Rapid degradation of newly synthesized tubulin in lithium-treated sensory neurons.
When cultured chick sensory neurons were labeled with [35S]methionine for 1 h or longer in the presence of 5-25 mM LiCl, we found a dose-dependent reduction in the level of radiolabeled tubulin, to one third of control levels, with no noticeable effect on other proteins. The magnitude of this response was identical after a 1-h or 72-h preincubation in 25 mM LiCl and returned to control values within 1 h after removal of LiCl. Short (5-min) pulse-chase experiments revealed that tubulin synthesis was not affected by Li+, but that newly synthesized tubulin was rapidly degraded, such that 50% of the labeled beta-tubulin was lost within 5 min. There was no enhanced degradation of tubulin present before exposure to Li+. Addition of LiCl at various times before and after a 10-min pulse suggested that tubulin becomes completely refractory to Li(+)-induced degradation within 10 min after translation. Although Li+ treatment resulted in a decrease in the fraction of extant tubulin present in the unassembled form, the Li(+)-induced degradation of nascent tubulin is not a consequence of shifts in assembly state, because colcemid or taxol treatment did not lead to rapid degradation of newly synthesized tubulin, and neither drug altered the response to Li+. We suggest that Li+ interferes with the correct folding of tubulin polypeptides, exposing sites, normally hidden, to the action of a protease(s).
Bennett
G S
GS
Department of Anatomy and Cell Biology, University of Florida, Gainesville 32610.
Hollander
B A
BA
Laskowska
D
D
DiLullo
C
C
D000470:Alkaloids / Q000494:pharmacology; D000596:Amino Acids / Q000378:metabolism; D000818:Animals; D003994:Bucladesine / Q000494:pharmacology; D002642:Chick Embryo; D003080:Cold Temperature; D003470:Culture Media; D003703:Demecolcine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D005470:Fluorometry; D008094:Lithium / Q000494:pharmacology*; D008715:Methionine / Q000494:pharmacology; D008870:Microtubules / Q000187:drug effects; D009475:Neurons, Afferent / Q000378:metabolism*; D017239:Paclitaxel; D013997:Time Factors; D014404:Tubulin / Q000096:biosynthesis / Q000378:metabolism*
D000470:Alkaloids; D000596:Amino Acids; D003470:Culture Media; D014404:Tubulin; D003994:Bucladesine; D008094:Lithium; D008715:Methionine; D017239:Paclitaxel; D003703:Demecolcine