Proteins preprotein

As described earlier, translation of the EPSPS mRNA of plants results in the formation of a protein which has an AJ-terminal extension. The AJ-terminal extension, referred to as the chloroplast transit peptide, is necessary and sufficient for the import of the preprotein by the chloroplast. Once imported by the chloroplast, the transit peptide is cleaved releasing the mature enzyme. As expected, introduction of the EPSPS transit peptide to other protein sequences results in the importation of the fusion protein by the chloroplast. 

Proteins that are destined to be secreted or to end up as transmembrane proteins are synthesized with an N-terminal signal peptide. Signal peptides are highly hydrophobic sequences of variable length. Proteins synthesized with signal peptides are termed preproteins. For example, insulin is a secreted protein. It is therefore synthesized as a preprotein. It is also synthesized in inactive form a proprotein. Insulin as initially synthesized is, in consequence, a preproprotein. 

Preprotein a protein synthesized with the N-terminal signal peptide targeting it to a membrane site. 

The two complexes that import cytosolic proteins through the mitochondrial inner membrane, TIM22 and TIM23 split the import pathways of hydrophobic inner membrane proteins from that of presequence-containing preproteins (Fig. 1). 

Becker L et al. (2005) Preprotein translocase of the outer mitochondrial membrane reconstituted Tom40 forms a characteristic TOM pore. J Mol Biol 353 1011-1020 Blobel G, Dobberstein (1975a) Transfer of proteins across membranes. I. Presence of proteolytically processed and unprocessed nascent immunoglobulin light chains on membrane-bound ribosomes of murine myeloma. J Cell Biol 67 835-851 Blobel G, Dobberstein (1975b) Transfer to proteins across membranes. II. Reconstitution of functional rough microsomes from heterologous components. J Cell Biol 67 852-862... 

Honlinger A et al. (1995) The mitochondrial receptor complex Mom22 is essential for cell viability and direcdy interacts with preproteins. Mol Cell Biol 15 3382-3389 Hoogenraad NJ, Ward LA, Ryan MT (2002) Import and assembly of proteins into mitochondria of mammalian cells. Biochim Biophys Acta 1592 97-105 Hoppins SC, Nargang FE (2004) The Tim8-Timl3 complex of Neurospora crassa functions in the assembly of proteins into both mitochondrial membranes. J Biol Chem 279 12396-12405... 

Murcha MW et al. (2007) Characterization of the preprotein and amino acid transporter gene family in Arabidopsis. Plant Physiol 143 199-212 Murcha MW, Millar AH, Whelan J (2005a) The N-terminal cleavable extension of plant carrier proteins is responsible for efficient insertion into the inner mitochondrial membrane. J Mol Biol 351 16-25... 

Truscott KN et al. (2001) A presequence- and voltage-sensitive channel of the mitochondrial preprotein translocase formed by Tim23. Nat Struct Biol 8 1074-1082 Truscott KN et al. (2003) A J-protein is an essential subunit of the presequence translocase-associated protein import motor of mitochondria. J Cell Biol 163 707-713 van der Giezen M et al. (2003) Fungal hydrogenosomes contain mitochondrial heat-shock proteins. Mol Biol Evol 20 1051-1061... 

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