INFLUENCE OF HORIZONTAL CLINOSTAT ROTATION ON PLANT-PROTEINS .1. EFFECTS ON UBIQUITINATED POLYPEPTIDES IN THE STROMA AND THYLAKOID MEMBRANES OF VICIA-FABA L CHLOROPLASTS

Proteins of the stroma and thylakoid membrane fractions of isolated ch loroplasts from leaves of Vicia faba L. were investigated after clinos tat rotation by SDS-PAGE and Western-immunoblot analysis with anti-ubi quitin antibodies. Two-week-old plants were rotated 15 and 24 hours on a slow clinostat. After a 24-h rotation, marked quantitative and qual itative changes in the polypeptide profile of thylakoid membrane prote ins were found. Proteins of the stroma fraction (18 kDa and 57 kDa) re vealed qualitative modifications. Thylakoid membrane proteins (15 kDa and about 20 and 30 kDa) exhibited changes in the immunoresponse to ub iquitin antibodies. The most obvious alterations of immunoreactive thy lakoid membrane proteins were detected after 24 hours of rotation in t he molecular range of about 20 and 30 kDa. The ubiquitination of these membrane-bound proteins is strongly diminished. This phenomenon proba bly indicates selective degradation of proteins through the ubiquitin pathway or of the appearance of new conjugates with unknown function. Protein samples of the stroma fraction showed an increased ubiquitinat ion of the large subunit of the RuBPCase after either 15 h or 24 h of clinostat rotation. Free ubiquitin could be proved only in the 24 h tr eated samples. A changed protein turnover in soluble and membrane boun d proteins of the chloroplasts after prolonged rotation on a slow clin ostat is assumed.