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The 26S proteasome plays essential roles in cell cycle progression in various types of cell. We previously reported that the inhibition of 26S proteasome activities by a proteasome inhibitor, MG-132, exclusively caused cell cycle arrest in synchronized tobacco BY-2 cells. Here we report a further observation of 26S proteasome involvement during M/G1 transition utilizing a transgenetic BY-2 cell line that stably expresses a GFP-alpha-tubulin fusion protein (BY-GT16). Interestingly, MG-132 treatment caused the arrest of cell cycle progression prior to entering the G1 phase. Indeed, phragmoplast-like structures were formed and cortical microtubules were not organized after the collapse of the original phragmoplasts. Additionally, actin microfilaments showed irregular rearrangements when further incubated with MG-132 and as the phragmoplast-like structures developed. Since these phragmoplast-like structures had a similar configuration and ability to form cell plates to that of the original phragmoplasts, we designated these phragmoplast-like structures as extra phragmoplasts. Furthermore, we showed that a tobacco kinesin-related polypeptide of 125 kDa (TKRP125) localized in the extra phragmoplasts and that its protein level remained unchanged during MG-132 treatment. We propose that TKRP125 might be one of the possible targets of the ubiquitin-proteasome degradation pathway during M/G1 transition.

Poly(A) + RNA was isolated from ripe tomato fruit and translated in a wheat germ cell-free translation system. A 74-kDa polypeptide was detected as a putative precursor of acid invertase by immunoprecipitation with antiserum raised against SDS-treated acid invertase (denatured form) from tomato fruit. The molecular mass of the mRNA for acid invertase was estimated to be about 8×10 5 Da (2.4 k nucleotides) by sucrose density gradient centrifugation. (...)

Mise en evidence du haut degre de similitude des nitrates reductases radiculaires ou caulinaires