Estudos sobre o duplo direcionamento de proteínas de plantas / Studies on the dual targeting of plant proteins

AUTOR(ES)

Carolina Vianna Morgante

DATA DE PUBLICAÇÃO

2008

RESUMO

Compartimentalization of the metabolic processes in organelles, each one having a characteristic protein pool and distinct functions, is a property of eukaryote cells. A highly specific cellular system directs proteins, which are synthesized in the cytosol, to the proper organelles. However, due to functional overlaps between organelles, a given protein may be needed in different compartments. This is the case of dual-targeted proteins, which are the product of single nuclear genes, but are somehow directed to different organelles. About 60 plant proteins have had their dual targeting demonstrated, most of them to mitochondria and cloroplasts, the phenomenon being not so rare as previously supposed. Investigations have focused on the mechanisms, which enable protein dual targeting, even in the presence of other cell mechanisms that guarantee the specific protein transport to each organelle. The present work on the subject can be divided in three parts. In the first part, evolutionary aspects of dual targeting were investigated. A comparative analysis of gene families that included members encoding dual-targeted proteins in Arabidopsis thaliana and Oryza sativa demonstrated that the dual targeting of monodehidro-ascorbate reductase, methyonine aminopeptidase and, problably, of the thiazole biosynthetic enzyme THI1 was evolutionary conserved between the two species. In addition, the data suggested the same pattern of evolution for families with members presenting dual targeting. The focus of the second part was the ambiguous sequence for dual targeting. After showing that the RNA-binding proteins RBP1a, RBP1b and RPS19 were dual-targeted to mitochondria and cloroplasts, sitedirected mutations were introduced in the targeting sequence of RBP1b. The importance of positive-charged amino acids for directing the protein to mitochondria was confirmed. Mutation of alanine at position 2, which is conserved in ambiguous sequences, was shown not to affect RBP1b dual targeting. Information for dual targeting was localized among the 17 first amino acids in the amino-terminal region. The signals for directing the protein to cloroplasts appeared distributed along the targeting sequence. While the amino-terminal half of the sequence was sufficient for RBP1b dual targeting, the sequence comprising the next 13 amino acids appeared to affect the efficiency of the transport. In these analyses, a quantitative method to measure the intensity of fluorescent signals of GFP had its efficacy demonstrated to be adopted for in vivo quantitative analysis of dual targeting to mitochondria and cloroplasts. In the third part, the focus was the translational mechanism enabling THI1 dual targeting to mitochondria and cloroplasts, in A. thaliana. It was hypothesized that an internal ribosomal entry site (IRES) was present in thi1 mRNA. However, a transient expression system could not be found that reproduced the literature data demonstrating THI1 dual targeting. In the tested systems the protein was addressed solely to cloroplasts, thus preventing the objective to be pursued.

ASSUNTO(S)

cloroplastos arabidopsis thaliana mitocôndrias vegetais chloroplast arroz mitochondria papaverales. protein dual targeting oryza sativa. proteínas de plantas

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