Besides functioning as the plasma transporter of retinol and thyroxine, TTR (transthyretin) is a protease, cleaving apoA-I (apolipoprotein A-I) after a phenylalanine residue. In the present study, we further investigated TTR substrate specificity. By using both P-diverse libraries and a library of phosphonate inhibitors, a TTR preference for a lysine residue in P1 was determined, suggesting that TTR might have a dual specificity and that, in addition to apoA-I, other TTR substrates might exist. Previous studies revealed that TTR is involved in the homoeostasis of the nervous system, as it participates in neuropeptide maturation and enhances nerve regeneration. We investigated whether TTR proteolytic activity is involved in these functions. Both wild-type TTR and TTR prot− (proteolytically inactive TTR) had a similar effect in the expression of peptidylglycine α-amidating mono-oxygenase, the rate-limiting enzyme in neuropeptide amidation, excluding the involvement of TTR proteolytic activity in neuropeptide maturation. However, TTR was able to cleave amidated NPY (neuropeptide Y), probably contributing to the increased NPY levels reported in TTR-knockout mice. To assess the involvement of TTR proteolytic activity in axonal regeneration, neurite outgrowth of cells cultivated with wild-type TTR or TTR prot− , was measured. Cells grown with TTR prot− displayed decreased neurite length, thereby suggesting that TTR proteolytic activity is important for its function as a regeneration enhancer. By showing that TTR is able to cleave NPY and that its proteolytic activity affects axonal growth, the present study shows that TTR has natural substrates in the nervous system, establishing further its relevance in neurobiology.

Our quest to understand the complex inner workings of the cell depends on the development of new technologies that allow the study of global regulatory events as they happen within their native cellular environment. Post-translational processing of proteins by proteases is one such regulatory process that can control many aspects of basic cell biology. In this issue of the Biochemical Journal , Timmer et al. describe a new proteomic approach that can be used to globally monitor constitutive proteolytic events in vivo . Using bacterial, human, yeast and mouse cells, the authors show that this methodology provides a comprehensive map of constitutive trimming events mediated by regulatory proteases such as methionine aminopeptidase. This study also identifies previously uncharacterized processing events that highlight potential novel regulatory mechanisms mediated by proteolysis.