Chloroplast-localized protein kinases: a step forward towards a complete inventory

Abstract

In addition to redox regulation, protein phosphorylation has gained increasing importance as a regulatory principle in chloroplasts in recent years. However, only very few chloroplast-localized protein kinases have been identified to date. Protein phosphorylation regulates important chloroplast processes such as photosynthesis or transcription. In order to better understand chloroplast function, it is therefore crucial to obtain a complete picture of the chloroplast kinome, which is currently constrained by two effects: first, recent observations showed that the bioinformatics-based prediction of chloroplast-localized protein kinases from available sequence data is strongly biased; and, secondly, protein kinases are of very low abundance, which makes their identification by proteomics approaches extremely difficult. Therefore, the aim of this study was to obtain a complete list of chloroplast-localized protein kinases from different species. Evaluation of protein kinases which were either highly predicted to be chloroplast localized or have been identified in different chloroplast proteomic studies resulted in the confirmation of only three new kinases. Considering also all reports of experimentally verified chloroplast protein kinases to date, compelling evidence was found for a total set of 15 chloroplast-localized protein kinases in different species. This is in contrast to a much higher number that would be expected based on targeting prediction or on the general abundance of protein kinases in relation to the entire proteome. Moreover, it is shown that unusual protein kinases with differing ATP-binding sites or catalytic centres seem to occur frequently within the chloroplast kinome, thus making their identification by mass spectrometry-based approaches even more difficult due to a different annotation.

The SnRK1 Kinase as Central Mediator of Energy Signaling between Different Organelles

“…Still, the exact mechanisms of this inhibition as well as the identity of other inhibitors remain unclear (Emanuelle et al, 2016). The kinase subunits AKIN10 and AKIN11 are found in the nucleus and the cytosol, and additionally, a chloroplast localization of AKIN10 has been reported (Fragoso et al, 2009) but could not be confirmed (Bayer et al, 2012). Nevertheless, the regulation of photosynthesis is intimately linked to energy metabolism, and accordingly, functional links between AKIN10/11 activity and chloroplast functions have been reported.…”

A Nucleus-Encoded Chloroplast Phosphoprotein Governs Expression of the Photosystem I Subunit PsaC in Chlamydomonas reinhardtii

“…Phosphorylation by cpCK2/PTK also regulates the activity of the chloroplast endoribonuclease p54 in vitro (Liere and Link, 1997).The targets of cpPK2 are not limited to proteins involved in gene expression and, for example, also include the b-subunit of ATP-synthase in the thylakoid membrane (Kanekatsu et al, 1998). The fact that the kinase STN7 is itself a phosphoprotein and the presence in the plastid of numerous other protein kinases are suggestive of a complex regulatory phosphorylation network in the chloroplast of vascular plants (Baginsky and Gruissem, 2004;Reiland et al, 2011;Bayer et al, 2012).…”

Meta-Analysis of Arabidopsis thaliana Phospho-Proteomics Data Reveals Compartmentalization of Phosphorylation Motifs

“…However, it is very likely that specific kinases and/or kinase families show differential subcellular distribution and activity. For instance, even if chloroplasts contain some 3000 predicted proteins, until recently, only four protein kinases have been conclusively demonstrated to localize to plastids (STN7, STN8, CSK, and PKT) (reviewed in Schliebner et al, 2008;Bayer et al, 2012;Schönberg and Baginsky, 2012). However, recently, a set of 17 atypical ABC1 kinases was discovered that appears to localize exclusively to mitochondria and chloroplasts (Lundquist et al, 2012).…”