Rab40 GTPases regulate AMBRA1-mediated transcription and cell migration.

The Rab40 subfamily of proteins consists of unique small monomeric GTPases that form CRL5-based ubiquitin E3 ligase complexes and regulate ubiquitylation of specific target proteins. Recent studies have shown that Rab40 proteins play an important role in regulating cell migration, but the underlying mechanisms of how the Rab40-CRL5 complex functions are still not fully understood. In this study, we identified AMBRA1 as a novel binding partner of Rab40 GTPases and show that this interaction mediates a bidirectional crosstalk between the CRL4 and CRL5 E3 ligases. Importantly, we found that Rab40-CRL5 ubiquitylates AMBRA1, which does not result in AMBRA1 degradation but, instead, appears to induce AMBRA1-dependent regulation of gene transcription. The global transcriptional profiles identified by RNA sequencing showed that AMBRA1 regulates transcription of genes related to cell adhesion and migration. Additionally, we show that AMBRA1-dependent transcription regulation does not require the enzymatic activity of AMBRA1-CRL4, and that Rab40-induced AMBRA1 ubiquitylation leads to dissociation of the AMBRA1-CRL4 complex. Taken together, our findings reveal a novel function of the Rab40-CRL5 complex as an important regulator of AMBRA1-dependent transcription of genes involved in cell migration.