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Eukaryotic Cell, December 2003, p. 1234-1245, Vol. 2, No. 6
1535-9778/03/$08.00+0     DOI: 10.1128/EC.2.6.1234-1245.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Cysteine Proteases and Cell Differentiation: Excystment of the Ciliated Protist Sterkiella histriomuscorum

Eduardo Villalobo, Clara Moch, Ghislaine Fryd-Versavel, Anne Fleury-Aubusson, and Loïc Morin^*

Laboratoire de Biologie Cellulaire 4, Université de Paris-Sud, Orsay, France

Received 4 April 2003/ Accepted 5 August 2003

The process of excystment of Sterkiella histriomuscorum (Ciliophora, Oxytrichidae) leads in a few hours, through a massive influx of water and the resorption of the cyst wall, from an undifferentiated resting cyst to a highly differentiated and dividing vegetative cell. While studying the nature of the genes involved in this process, we isolated three different cysteine proteases genes, namely, a cathepsin B gene, a cathepsin L-like gene, and a calpain-like gene. Excystation was selectively inhibited at a precise differentiating stage by cysteine proteases inhibitors, suggesting that these proteins are specifically required during the excystment process. Reverse transcription-PCR experiments showed that both genes display differential expression between the cyst and the vegetative cells. A phylogenetic analysis showed for the first time that the cathepsin B tree is paraphyletic and that the diverging S. histriomuscorum cathepsin B is closely related to its Giardia homologues, which take part in the cyst wall breakdown process. The deduced cathepsin L-like protein sequence displays the structural signatures and phylogenetic relationships of cathepsin H, a protein that is known only in plants and animals and that is involved in the degradation of extracellular matrix components in cancer diseases. The deduced calpain-like protein sequence does not display the calcium-binding domain of conventional calpains; it belongs to a diverging phylogenetic cluster that includes Aspergillus palB, a protein which is involved in a signal transduction pathway that is sensitive to ambient pH.

Present address: Departamento de Microbiologia, Facultad de Biologia, Universidad de Sevilla, Seville, Spain.