Submitted to: The Plant Cell
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/26/2004
Publication Date: 11/14/2004
Citation: Hwang, Y.T., Pelitire, S.M., Henderson, M.P.A., Andrews, D.W., Dyer, J.M., and Mullen, R.T. 2004. Novel targeting signals mediate the sorting of different isoforms of the tail-anchored membrane protein cytochrome b5 to either endoplasmic reticulum or mitochondria. Plant Cell 16(11):3002-3019. Interpretive Summary: The cells of all living organisms are in a constant state of renewal, continuously replacing parts that have broken down or building new structures to perform needed biological functions. One of the great challenges of modern science is to figure out how the various parts of a cell are put together. Much like the construction jobs in modern cities, the necessary building materials must first be transported to the correct job site where they are subsequently assembled into the final form. In the current manuscript, which is accepted for publication in the world's top plant research journal, scientist at ARS and collaborators at the University of Guelph and McMaster University describe in great detail the underlying mechanisms that are involved in the transport of proteins to two specific locations in plant cells. This information illuminates two of the major routes by which proteins are transported to different places within the same cell. As such, this knowledge will impact many different areas of plant science research, since there are likely to be hundreds of different proteins that travel along these same routes to reach their final destination.
Technical Abstract: Tail-anchored membrane proteins are a class of proteins that are targeted post-translationally to various organelles and integrated by a single segment of hydrophobic amino acids located near the C terminus. While the localization of tail anchored proteins in specific subcellular compartments in plant cells is essential for their biological function, the molecular targeting signals responsible for sorting these proteins are not well defined. Here we describe the biogenesis of four closely related tung (Aleurites fordii) cytochrome b5 isoforms (Cb5-A, -B, -C, and -D), which are small tail-anchored proteins that play an essential role in many cellular processes, including lipid biosynthesis. Using a combination of in vivo and in vitro assays, we show that Cb5-A, -B and -C are targeted exclusively to the endoplasmic reticulum (ER) while Cb5-D is targeted specifically to mitochondrial outer membranes. Comprehensive mutational analysis of ER and mitochondrial Cb5s revealed that their C termini, including transmembrane domains (TMD) and tail regions, contained several unique physicochemical and sequence-specific characteristics that defined organelle-specific targeting motifs. Mitochondrial targeting of Cb5 was mediated by a combination of hydrophilic amino acids along one face of the TMD, an enrichment of branched beta-carbon-containing residues in the medial portion of the TMD, and a dibasic -R-R/K/H-x motif in the C-terminal tail. In contrast, ER targeting of Cb5 depended primarily upon the overall length and hydrophobicity of the TMD, although an -R/H-x-Y/F-motif in the tail was also a targeting determinant. Collectively, the results presented provide significant insight to the early biogenetic events required for entry of tail-anchored proteins into either the ER or mitochondrial targeting pathways.