KatayaProject

Identification of peroxisomal kinases

Numerous peroxisome-targeted protein phosphatases and kinases have been identified, establishing a framework of key players that control peroxisomal phosphorylation.

It is not known yet how peroxisomal phosphoproteins are regulated, hence it is important to identify peroxisomal protein kinases and phosphatases, as part of our research. Only two protein kinases (calcium dependent protein kinase 1 (CPK1) and glyoxysomal protein kinase 1 (GPK1)) are known to target peroxisomes (Dammen et al., 2003; Fukao et al., 2003; Coca and San Segundo, 2010). Moreover, three additional protein kinases were reported to harbor a functional PTS1, but their full-length proteins did not target peroxisomes (Ma and Reumann et al., 2008).

Our goal is to identify all the protein phosphatases and kinases that are present in peroxisomes. This will supply a foundation for understanding peroxisomes in greater details. These identifications will also be a template for other organisms, because so far information about peroxisomal phospho-regulation is scarcely present.

As a follow up study, it would be of great interest to investigate the presence of potential peroxisomal kinases. So far, two protein kinases were identified in peroxisomes. From bioinformatics screening we were able to identify >10 Arabidopsis kinases that have non-canonical PTS1s. The C-terminal 10 residues of these candidates were fused N-terminally with the fluorescent reporter protein, and investigated for their functionality as peroxisomal targeting domain. The full-length proteins then were fused N-terminally with the reporter protein and investigated in vivo. We care currently working on positive candidates to start the determination of their putative involvment in peroxisomal metabolic regulation. Notably, the kinase CPK1 that targeted peroxisomes was also reported by the same study to target lipid droplets. We recently identified, in the PP2A B’θ mutant seedlings, two lipases that are phosphorylated (Kataya et al., 2015a). Probably, CPK1 has a dual role in triacylglyceride degradation in lipid bodies as well as in fatty acid β-oxidation in peroxisomes (Coca and Segundo, 2010). As a long-term goal it would be of interest to identify the substrates of CPK1 and possibly other peroxisomal kinases (if their mutants show β-oxidation phenotypes). For example, transgenic lines and recombinant proteins can be employed for identification of CPK1 substrates.

Peroxisomal kinases include both soluble and receptor-like kinases. Their gene expression show up-and down-regulations upon different biotic and abiotic stresses.