In Cellulo Kinase Screen Identifies Novel MASTL Substrates

MASTL (microtubule-associated serine/threonine kinase-like) has emerged as a critical regulator of mitosis and as a potential oncogene in a variety of cancer types. To date, Arpp-19/ENSA are the only known substrates of MASTL. With the cellular roles of MASTL expanding and increased interest in development of MASTL inhibitors, it has become critical to determine if there are any additional substrates and if so, what the optimal consensus motif for MASTL is. Here we utilised a whole cell lysate (in cellulo) kinase screen approach combined with stable isotope labelling of amino acids in cell culture (SILAC) to identify novel substrates and a consensus motif of MASTL. Using the related AGC kinase family members AKT1/2, the in cellulo assay identified several known and new substrates highly enriched with the validated consensus motif for AKT. Applying this method to MASTL identified 59 phosphosites on 26 novel proteins significantly increased in the presence of active MASTL. Subsequent in vitro kinase assays confirmed that MASTL was capable of phosphorylating YB1, RPS6, TUBA1C, and RPL36A under some conditions, and hnRNPM specifically on a single site (S-633/7). Taken together, these data suggest that MASTL can phosphorylate several additional substrates, which may help provide greater insight into the ever-increasing biological functions and roles MASTL plays in driving cancer progression and therapy resistance.