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Abstract Detail

Borrowed Chloroplasts: Secondary Endosymbiosis and the Chromalveolates

Gould, Sven B. [1], Sommer, Maik S. [2], Maier, Uwe-G. [2].

Getting there is all the fun: pre-protein targeting and transport in chromalveolates.

Plastids of cryptophytes, heterkontophytes and apicomplexa trace back to a red algal endosymbiont and are surrounded by four membranes, turning targeting of plastid pre-proteins into quite a hurdle-race. Not much is known about factors involved, but for correct targeting stromal proteins encoded in the nucleus harbour a bipartite leader, consisting of a signal- and transit peptide.
We focused on the cryptophyte Guillardia theta, which between the outer and inner membrane pair still harbours the remnant red algal nucleus, the nucleomorph, inside the periplastidial compartment (PPC). This offers the opportunity to discriminate between proteins being translocated across only the outer two membranes into the PPC and those crossing all four membranes to reach the stroma. In an EST-library we identified several nuclear encoded PPC proteins, which surprisingly all harbour a bipartite leader just like stromal pre-proteins. The +1 position of the transit peptide alone turned out to be the crucial trigger for correct targeting of the two subsets of pre-proteins.
Furthermore we investigated the transport of nuclear-encoded phycoerythrin alpha (PEα) and could show that the TAT pathway transports these light-harvesting pigments into the thylakoid lumen after they reached the stroma. They get processed three times, and cross five biological membranes, before they are released into the thylakoid lumen.
Transferring this knowledge we also identified PPC proteins in other chromalveolates, such as e.g. Phaeodactylum tricornutum and Plasmodium falciparum. Amongst them were proteins of the ER-associated degradation machinery, which in cryptophytes is still encoded on the nucleomorph. We suggest this machinery to be the long-searched-for translocator for nuclear-encoded endosymbiont proteins in the periplastidial membrane, challenging the vesicle-transport theory.
Hence, its fun being a nuclear encoded plastid protein in a chromalveolate as you get to see many places, and loose lots of kDa, before you actually reach your destination compartment.

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1 - University of Melbourne, School of Botany, Professors Walk, Melbourne, Victoria, 3010 Parkville, Australia
2 - Philipps University marburg, Cell Biology

complex plastid
periplastidial compartment

Presentation Type: Symposium or Colloquium Presentation
Session: SY15
Location: Waldorf Room/Hilton
Date: Wednesday, July 11th, 2007
Time: 9:30 AM
Number: SY15004
Abstract ID:2342

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