engleski

The molecular concept of protein translocation across the outer membrane of chloroplasts

Chloroplasts originated in an endosymbiotic event more than 1.2 billion years ago (Butterfield, 2000), when a photosynthetic cyanobacterium was taken up by a heterotrophic cell. This event was followed by a massive transfer of genetic material to the host nucleus (Martin, 2003), which led to a problem for the newly established endosymbiotic relationship: proteins encoded by the transferred genes were now being synthesized in the cytosol of the host cell, and had to be transferred back to their place of action, the newly acquired chloroplast. Here, the chloroplast outer envelope is the first barrier for the cytosol-synthesized proteins. For a long time, the prevailing opinion was that the outer envelope of chloroplasts is like a molecular sieve – full of holes and leaky, with maybe a limited selectivity (Flügge, 2000). However, a significant body of recently accumulated evidence points to a tightly controlled regulation of the traffic across the outer envelope, previous picture of a molecular sieve being attributed to artifacts of at that time widely used patch-clamping technique (Soll et al., 2000). One of the molecular gates controlling the entry of molecules into chloroplasts is the protein translocon, responsible for selection and import of protein molecules. The first task of this complex is the recognition and binding of chloroplastic precursor proteins which have to be imported. Most of such proteins are synthesized with a cleavable Nterminal transit sequence, which is both necessary and sufficient to target them for chloroplast import (Sveshnikova et al, 2000 ; Bruce, 2001). The transit sequence consists of 20 to 150 amino acids, has an overall positive charge and is rich with hydroxylated amino acids (Soll and Schleiff, 2004). Its variable length and divergent primary structure suggest that it is not a specific sequence motif, but rather a certain structural characteristic, which is recognized by the receptors. For a long time it was believed that only outer envelope proteins lack an Nterminal transit sequence and some authors suggest that they might have an internal signal instead (Bruce et al., 2001 ; Schleiff and Klösgen, 2001). However, recent proteomic approaches identified many plastid proteins without an obvious signal with the above outlined characteristics within the coding region (Kleffmann et al., 2004). Therefore, the characteristics of the targeting signal and translocation events will be revisited for these proteins in future. The first hint for an alternative route in Arabidopsis thaliana came from the observed endoplasmic reticulum intermediate of α – carbonic anhydrase before its translocation into plastids (Villarejo et al., 2005).

TOC ; TIC ; preprotein

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