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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We present a detailed analysis of strand‐specific transcription in different regions of the murine c‐myc locus. In normal and transformed cell lines, RNA polymerase II directed transcription occurs in the sense and anti‐sense direction. Three noncontiguous regions show a high level of transcription in the anti‐sense orientation: upstream of the first exon, within the first intron and in the 3′ part of the gene (intron 2 and exon 3). In a cell line carrying a c‐myc amplification (54c12), anti‐sense transcription is not uniformly increased throughout the locus and is differentially affected by inhibition of protein synthesis. These results suggest that anti‐sense transcription in various parts of the locus is independently regulated. In the sense orientation, transcriptional activity is higher in the first exon than in the rest of the gene indicating that transcription pauses near the 3′ end of the first exon. The extent of this intragenic pausing varies among different cell lines and is most severe in cells with a c‐myc amplification. Transcription initiation and pausing are both negatively regulated by labile proteins.