Using biochemical means, Nowack and Grossman [4
] have convincingly demonstrated that at least some of the nucleus-encoded, chromatophore-derived proteins in Paulinella
function in the chromatophore. Specifically, the authors carried out western blot analysis and amino-terminal sequencing to confirm that the product of the nuclear psaE
gene and two distinct PsaK proteins are assembled into chromatophore-derived PSI. Further, inhibition experiments targeted at cytosolic- and chromatophore-derived ribosomes indicate that the PsaE and PsaK proteins are synthesized by the host's 80S ribosomes. Immunogold electron microscopy using a PsaE-specific antibody revealed a clear accumulation of gold particles on the chromatophore thylakoid membranes, suggesting the existence of a selective protein import system. Even more interesting is the fact that gold particles were also seen decorating the Golgi apparatus of the host, in addition to the chromatophore itself. This result is consistent with the intriguing possibility that the PsaE protein of Paulinella
is targeted to the chromatophore by an endoplasmic reticulum-Golgi-based system. The host cell secretion system has on multiple occasions been co-opted to function in the targeting of plastid proteins in organisms with secondary plastids (for example, dinoflagellates, euglenophytes and heterokontophytes [1
]), and also appears to be used to target a minority of proteins to primary plastids [1
]. The 'recycling' of pre-existing protein trafficking machinery is thus an emerging theme in the evolution of photosynthetic organelles.
The work of Nowack and Grossman [4
] effectively puts to rest doubts over whether the chromatophore of Paulinella
is an organelle. Yet there is still much to learn. Despite the clear localization of EGT-derived proteins to the Paulinella
chromatophore, the exact nature of the targeting pathway is still far from clear. Considering currently available bioinformatic, transcriptomic, and amino-terminal protein sequence data, there is no consistent picture as to whether nucleus-encoded, chromatophore-localized proteins such as PsaE and PsaK have amino-terminal extensions that could mediate their transport [4
], although the presence of signal peptide-like sequences has been proposed [21
]. In the case of PsaE at least, transport presumably requires other protein factors and elucidating these factors will be an important next step. Although the PsaE protein of Paulinella
was shown to pass through the Golgi, the possibility that PsaK and other endosymbiotically derived proteins are targeted to the chromatophore in a Golgi-independent manner cannot be ruled out. Considering the extent of genome reduction and the apparent paucity of transferred genes in the Paulinella
nuclear genome compared to organisms with canonical plastids, the current state of the chromatophore should be regarded as an early step in organelle evolution. It will thus perhaps not be surprising if its protein import system turns out not to be highly tuned. Ultimately, the extent to which the Paulinella
chromatophore can shed light on the evolution of canonical plastids will depend on the similarities and differences inferred about their independent evolutionary trajectories.