Novel protein-protein interactions of TPPII, p53, and SIRT7

Earlier, the biological effect of tripeptidylpeptidase II (TPPII) in cells was often associated only with a protein degradation, since the enzyme functions downstream of the ubiquitin-proteasome system and provides together with other amino- and exopeptidases free amino acid pool for biosynthesis. However, several previous functional studies including cancer cells, knockout mouse models and specific TPPII substrates suggested, that the enzyme plays additional cellular functions. In cancer cells, TPPII was shown to be required for their survival and a previous experimental work also confirmed its regulatory effects on apoptosis, the cell cycle and DNA damage responses. It also regulated the cellular level of important neuropeptides like cholecystokinin and somatostatin and the deficiency in experimental animals was manifested by a shortened lifespan, early ageing and immunosenescence. Since the detail molecular mechanisms how TPPII controls these cellular events are not known yet, here protein interaction partners were analysed with the aim to identify corresponding pathways. Both conventional co-IP and very sensitive method in situ Proximity Ligation Assay (PLA) were exploited for the identification. In situ PLA is an established method for the detection of single molecule interactions based on the principle, that only the specifically bound probes localized in very close proximity (< 16 nm) can form circular DNA. The formed circular template can be further amplified by the rolling-circle amplification and be detected by fluorescent markers within cells. The used methods identified a novel protein-protein interactions with p53 and SIRT7, which are important targets for development of anti-tumour therapeutics, neuroprotective treatments and for improvement of the symptoms of diseases of ageing. TPPII was also implicated in the anti-cancer drug development, since inhibitors of the enzymatic activity were proposed as potential tumor-suppressing agents. One of the TPPII interacting proteins SIRT7 belongs to the sirtuin family, which is currently under detailed investigation due to neuroprotective effects of several members. Another interacting protein p53 is the key protein, which regulation represents one of the main challenges for both cancer research and for the treatment of neurodegenerative diseases. Recently, neuroprotective strategies towards inactivation of p53 and/or decreasing of its expression in neurons were very seriously considered. Its transcription is activated by binding to MYBBP1A, which interaction with TPPII was very recently discovered (Nahálková & Tomkinson, 2014). Reference: Nahálková, J., & Tomkinson, B. (2014). Arch Biochem Biophys, 564, 128–35.