Small heat shock/
alpha-crystallin proteins function as
molecular chaperones, protecting other
proteins from irreversible denaturation by an energy-independent process. The brine shrimp, Artemia franciscana, produces a small heat shock/
alpha-crystallin protein termed p26, found in embryos undergoing encystment, diapause, and metabolic arrest. These embryos withstand long-term
anoxia and other stresses normally expected to cause death, a property likely dependent on
molecular chaperone activity. The association of p26 with
tubulin in unfractionated cell-free extracts of Artemia embryos was established by affinity chromatography, suggesting that p26 chaperones
tubulin during encystment. To test this possibility, both
proteins were purified by modifying published protocols, thereby simplifying the procedures, enhancing p26 yield about 2-fold, and recovering less
tubulin than before. The denaturation of purified
tubulin as it "aged" and exposed hydrophobic sites during incubation at 35 degrees C was greatly reduced when p26 was present; however,
tubulin polymerization into microtubules was reduced. On incubation at 35 degrees C, centrifugation in
sucrose density gradients demonstrated the association of purified p26 with
tubulin. This is the first study where the relationship between a small heat shock/
alpha-crystallin protein and
tubulin from the same physiologically stressed organism was examined. The results support the proposal that p26 binds
tubulin and prevents its denaturation, thereby increasing the resistance of encysted Artemia embryos to stress. Additional factors are apparently required for release of
tubulin from p26 and restoration of efficient assembly, events that would occur as embryos resume development and the need for microtubules is established.