Abstract
Proteins of the stroma and thylakoid membrane fractions of isolated chloroplasts from leaves of Vicia faba L. were investigated after clinostat rotation by SDS-PAGE and Western-immunoblot analysis with anti-ubiquitin antibodies. Two-week-old plants were rotated 15 and 24 hours on a slow clinostat. After a 24-h rotation, marked quantitative and qualitative changes in the polypeptide profile of thylakoid membrane proteins were found. Proteins of the stroma fraction (18 kDa and 57 kDa) revealed qualitative modifications. Thylakoid membrane proteins (15 kDa and about 20 and 30 kDa) exhibited changes in the immunoresponse to ubiquitin antibodies. The most obvious alterations of immunoreactive thylakoid membrane proteins were detected after 24 hours of rotation in the molecular range of about 20 and 30 kDa. The ubiquitination of these membrane-bound proteins is strongly diminished. This phenomenon probably 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 ubiquitination 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 treated samples. A changed protein turnover in soluble and membrane bound proteins of the chloroplasts after prolonged rotation on a slow clinostat is assumed.
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