Translocation of proteins across and integration of membrane proteins into the rough endoplasmic reticulum

mRNA Localization to the Endoplasmic Reticulum in Plant Endosperm Cells

Subcellular mRNA localization is an evolutionarily conserved mechanism to spatially and temporally drive local translation and, in turn, protein targeting. Hence, this mechanism achieves precise control of gene expression and establishes functional and structural networks during cell growth and development as well as during stimuli response. Since its discovery in ascidian eggs, mRNA localization has been extensively studied in animal and yeast cells. Although our knowledge of subcellular mRNA localization in plant cells lags considerably behind other biological systems, mRNA localization to the endoplasmic reticulum (ER) has also been well established since its discovery in cereal endosperm cells in the early 1990s. Storage protein mRNA targeting to distinct subdomains of the ER determines efficient accumulation of the corresponding proteins in different endosomal storage sites and, in turn, underlies storage organelle biogenesis in cereal grains. The targeting process requires the presence of RNA localization elements, also called zipcodes, and specific RNA-binding proteins that recognize and bind these zipcodes and recruit other factors to mediate active transport. Here, we review the current knowledge of the mechanisms and functions of mRNA localization to the ER in plant cells and address directions for future research.

Molecular genetic characterization and subcellular localization of a putative Theileria annulata membrane protein

A Theileria annulata protein (TaD) exhibiting an N-terminal signal sequence for endoplasmic reticulum membrane translocation and a conserved cysteine-rich region was isolated by screening the mRNA of a T. annulata-infected bovine lymphoblastoid cell line with degenerated primers directed against T. annulata-targeting sequences. The TaD-coding sequence was found to be most closely related to the genomic DNA sequence of T. parva (TIGR database, 72%) and the amino acid sequence of Plasmodium falciparum (41%), P. yoelii yoelii (38%) and Cryptosporidium parvum (36%). The TaD mRNA is expressed within the sporozoite, schizont and merozoite stages of the parasite, implying that it is constitutively transcribed throughout the parasite's life cycle. Allelic variants were found between isolates originating from different geographical regions, however not affecting conserved cysteines. The open reading frame encoded a protein of 19.5 kDa and non-reducing SDS-PAGE analysis demonstrated a homodimeric protein. Using confocal microscopy, the protein was found to be both located in the parasite cytoplasm and to colocalize with a transmembrane protein of the schizonts within infected cells.