University of Cambridge > Talks.cam > Plant Sciences Departmental Seminars > Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system

Protein import into chloroplasts and its regulation by the ubiquitin-proteasome system

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The development and operation of plant chloroplasts (or plastids) requires the participation of thousands of organellar proteins. Most chloroplast proteins are nucleus-encoded and synthesized in the cytosol in precursor form. These preproteins pass through multiprotein import machines in the organelle’s outer and inner envelope membranes, called the TOC and TIC translocons. Receptors components of the TOC complex exist in multiple isoforms, and genetic studies in the model plant Arabidopsis have indicated that the isoforms have distinct preprotein recognition specificities and function in different import pathways. Operation of such client-specific import pathways controls the organelle’s proteome and functions, and plays a role in the differentiation of different plastid types (e.g., the chromoplasts formed in ripening tomato fruit). Our work has shown that the chloroplast protein import machinery is proteolytically regulated by direct action of the ubiquitin-proteasome system (UPS), in a process termed “chloroplast-associated protein degradation” (CHLORAD). The CHLORAD machinery has three key components: the SP1 ubiquitin E3 ligase, the SP2 β-barrel channel protein, and the CDC48 AAA + ATPase. The SP1 and SP2 proteins form a complex in the outer envelope membrane, and respectively mediate the ubiquitination and extraction (or retrotranslocation) of TOC protein targets. The CDC48 protein, located in the cytosol, provides the energy that drives the retrotranslocation step, delivering the target proteins to the cytosolic 26S proteasome for degradation. Recent advances in our understanding of the mechanisms and functions of CHLORAD will be discussed.

This talk is part of the Plant Sciences Departmental Seminars series.

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