Preproteins

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. 

A furin-like protease is a recursor (prehormone, preprotein) conveitase (PC). 

The details of how preproteins are translocated have not been fully elucidated. It is possible that the electric potential associated with the inner mitochondrial membrane causes a conformational change in the unfolded preprotein being translocated and that this helps to puU it across. Furthermore, the fact that the matrix is more negative than the intermembrane space may attract the positively charged amino terminal of the preprotein... 

Pfanner, N., Craig, E., and Honlinger, A. (1997). Mitochondrial preprotein translocase. Annu. Rev. Cell Biol. Dev. 13, 25-51. 

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. 

Signal peptide an N-terminal amino acid sequence that targets preproteins to membrane sites. 

Three of the frequently occurring amino acid residues in hydrogenosomal presequences are positionally conserved as well. Of the 13 hydrogenosomal matrix preproteins for which presequence cleavage sites have been experimentally determined, 12 have Leu at position 2 of the presequence, and the exception has a Leu residue at position 3 (Table 1). Thus, not only the presence, but also the position of the Leu residue is conserved. This is even more striking when we... 

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). 

---