Regulation of microtubule dynamics by extracellular signals: cAMP-dependent protein kinase switches off the activity of oncoprotein 18 in intact cells.
Oncoprotein 18 (Op18, also termed p19, 19K, metablastin, stathmin, and prosolin) is a recently identified regulator of microtubule (MT) dynamics. Op18 is a target for both cell cycle and cell surface receptor-coupled kinase systems, and phosphorylation of Op18 on specific combinations of sites has been shown to switch off its MT-destabilizing activity. Here we show that induced expression of the catalytic subunit of cAMP-dependent protein kinase (PKA) results in a dramatic increase in cellular MT polymer content concomitant with phosphorylation and partial degradation of Op18. That PKA may regulate the MT system by downregulation of Op18 activity was evaluated by a genetic system allowing conditional co-expression of PKA and a series of kinase target site-deficient mutants of Op18. The results show that phosphorylation of Op18 on two specific sites, Ser-16 and Ser-63, is necessary and sufficient for PKA to switch off Op18 activity in intact cells. The regulatory importance of dual phosphorylation on Ser-16 and Ser-63 of Op18 was reproduced by in vitro assays. These results suggest a simple model where PKA phosphorylation downregulates the MT-destabilizing activity of Op18, which in turn promotes increased tubulin polymerization. Hence, the present study shows that Op18 has the potential to regulate the MT system in response to external signals such as cAMP-linked agonists.
Gradin
H M
HM
The Department for Cell and Molecular Biology, University of Umeâ, S-901 87 Sweden.
Larsson
N
N
Marklund
U
U
Gullberg
M
M
D017868:Cyclic AMP-Dependent Protein Kinases / Q000096:biosynthesis / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008868:Microtubule Proteins; D008870:Microtubules / Q000502:physiology*; D010750:Phosphoproteins / Q000096:biosynthesis / Q000378:metabolism*; D010766:Phosphorylation; D010768:Phosphoserine / Q000378:metabolism; D011401:Promoter Regions, Genetic; D011993:Recombinant Fusion Proteins / Q000096:biosynthesis / Q000378:metabolism; D015398:Signal Transduction; D050777:Stathmin; D014162:Transfection; D014404:Tubulin / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D008868:Microtubule Proteins; D010750:Phosphoproteins; D011993:Recombinant Fusion Proteins; C493995:STMN1 protein, human; D050777:Stathmin; D014404:Tubulin; D010768:Phosphoserine; D017868:Cyclic AMP-Dependent Protein Kinases