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Mol Cell. 2004 Apr 23;14(2):207-19.

A cathepsin L isoform that is devoid of a signal peptide localizes to the nucleus in S phase and processes the CDP/Cux transcription factor.

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Department of Biochemistry, McGill University, 687 Pine Avenue West, Montreal H3A 1A1, Canada.


The subclass of cysteine proteases termed lysosomal cathepsins has long been thought to be primarily involved in end-stage protein breakdown within lysosomal compartments. Furthermore, few specific protein substrates for these proteases have been identified. We show here that cathepsin L functions in the regulation of cell cycle progression through proteolytic processing of the CDP/Cux transcription factor. CDP/Cux processing in situ was increased following ectopic expression of cathepsin L but was reduced in Cat L(-/-) cells. Furthermore, catalytically active cathepsin L was localized to the nucleus during the G1-S transition as detected by immunofluorescence imaging and labeling using activity-based probes. Trafficking of cathepsin L to the nucleus is accomplished through a mechanism involving translation initiation at downstream AUG sites and the synthesis of proteases that are devoid of a signal peptide. Overall, these results uncover an as yet unsuspected role for cysteine proteases in the control of cell cycle progression.

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