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Volume 271, Number 28, Issue of July 12, 1996 pp. 16553-16558
©1996 by The American Society for Biochemistry and Molecular Biology, Inc.

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Pathways for Degradation of the Catalytic Subunit of cAMP-dependent Protein Kinase Differ in Wild-type and Kinase-negative S49 Mouse Lymphoma Cells

(Received for publication, February 6, 1996, and in revised form, March 25, 1996)

From the Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190

The catalytic subunit of cAMP-dependent protein kinase radiolabeled with [³⁵S]methionine in wild-type S49 mouse lymphoma cells was degraded with half-lives of ~9.2 h in unstimulated cells and ~4.5 h in cells stimulated with a membrane-permeable cAMP analog. Turnover in kinase-negative mutant cells was about three times faster than in stimulated wild-type cells and appeared to involve a unique 47-kDa intermediate. Levels of catalytic subunit protein revealed by Western immunoblotting were consistent with the measured differences in turnover, but whereas the protein was mostly soluble in wild-type cell extracts, it was almost entirely insoluble in the mutant cell extracts. A substantial fraction of the catalytic subunit labeled in a 5-min pulse was soluble in kinase-negative cell extracts, but most of this material was rendered insoluble by incubating the cells for an additional 30 min before extraction. Degradation of the catalytic subunit in kinase-negative, but not in wild-type, cells was inhibited strongly by two specific peptide aldehyde inhibitors of the proteasomal chymotrypsin-like activity. An inhibitor of the proteasomal protease that prefers branched-chain amino acids had less of an effect on catalytic subunit degradation in the mutant cells.

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