Phase-separation of EML4-ALK variant 3 is dependent upon an active ALK conformation

Abstract

Oncogenic fusions involving tyrosine kinases are common drivers of non-small cell lung cancer (NSCLC). There are at least 15 different variants of the EML4-ALK fusion, all of which have a similar portion of ALK that includes the kinase domain, but different portions of EML4. Targeted treatment with ALK tyrosine kinase inhibitors (TKIs) has proven effective but patient outcomes are variable. Here, we focus on one common variant, EML4-ALK V3, which drives an aggressive form of the disease. EML4-ALK V3 protein forms cytoplasmic liquid droplets that contain the signalling proteins GRB2 and SOS1. The TKIs ceritinib and lorlatinib dissolve these droplets and the EML4-ALK V3 protein re-localises to microtubules, an effect recapitulated by an inactivating mutation in the ALK catalytic site. Mutations that promote a constitutively active ALK stabilise the liquid droplets even in the presence of TKIs, indicating that droplets do not depend on kinase activity per se. Uniquely, the TKI alectinib promotes droplet formation of both the wild-type and catalytically inactive EML4-ALK V3 mutant, but not in a mutant that disrupts a hallmark of the kinase activity, the Lys-Glu salt-bridge. We propose that EML4-ALK V3 liquid droplet formation occurs through transient dimerization of the ALK kinase domain in its active conformation in the context of stable EML4-ALK trimers. Our results provide insights into the relationship between ALK activity, conformational state and the sub-cellular localisation of EML4-ALK V3 protein, and reveal the different effects of structurally divergent ALK TKIs on these properties.