Expression Regulation, Enzymologic Gene Expression Regulation, Plant Germination Immunoblotting Immunohistochemistry Solanum lycopersicum (enzymology, genetics) Plant Proteins Plant Structures (genetics, metabolism) Plants, Genetically Modified Protein Sorting Signals Protoplasts Recombinant Fusion Proteins (genetics) Ribonucleases (genetics, physiology)

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