Protein import into the photosynthetic organelles of Paulinella chromatophora and its implications for primary plastid endosymbiosis

Abstract

The rhizarian amoeba Paulinella chromatophora harbors two photosynthetically active organelles of cyanobacterial origin that have been acquired independently of classic primary plastids. Because their acquisition did take place relatively recently, they are expected to provide new insight into the ancient cyanobacterial primary endosymbiosis. During the process of Paulinella endosymbiont-to-organelle transformation, more than 30 genes have been transferred from the organelle to the host nuclear genome via endosymbiotic gene transfer (EGT). The article discusses step-by-step protein import of EGT-derived proteins into Paulinella photosynthetic organelles with the emphasis on the nature of their targeting signals and the final passage of proteins through the inner organelle membrane. The latter most probably involves a simplified Tic translocon composed of Tic21- and Tic32-like proteins as well as a Hsp70-based motor responsible for pulling of imported proteins into the organelle matrix. Our results indicate that although protein translocation across the inner membrane of Paulinella photosynthetic organelles seems to resemble the one in classic primary plastids, the transport through the outer membrane does not. The differences could result from distinct integration pathways of Paulinella photosynthetic organelles and primary plastids with their respective host cells.

Keywords: Endosymbiosis; Molecular motor; Plastid; Protein import; Signal peptide; Vesicular trafficking.

Fig. 1

Paulinella chromatophora viewed under the optical (a) and electron (b) microscope. This testate filose amoeba is surrounded by the cell wall called theca (T), which is composed of silica scales. Apart from typical eukaryotic organelles such as nucleus (N) and mitochondria (M), it harbors two cyanobacterium-derived endosymbionts (E). The endosymbionts are photosynthetically active, deeply integrated with the host cell, and their genome have been reduced to one third in comparison to their cyanobacterial ancestors. Moreover, the endosymbionts import proteins encoded by genes that were transferred from the endosymbiont to the host nuclear genome. This import proceeds co-translationally via the host endomembrane system, involving the Golgi apparatus (G). All these features justify to call Paulinella photosynthetic endosymbionts true cell organelles. The Paulinella images were kindly supplied by Dr Eva Nowack

Fig. 2

Protein import into classic primary plastids (a) and Paulinella photosynthetic organelles (b). (a) The majority of nucleus-encoded plastid proteins are imported into classic primary plastids post-translationally using N-terminal transit peptide and Toc-Tic supercomplex. The latter consists of many specialized protein subunits that function as transit peptide receptors (Toc34, Toc64, and Toc159), protein-conducting channels (Toc75, Tic20, Tic21, and Tic110), regulatory elements (Tic55, Tic62, and Tic32), scaffold proteins (Tic110), Toc-Tic translocons connecting subunits (Toc12, Tic22), chaperones (Hsp70, Hsp93), and co-chaperones (Tic40). There are two independent molecular motors pulling imported proteins into the stroma: one consists of Tic110, Tic40, and Hsp93, whereas the second engages Hsp70, possibly GrpE, and some unknown J-domain protein (J?). (b) In contrast to classic primary plastids, nucleus-encoded photosynthetic proteins of Paulinella chromatophora are usually equipped with a signal peptide-like domain and are targeted to its photosynthetic organelles via the endomembrane system involving the endoplasmic reticulum (ER) and Golgi apparatus (GA). After the release of imported proteins into the intermembrane space (IMS), their further trafficking through the peptidoglycan wall could be facilitated by molecular chaperones, such as DegP, FkpA, PpiA, and Hsp70. The final import step, translocation across the inner membrane, may proceed via the Tic21 channel homolog with the help of Tic32-like protein, a calcium- and redox-sensing regulatory subunit. The imported proteins are hypothesized to be finally pulled into the organelle matrix by a molecular motor composed of Hsp93, Hsp70, Hsp40, and GrpE

Fig. 3

Alignment of PsaE proteins from two Paulinella strains. PsaE from FK01 strain is equipped with an N-terminal extension (yellow box) that was predicted with high confidence as a signal peptide by 27 from 30 algorithms with an unambiguous cleavage site (Mackiewicz and Bodył 2010). Eight non-conserved substitutions according to Blosum62 matrix (in red), which change biochemical properties of residues, may constitute some internal PsaE targeting signal in the CCAC0185 strain