The major protein translocation pathway in bacteria, the general secretory (Sec) pathway, transports proteins across both the thylakoid membranes and the cytoplasmic membrane, as demonstrated for Synechococcus PCC 7942 (Nakai et al., 1993). Synechococcus PCC 7942 has just a single gene for each of the Sec translocase components (secA, secY, secE and secG) and so an identical translocase must be operating in both membrane locations (Nakai et al., 1993). The Tat pathway also operates in both membrane systems in Synechocystis sp. (Aldridge et al., 2008) and this raises the question of how Tat substrates are targeted to a particular membrane. Two main hypotheses have been proposed: one hypothesis is that proteins are sorted
before translocation occurs. This would require the same translocation machinery recognizing a specific subset of proteins in different membrane systems, and there is some limited evidence in favour of this model. Lumacaftor mouse Thus, the signal sequences of noncytoplasmic proteins have different chemical properties depending on the final localization of the cargo protein (Rajalahti et al., 2007). This would suggest that membrane targeting is at least in part dictated by the signal peptide. Furthermore, the signal peptide of Tat substrates interacts with membranes as an early step in the translocation process (Hou et al., 2006; Bageshwar et al., 2009). Differences in the composition of the two membranes could provide one possible
mechanism for this pretranslocation sorting.
An alternative ABT 199 hypothesis is that proteins are sorted post-translocation and again second some evidence suggests that this might be the case. For example, cyanobacterial photosystems have been found to partially assemble within the plasma membrane before being translocated to the thylakoid membrane in a mechanism that might involve vesicular transport (Zak et al., 2001; Nevo et al., 2007). The actual mechanism of sorting is likely to be complex and may even involve both of the presented models to some extent. Approximately one in three cellular proteins are predicted to use metal ions for either a structural or functional role (Holm et al., 1996). Amongst the so-called trace-metals, iron and zinc are the two most frequently utilized (Maret, 2010). Cyanobacteria are likely to have played a major role in the bioavailability of metal ions through the evolution of oxygenic photosynthesis and the consequent oxygenation of the Earth’s atmosphere, roughly 2.75 billion years ago (Saito et al., 2003). Once soluble forms of Fe(II) were oxidized to more insoluble Fe(III) compounds, this is thought to have resulted in the evolution of sophisticated iron acquisition systems. Other metals, such as copper and zinc were liberated from insoluble sulphides and whilst this would have initially presented a challenge because of toxicity, it also presented cells with an opportunity to acquire and utilize ‘new’ metals (Cavet et al.
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