Out-of-frame start codons prevent translation of truncated nucleo-cytosolic cathepsin L in vivo
15.09.2014
Tholen M, Hillebrand LE, Tholen S, Sedelmeier O, Arnold SJ, Reinheckel T.
Nat Commun. 2014 Sep 15;5:4931
Cathepsin L (CTSL) is a typical lysosomal enzyme executing bulk proteolysis inside the acidic cellular compartment. Its biosynthesis has been suggested to be under extensive posttranscriptional modulation, namely during mRNA splicing and translation initiation. These processes are thought to affect expression levels, but also cellular localization and function of the protease. Our work aims to dissect these processes based on genetic mouse models in order to understand CTSL regulation in physiology and in disease states such as cancer.
To explain nucleo-cytosolic localization of CTSL it had been hypothesized that skipping of the first start codon during translation initiation results in an N-terminally truncated protein lacking the ER-import signal. However, here we report that translation of truncated CTSL isoforms is prevented by hindering out-of-frame AUGs in vitro as well as in a knock-in mouse model. In addition, this new mouse model proves that spontaneous phenotypes of cathepsin L deficiency can be fully assigned to the lack of canonically targeted CTSL. We further re-evaluated a potential role of nuclear CTSL as a histone processing enzyme in mouse embryonic stem cells early embryonic development. Here we observed no general defect in stem cell differentiation or epiblast development upon CTSL deficiency but a strong lysosomal storage phenotype in the visceral endoderm. This strengthens the crucial importance of CTSL in lysosomal degradation, while the biogenesis of bona fide nucleo-cytosolic CTSL remains elusive.
