Red light-induced formation of ubiquitin-phytochrome conjugates: Identification of possible intermediates of phytochrome degradation

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Phytochrome is the photoreceptor that controls red light-mediated morphogenesis in higher plants. It exists in two photointerconvertible forms, a red light-absorbing form, Pr, and a far-red light-absorbing form, Pfr. Because photoconversion of Pr to Pfr by a brief light pulse decreases the in vivo half-life of this chromoprotein by a factor of ≈100, this system offers a unique way to modulate the turnover rate of a specific protein and hence study the mechanisms responsible for selective protein degradation. In etiolated oat [Avena sativa (L.)] seedlings, degradation of phytochrome as Pfr follows zero-order kinetics as measured both spectrally and by ELISA, with 50% of Pfr lost in ≈130 min at 27°C. Immunoblot analysis of the destruction process with anti-oat phytochrome immunoglobulins reveals that degradation involves the loss of the 124-kDa phytochrome monomer and that proteolytic intermediates of apparent molecular mass lower than 124 kDa do not accumulate to detectable levels in vivo (<0.015% of total phytochrome). The latter observation suggests that proteolytic breakdown of the protein is extremely rapid. However, a series of polypeptides with higher apparent molecular mass and recognized by anti-phytochrome immunoglobulins (principally 129 and 134 kDa) appears after photoconversion to Pfr. These polypeptides represent no more than a few percent of the total immunologically detectable phytochrome pool and have incremental differences in apparent molecular mass of 5 kDa. They appear within 5 min after Pfr formation, reach maximal levels between 90 and 180 min, and decline thereafter. These polypeptides and others of apparent molecular mass up to 160 kDa are also detectable with immunoglobulins directed against either oat or human ubiquitin, indicating that they are ubiquitin-phytochrome conjugates. Since ubiquitin conjugation is involved in intracellular protein turnover and since formation and degradation of Pfr-ubiquitin conjugates coincide with the turnover of Pfr, these data suggest that the Pfr form of phytochrome is degraded via a ubiquitin-dependent proteolytic pathway.

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