Evidence for the probable oil body association of a thiol-protease, leading to oleosin degradation in sunflower seedling cotyledons

The activity of a 65 kDa, cytosolic protease from sunflower seedling cotyledons coincides with the degradation of oleosins during seed germination. Further investigations carried out in this laboratory have demonstrated the probable association of a thiol-protease with oil bodies, leading to gradual degradation of oleosins during seedling growth. Evidence to this effect have been brought out through zymographic detection of protease activity from oil bodies, degradation of oleosins by electrophoretically eluted protease from the seedling cotyledons and inhibition of protease activity by thiol-protease inhibitor, such as N-ethylmaleimide (NEM). In addition to these biochemical evidence, visualization of thiol-protease activity has also been achieved by a novel fluorescence microscopic method and confocal imaging. It involves the uptake and binding of a fluorogenic thiol-protease inhibitor (fluorescein mercuric acetate, FMA) at the intracellular thiol-protease activity sites in protoplasts, leading to fluorescence emission at 523 nm following excitation at 499 nm. Maximum protease activity is observed in 4-d-old seedling cotyledons, coinciding with the phase of active triacylglycerol (TAGs) hydrolysis. All these observations provide evidence for the expression of the said thiol-protease activity on the oil body surface, leading to gradual proteolysis of oleosins during seed germination.