Rapid degradation of apoplastocyanin in Cu(II)-deficient cells of Chlamydomonas reinhardtii.

Although plastocyanin is not detected in Cu(II)-deficient cells of Chlamydomonas reinhardtii, accumulation of messenger RNA for pre-apoplastocyanin is independent of the concentration of Cu(II) in the medium (Merchant, S., and Bogorad, L. (1986) Mol. Cell. Biol. 6, 462-469). This work shows that the synthesis, transport, and processing of pre-apoplastocyanin also appear to be unaffected in cells grown in Cu(II)-deficient medium. However, the mature protein, presumably formed after import of the precursor into the chloroplast, is rapidly degraded in Cu(II)-deficient cells. The half-life of the mature protein is estimated to be between 16 and 18 min in cells grown in Cu(II)-deficient medium. In cells grown in medium containing Cu(II), the mature protein is stable. The proteolytic activity thus appears to be specific for apoplastocyanin versus plastocyanin and thereby accounts for the absence of accumulated plastocyanin in Cu(II)-deficient cells. This process may be part of a general mechanism designed to remove chloroplast proteins which cannot be utilized.