Characterization of a SRS13: a new cyst wall mucin-like domain containing protein

Toxoplasma gondii, an obligate intracellular apicomplexan, causes latent infection in about one third of the human population. During latent infection, T. gondii bradyzoites are found within cysts, a modified parasitophorous vacuole. This parasite has a large family of SRS (surface antigen-1 related sequence) proteins which are reported to be involved in attachment of these organisms to their mammalian host cells and in immune subversion during latent infection. We have identified a novel mucin domain containing SRS protein, using a glycoepitope-specific antibody, which recognizes the cyst wall. SRS13 has two SRS domains and between these domains is a threonine-rich tandem repeat mucin-like domain that is similar to the mucin-like domain seen in another cyst wall specific SRS protein CST1 (SRS44). SRS13 is upregulated in bradyzoites and O-GalNAc glycosylated by ppGalNAc-T2 and T3. Similar to the cyst wall protein CST1, SRS13 localizes to the cyst wall, but unlike CST1, SRS13 is dispensable for normal cyst wall formation. Together, these findings elucidate the role of a second SRS mucin domain protein, SRS13, in bradyzoite biology and expands the previously reported functions of the SRS protein family.