Caspase-dependent Cleavage Disrupts the ERK Cascade Scaffolding Function of KSR1^*

Melissa M. McKay, and Deborah K. Morrison¹

Laboratory of Cell and Developmental Signaling, Center for Cancer Research, NCI-Frederick, National Institutes of Health, Frederick, Maryland 21702

Abstract: Kinase suppressor of Ras 1 (KSR1) is a protein scaffold that facilitates ERK cascade activation at the plasma membrane, a critical step in the signal transduction process that allows cells to respond to survival, proliferative, and differentiative cues. Here, we report that KSR1 undergoes caspase-dependent cleavage in apoptotic cells and that cleavage destroys the scaffolding function of the full-length KSR1 protein and generates a stable C-terminal fragment that can inhibit ERK activation. KSR1 is cleaved in response to multiple apoptotic stimuli and occurs in vivo during the involution of mouse mammary tissues, a morphogenic process requiring cellular apoptosis. In addition, we find that in comparison with KSR1^-/- mouse embryonic fibroblasts expressing wild type KSR1 (WT-KSR1), cells expressing a cleavage-resistant KSR1 protein (DEVA-KSR1) exhibit reduced apoptotic signaling in response to tumor necrosis factor-/cycloheximide treatment. The effect of DEVA-KSR1 expression was found to correlate with increased levels of active phosphoERK and could be significantly reversed by treating cells with the MEK inhibitor U0126. In contrast, reduced phosphoERK levels and enhanced apoptotic signaling were observed in cells constitutively expressing the C-terminal KSR1 fragment (CTF-KSR1). Moreover, we find that cleavage of WT-KSR1 correlates with a dramatic reduction in active phosphoERK levels. These findings identify KSR1 as a caspase target and suggest that cleavage of the KSR1 scaffold represents another mechanism whereby caspases down-regulate ERK survival signaling to promote cellular apoptosis.

Received for publication March 29, 2007. Revision received June 20, 2007.

^* This work was supported by a grant from the NCI, National Institutes of Health/Department of Health and Human Services. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

This article was selected as a Paper of the Week.

¹ To whom correspondence should be addressed: NCI-Frederick, National Institutes of Health, P. O. Box B, Frederick, MD 21702. Fax: 301-846-1666; E-mail: dmorrison{at}ncifcrf.gov.

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