Tsp36, a tapeworm small heat-shock protein with a duplicated alpha-crystallin domain, forms dimers and tetramers with good chaperone-like activity

Small heat shock proteins (sHSPs), which range in monomer size between 12 and 42 kDa, are characterized by a conserved C-terminal alpha-crystallin domain of 80-100 residues. They generally form large homo- or heteromeric complexes, and typically have in vitro chaperone-like activity, keeping unfolding proteins in solution. A special type of sHSP, with a duplicated alpha-crystallin domain, is present in parasitic flatworms (Platyhelminthes). Considering that an alpha-crystallin domain is essential for the oligomerization and chaperone-like properties of sHSPs, we characterized Tsp36 from the tapeworm Taenia saginata. Both wild-type Tsp36 and a mutant (Tsp36C-->R) in which the single cysteine has been replaced by arginine were expressed and purified. Far-UV CD measurements of Tsp36 were in agreement with secondary structure predictions, which indicated alpha-helical structure in the N-terminal region and the expected beta-sandwich structure for the two alpha-crystallin domains. Gel permeation chromatography and nano-ESI-MS showed that wild type Tsp36 forms dimers in a reducing environment, and tetramers in a non-reducing environment. The tetramers are stabilized by disulfide bridges involving a large proportion of the Tsp36 monomers. Tsp36C-->R exclusively occurs as dimers according to gel permeation chromatography, while the nondisulfide bonded fraction of wild type Tsp36 dissociates from tetramers into dimers under nonreducing conditions at increased temperature (43 degrees C). The tetrameric form of Tsp36 has a greater chaperone-like activity than the dimeric form.