Localization of sulfolipid labeling within cells and chloroplasts

Spinach chloroplasts were purified on gradients of Percoll which preserved envelope impermeability and CO2-dependent oxygen evolution in the light. Application of (35)SO4″ to purified chloroplasts resulted in a light-dependent labeling of a lipid component which was indentified as sulfoquinovosyl diacylglycerol. Fractionation of chloroplasts showed that after 5 min of labeling most of the newly synthesized sulfolipid was present in thylakoids. Only a small percentage was recovered from the envelopes. Molecular species from envelopes and thylakoids were identical. The molecular species did not change during incubation times ranging from 5 min up to 4.5 h. Mesophyll protoplasts from (35)SO4″-labeled oat primary leaves were gently disrupted and separated into organelles by sucrose gradient centrifugation. Labeled sulfolipid was located almost exclusively in the chloroplasts. This, in combination with the experiments carried out with isolated chloroplasts, indicates that the final assembly steps in the biosynthesis of sulfolipid are confined to the chloroplasts.