heat shock proteins, RNA, and RNPs in plants

Uncapped mRNA introduced into tobacco protoplasts can be imported into the nucleus and is trapped by leptomycin B

Rogier Stuger and Christoph Forreiter

Plant Cell Rep 23 (2004) 99-103

The mechanism of nuclear export of RNAs in yeast and animal cells is rapidly being uncovered, but RNA export in plants has received little attention. We introduced capped and uncapped fluorescent mRNAs into tobacco ( Nicotiana plumbaginifolia) protoplasts and studied their cellular localization. Following insertion, capped transcripts were found in the cytoplasm, while uncapped messengers transiently appeared in the nucleus in about one-quarter to one-third of the cells. These mRNAs were trapped by the nuclear export-inhibiting drug leptomycin B, pointing to an export mechanism in plants similar to Rev-NES-mediated RNP export in other organisms. [full-text pdf]

Partially denatured proteins are kept in a folding competent state by heat stress granules built from two classes of cytoplasmic small Hsps

Christoph Forreiter, Daniela Löw, and Rogier Stuger

Eur J Biochem 268 (2001) S111

poster presented at the 27th FEBS meeting, 30 June-5 July 2001, Lisboa, Portugal

During heat stress plants accumulate large amounts of small heat stress proteins (sHsps) in all cellular compartments. These proteins are encoded by a multigene family. There are two different classes of cytoplasmic sHsps, both of which are required to build large complexes coined heat stress granules (HSGs). Proposed functions of these complexes include storage of silent mRNAs released from polysomes during stress, and repair of denatured proteins. Using immunofluorescence and peptide library analysis we found no evidence for a role of sHsps in mRNA storage. Denatured proteins associate with HSGs. sHsps keep these proteins in a state suitable for poststress refolding.

Messenger RNA-binding properties of nonpolysomal ribonucleoproteins from heat-stressed tomato cells

Rogier Stuger, Sigrid Ranostaj, Tilo Materna, and Christoph Forreiter

Plant Physiology 120 (1999) 23-31

biocenter, dept mol cell biol, Goethe University, Frankfurt.

Most cells experiencing heat stress reprogram their translational machinery to favor the synthesis of heat-stress proteins. Translation of other transcripts is almost completely repressed, but most untranslated messengers are not degraded. In contrast to yeast, Drosophila melanogaster, and HeLa cells, plant cells store repressed messengers in cytoplasmic nonpolysomal ribonucleoproteins (RNPs). To follow the fate of untranslated transcripts, we studied protein composition, mRNA content, and RNA-binding properties of nonpolysomal RNPs from heat-stressed tomato (Lycopersicon peruvianum) cells. Contrary to the selective interaction in vivo, RNPs isolated from tomato cells bound both stress-induced and repressed messengers, suggesting that the selection mechanism resides elsewhere. This binding was independent of cap or a poly(A) tail. The possible role of proteasomes and heat-stress granules (HSGs) in mRNA storage is a topic of debate. We found in vitro messenger-RNA-binding activity in messenger RNP fractions free of C2-subunit-containing proteasomes and HSGs. In addition, mRNAs introduced into tobacco (Nicotiana plumbaginifolia) protoplasts were found in the cytoplasm but were not associated with HSGs. [full-text html and pdf at pubmedcentral]

Wikipedia on stress granules