Cellular actin assembly is controlled at the barbed ends of actin filaments, where capping protein (CP) limits polymerization. Twinfilin is a conserved in vivo binding partner of CP, yet the significance of this interaction has remained a mystery. Here, we discover that the C-terminal tail of Twinfilin harbors a CP-interacting (CPI) motif, identifying it as a novel CPI-motif protein. Twinfilin and the CPI-motif protein CARMIL have overlapping binding sites on CP. Further, Twinfilin binds competitively with CARMIL to CP, protecting CP from barbed-end displacement by CARMIL. Twinfilin also accelerates dissociation of the CP inhibitor V-1, restoring CP to an active capping state. Knockdowns of Twinfilin and CP each cause similar defects in cell morphology, and elevated Twinfilin expression rescues defects caused by CARMIL hyperactivity. Together, these observations define Twinfilin as the first ‘pro-capping’ ligand of CP and lead us to propose important revisions to our understanding of the CP regulatory cycle.

Vibrio natriegens, a fast-growing Gram-negative bacterium, is gaining interest as a platform for rapid biotechnology applications and metabolic engineering. Only a few bacteriophages that infect this bacterium have been identified. Here, we describe the isolation and characterization of two V. natriegens bacteriophages isolated from Hatches Creek, Wellfleet, Massachusetts.

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Protein synthesis is tightly regulated under stress conditions where energy may be scarce. Despite global repression of translation, some transcripts remain actively translated in order for the cell to be able to respond to the insult or prepare to quickly return normal cellular function after the stress ends.  For the insulin receptor (Insr) and insulin-like growth factor receptor (Igf1r) transcripts this translation is mediated by an internal ribosome entry site (IRES) in their 5’UTRs that functions independently of eukaryotic initiation factor 4A (eIF4A) and eIF4E. Here we show that these cellular IRESes are also able to promote translation independently of the scaffolding protein eIF4G1 both in vitro and in the cell.