Background: Tic22 is a core protein import machinery component of chloroplasts and apicoplasts. Results: Tic22 is a chaperone, required for parasite survival and for protein import in the apicoplast. Conclusion: Tic22 lies between the plastid membranes, maintaining imported proteins unfolded for translocation. Significance: Chaperones which hold proteins in an unfolded state are central to membrane translocation systems and Tic22 plays this role in plastids.

Tic22 is a component of the protein-import apparatus of the chloroplasts of plants and algae and the apicoplasts of the Apicomplexa, a large group of organisms that includes the parasites that cause malaria. Tic22 is important for protein import into these organelles and for organelle biogenesis. It lies between the two membranes of chloroplasts, making interactions with components of both the TIC and TOC complexes. In the apicoplast, it is predicted to be located between the inner two membranes and to play a similar role in import. Although Tic22 is ubiquitous, its function is as yet uncertain. Tic22 from Plasmodium falciparum was therefore overproduced, purified and crystallized. A data set extending to 2.15 Å resolution has been collected from a crystal containing selenomethionine-labelled protein and structure determination is under way.

For biophysical investigations on viral proteins, in particular for structure determination by X-ray crystallography, relatively large quantities of purified protein are necessary. However, expression of cDNAs coding for viral proteins in prokaryotic or eukaryotic systems is often not straightforward, and frequently the amount and/or the solubility of the protein obtained are not sufficient. Here, we describe a number of protocols for production of nonstructural proteins of coronaviruses that have proven to be efficient in increasing expression yields or solubilities.