Mitogen-activated protein kinases (MAPKs) regulate a diverse array of cellular functions. Among these, a cascade involving extracellular-signal regulated kinases (ERKs) has crucial roles in development and oncogenesis, with ERKs acting both in the cytoplasm and the nucleus. Several reporters have been developed to study ERK activity in live cells, but the subcellular localisation of endogenous ERK in the context of an animal remained elusive. In this study, Neal Rasmussen and David Reiner (Rasmussen and Reiner, 2021) use CRISPR/Cas9-dependent genome editing to insert sequences coding for a fluorescent epitope tag into the endogenous mpk-1 gene, the C. elegans homolog of the ERK proteins. Using this system, they are able to visualise ubiquitous expression of endogenous MPK-1 throughout the animal, including in the germ line, where it has a transient nuclear localisation in the most proximal developing oocyte. However, surprisingly, the authors observe a translocation of MPK-1 from the cytosol to nucleus (as a read-out of upstream kinase activation) in all six developing vulval precursor cells (VPCs), not just the presumptive primary (P6.p) cell, as had been assumed based on the use of reporters that assessed substrate activation as a proxy of ERK function. The findings presented here suggest that the regulation of MPK-1 activity is more complex than anticipated, and demonstrate the usefulness of this reporter system in elucidating endogenous ERK function and its regulation.
