Highways for protein delivery to the mitochondria Trevor Lithgow,Jos6 M. Cuezva and Pamela A. Silver Messenger RNA (mRNA) Iocalisation is one of the prime mechanisms to ensure protein Iocalisation in the cytoplasm of polarised embryonic cells, and has been well-studied in the development of Xenopus and Drosophila embryos. But what of other cells? Here, we discuss whether the directed transport of mRNA out of the nucleus, following cytoplasmic highways to a specified organelle in the cytoplasm, might also contribute to the exquisite fidelity of protein targeting observed in all eukaryotic ceils. HOW DO PROTEINSencoded by nuclear DNA, but whose function is in another cellular organelle, ensure that they are targeted to the right place? One way would be transport of the mRNA encod- ing that protein from the nucleus to a ribosome in the vicinity of the correct subcellular compartment, for localised protein synthesis. In the case of protein delivery to the mitochondria, both mRNA.binding proteins and targeting factors that bind the nascent precursor proteins seem to assist in the journey to the mitochondrla. Mitochondrial precursor proteins carry amino4erminal targeting sequences to ensure import into the mitochondria. Precursor proteins specifically bind to the surface of the mitochondrla and are then translocated across one or both membranes and sorted to one of the intramitochondrial compartments ~-3.The protein import machinery in the mito- chondrla is well<haracterised, but the delivery of newly made precursor pro- teins to the mitochondria =¢mains to be understood. While it is possible that mRNAs encoding mitochondrial precur- sors are released and transferred ran- domly to uniformly distributed ribo- somes in the cytosol, and that the newly made precursor proteins encounter mitochondria only by chance, there is now mounting evidence to suggest that the mRNA export machinery can 'last- track' the early stages of mitochondrial protein sorting. Genetic screens for yeast mutants compromised in mitochondrial protein import suggest a directed pathway for the delivery of precursor proteins. By one approach, a fusion protein consist- ing of a non-mitochondrial passenger protein ([3-galactosidase) was directed 1', Uthgow is at the School of Biochemistry, La Trobe University,Bundoora 3083, Australia; ~. M, Cuezva is at the Centro de Biologa Molecular, UniversidadAutOnoma de Madrid, 28049 Madrid, Spain; and P, A, $11v~ is at the Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School and Dana Farber Cancer Institute, Boston, MA 02115, USA. Email: t.lithgow@latrobe.edu.au TmBS 22 - APRIL 1997 to the mitochondda by the amino- terminal targeting sequence from F~[~, the fi-subunit of the mitochgndrial ATPase4. Mutant yeast ceils that failed to deliver the fusion protein were iso- lated and are referred to as raft mutat~ts (for mitochondrial-fusion-targeting de- fects). One of these mutants, raft/, has a defect in the gene encoding the cyto- solic factor Mft52p. Purified Mft52p was found to bind mitochondrial targeting sequences, the first part of the precur- sor to emerge from the ribosome s, sug- gesting a role for Mft52p in directing the delivery of nascent precursors to the mitochondrial surface (Fig. 1). While fusion proteins that rely solely on their amino=terminal targeting infor- mation remain stranded in the cytosol of mftl cells, 'natural' precursor proteins evidently contain additional targeting information, as they can be transported to the mitochondna without assistance from Mft52p (Refs 4, 5). This additional targeting information might be a func- tion of the conformation of the mature part of the precursor protein, which might mean that molecular chaperones such as HspT0 and the mitochondrial- import stimulating factor (lVlSF) assist in the targeting of genuine precursor proteins 6, but not fusion proteins i, which the bulk o| the protein is non- mitochondriai. Mft52p ~!~gAA Hsp70 TOM Figure 1 A model for precursorprotein targeting to the mitochondria.Signals in the untranslatedre- gion of mRNAsthat encode mitochondrialprecursors direct mRNAtransport from the nu- cleus, mediated in pa~t by mRNA-binding proteins such as Npl3p. As a result, 80S ribo- somes could be targeted to the vicinity of the mitochondria. Recognitionof the nascent precursor protein by targeting factors [Mft52p (Re[. 7)] and molecularchaperones[such as Hsp70 and mitochondriaHmport stimulatingfactor (MSF)(Re[. 5)] ensures protein delivery to the translocase of the outer mitochondrial membrane (TOM)complex,the import recep- tor on the mitochondrial surface 1. 110 Copyright © 1997, Elsevier Science Ltd.Allrights reserved.0968-0004/97/$17.00 PII: S0968-0004(97)01007-4