Plastid Protease Activity and Prolamellar Body Transformation during Greening

Two proteases active in and specific to oat etioplasts and up to 24-hour etiochloroplasts, only very slightly contaminated by other cellular compartments are described. The enzyme showed pH optima of 4.2 (acid) and 6.8 (neutral), temperature optima of 50 C and the highest level of enzyme activity was with prolamellar bodies (PLBs) as substrate. Both enzymes showed evidence of a sulfhydryl reagent requirement, particularly for the neutral enzyme. Levels of both proteases increased up to 4 hours of illumination of leaves, and then sharply decreased with the largest differences exhibited by the neutral protease. The pH values in the plastid stroma indicated that the neutral enzyme was likely to be the most important in PLB transformation. A comparison between plastid-associated and extra-plastidic protease activities showed similar properties, except the affinity toward PLBs, which was much higher for plastid proteases (K(m): 0.2 and 1.1 milligrams protein per milliliter, respectively).