Cellular phospho-signaling map of the enigmatic serine/threonine kinase MAST2.

Background: MAST2 (Microtubule-Associated Serine/Threonine Kinase 2) is an enigmatic serine/threonine kinase that is considered to bridge microtubule-associated cytoskeletal architecture through its phospho-regulatory networks. Yet, MAST2 remains a dark horse in the human kinome, as the molecular details on its structure, upstream regulators, and downstream phosphorylation targets remain unknown. Methods and results: To interpret MAST2-linked phospho-signaling dynamics, PubMed-indexed articles were curated based on predefined MeSH terms to obtain global cellular phosphoproteomics datasets. Among 105 class I phosphosites in MAST2 identified across cellular phosphoproteomics datasets, 2 phosphosites, S74 and S148, were represented more abundantly compared to other phosphosites, making them predominant and functionally significant. Expression coregulation analysis, aimed at interpreting phosphosites that consistently show either similar or opposite patterns of expression, was performed by computing the expression patterns of phosphosites in parallel with the predominant MAST2 phosphosite. Their frequencies were then ranked across differential datasets to ensure consistency, and Fisher's exact test was performed to assess the likelihood of the coregulation pattern. Potential biases within the datasets were mitigated using additional cutoffs. Interpreting these datasets, we identified the majority of high-confidence coregulated protein phosphosites of both predominant phosphosites to be involved in transcriptional regulation. This is consistent with reports that a nearby Arg89Gln mutation in MAST2 disrupts its transcriptional regulatory activity. Co-occurrence analysis of phosphosites within MAST2 revealed that these predominant sites tend to co-occur positively and share similarity in expression coregulation patterns with phosphosites in other proteins. Finally, novel upstream kinases that potentially phosphorylate the predominant phosphosites of MAST2, as well as potential downstream substrates that are phosphorylated by MAST2, were also identified from the high-confidence coregulation dataset. Conclusions: We propose that phosphorylations at S74 and S148 of MAST2 are functionally similar, and that these phosphosites are candidate regulatory sites influencing the transcriptional regulatory activity of MAST2.