RNA targeting to a specific ER sub‐domain is required for efficient transport and packaging of α‐globulins to the protein storage vacuole in developing rice endosperm

Abstract

SUMMARYStudies focusing on the targeting of RNAs that encode rice storage proteins, prolamines and glutelins to specific sub-domains of the endoplasmic reticulum (ER), as well as mis-localization studies of other storage protein RNAs, indicate a close relationship between the ER site of RNA translation and the final site of protein deposition in the endomembrane system in developing rice endosperm. In addition to prolamine and glutelin, rice accumulates smaller amounts of a-globulins, which are deposited together with glutelin in the protein storage vacuole (PSV). In situ RT-PCR analysis revealed that a-globulin RNAs are not distributed to the cisternal ER as expected for a PSV-localized protein, but instead are targeted to the protein body-ER (PB-ER) by a regulated process requiring cis-sorting sequences. Sequence alignments with putative maize d-zein cis-localization elements identified several candidate regulatory sequences that may be responsible for PB-ER targeting. Immunocytochemical analysis confirmed the presence of a-globulin on the periphery of the prolamine protein bodies and packaging in Golgi-associated dense vesicles, as well as deposition and storage within peripheral regions of the PSV. Mis-targeting of a-globulin RNAs to the cisternal ER dramatically alters the spatial arrangement of a-globulin and glutelin within the PSV, with the accompanying presence of numerous small a-globulin particles in the cytoplasm. These results indicate that a-globulin RNA targeting to the PB-ER sub-domain is essential for efficient transport of a-globulins to the PSV and its spatial arrangement in the PSV. Such RNA localization prevents potential deleterious protein-protein interactions, in addition to performing a role in protein targeting.

Biogenesis, Turnover, and Mode of Action of Plant MicroRNAs

“…Some AGO1 subcellular foci reside at points along the ER network and may represent sites where translationally repressed mRNAs accumulate. Localization of specific mRNAs to subdomains of the ER has been observed in plants (Li et al, 1993;Washida et al, 2012), and compartmentalization of miRNA target mRNAs within AMP1 compartments might provide a mechanistic basis for the separation of slicing and translational repression.…”

Localized translation near the mitochondrial outer membrane: An update

“…Additional nonexclusive contacts have been reported, but all involve a translational process. The interaction between the mitochondria targeting sequence (MTS) undergoing translation and Tom20 (2) 51 is likely to be important for specifying which ribosomes should be maintained near the mitochondria. Nevertheless, once the MTS is imported into the mitochondria other anchoring factors are necessary.…”

“…Furthermore, the synthesis of proteins with similar roles in proximity to each other may provide on-site regulation of expression and a faster response to local needs. [1][2][3][4] Segregating the translation process to distinct loci throughout the cell may also be beneficial in cases of stress, in that some translation sites are more protected from stress than others and maintain some protein synthesis in adverse conditions, 5,6 presumably to support cell survival. Thus, localized translation further compartmentalizes cells into small functional units, which provide better response to local needs and improves overall physiological efficiency.…”

Translational Regulation of Cytoplasmic mRNAs

“…In plants, the targeting of two types of storage protein mRNA has been studied extensively in rice (Crofts et al, 2005;Washida et al, 2012). Prolamine mRNAs are targeted to a special subdomain of the ER, where prolamine protein bodies form.…”