CYRI/ Fam49 Proteins Represent a New Class of Rac1 Interactors

Abstract

Fam49 proteins, now referred to as CYRI (CYFIP-related Rac Interactor), are evolutionarily conserved across many phyla. Their closest relative by amino acid sequence is CYFIP, as both proteins contain a domain of unknown function DUF1394. We recently showed that CYRI and the DUF1394 can mediate binding to Rac1 and evidence is building to suggest that CYRI plays important roles in cell migration, chemotaxis and pathogen entry into cells. Here we discuss how CYRI proteins fit into the current framework of the control of actin dynamics by positive and negative feedback loops containing Rac1, the Scar/WAVE Complex, the Arp2/3 Complex and branched actin. We also provide data regarding the interaction between Rac1 and CYRI in an unbiassed mass spectrometry screen for interactors of an active mutant of Rac1.

Structure of CYRI-B (FAM49B), a key regulator of cellular actin assembly

“…In summary, we have revealed the structure of CYRI-B, also called FAM49B, the only described negative regulator of cellular actin assembly mediated by the WAVE regulatory complex (WRC;Fort et al, 2018;Whitelaw et al, 2019). Our structure, the first to be reported of any member of the CYRI family, reveals a protein that is entirely composed of -helices and organized into three distinct subdomains.…”

“…FAM49B was consequently renamed CYRI-B for CYFIP-related Rac1 interactor (Fort et al, 2018). CYRI-B binding has been reported to block various Rac1-dependent signalling cascades in the cell, thereby controlling multiple critical cellular functions including T-cell activation (Shang et al, 2018), membrane protrusion, chemotaxis and cell migration (Fort et al, 2018;Whitelaw et al, 2019). By negatively regulating Rac1 signalling, CYRI-B also reduces the entry of several intracellular bacterial pathogens into both phagocytic and non-phagocytic host cells, and indeed is targeted for ubiquitin-mediated destruction by the action of an injected Salmonella virulence protein (Yuki et al, 2019).…”

Structural Basis of CYRI-B Direct Competition with Scar/WAVE Complex for Rac1

“…Rac1 P29S is a clinically important mutation ( Hodis et al., 2012 ; Krauthammer et al., 2012 ), as it allows Rac1 to undergo spontaneous nucleotide exchange in the absence of a GEF, producing a shift to active Rac1 ( Davis et al., 2012 ). It has been reported that the dual mutation P29S Q61L increases the affinity of Rac1 to CYRI-B and not other effectors such as the CRIB domain ( Fort et al., 2018 ; Whitelaw et al., 2019 ). As P29 is located at the binding interface between Rac1 and CYRI-B ( Figure 3 A), one could speculate that mutation to a serine could result in additional hydrogen bonds, perhaps with R161 to increase the affinity of the complex.…”

Transcriptomics-Based Phenotypic Screening Supports Drug Discovery in Human Glioblastoma Cells

“…The FAM49A gene, also known as CYRI-A, encodes CYFIP-related Rac1 interactor A, a highly conserved regulator of the small GTPase RAC1, to which it binds [38]. FAM49A is expressed in the brain where the protein regulates chemotaxis, cell migration and epithelial polarization [39].…”