Predicted but unconfirmed phosphorylation sites on YBX1
As described above, very few phosphorylation sites on YBX1 have been confirmed in vivo. However, as shown in Table 2, multiple potential sites have been identified by MS analysis, but have yet to be confirmed by in vivo experiments.
Several research groups carried out MS analyses to identify substrates of various kinases and other cellular factors in the proteomes of normal human as well as cancer cell lines. For instance, Blasius et al used MS analysis to identify substrates of cell-cycle checkpoint kinase 1 (Chk1), which plays an essential role in controlling cellular processes such as DNA replication, mitosis, and response to DNA-damage. One of the substrates Chk1 phosphorylates is threonine 80 (T80), which is present in the CSD domain of YBX1.39 Regulation of YBX1 by Chk1 seems plausible since the functions of both Chk2 and YBX1 are mostly similar and it is possible that Chk1 exerts some of its effects through post-translationally modifying YBX1. However, the function of T80 is yet to be confirmed in vivo.
The most recent advancement can be seen in the study by Sharma et al. In this study, a large-scale quantitative MS study of phosphoproteome in cervical cancer HeLa cells allowed for global identification of protein modifications.40 Among the various proteins that were modified, several sites were also found in YBX1 (Table 2). All the phosphorylation sites that were identified in this study were mostly in the A/P and CTD domains. The majority of these sites have never been defined before. Thus, this study opens an extensive area of research to explore the importance of these sites in contribution to cervical cancer.
It is worth noting that until these sites are confirmed by in vivo experiments, they are simply potential sites. Nevertheless, this information could guide any future endeavor to better understand the role of post-translational modifications in YBX1 regulation.