Interaction of E. coli Ffh/4.5S ribonucleoprotein and FtsY mimics that of mammalian signal recognition particle and its receptor

Abstract

The mechanism of protein translocation across the endoplasmic reticulum membrane of eukaryotic cells and the plasma membrane of prokaryotic cells are thought to be evolutionarily related. Protein targeting to the eukaryotic translocation apparatus is mediated by the signal recognition particle (SRP), a cytosolic ribonucleoprotein, and the SRP receptor, an endoplasmic reticulum membrane protein. During targeting, the 54K SRP subunit (M(r) 54,000; SRP54), a GTP-binding protein, binds to signal sequences and then interacts with the alpha-subunit of the SRP receptor (SR alpha), another GTP-binding protein. Two proteins from Escherichia coli, Ffh and FTsY, structurally resemble SRP54 and SR alpha. Like SRP54, Ffh is a subunit of a cytosolic ribonucleoprotein that also contains the E. coli 4.5S RNA. Although there is genetic and biochemical evidence that the E. coli Ffh/4.5S ribonucleoprotein has an SRP-like function, there is no evidence for an SR alpha-like role for FtsY. Here we show that the Ffh/4.5S ribonucleoprotein binds tightly to FtsY in a GTP-dependent manner. This interaction results in the stimulation of GTP hydrolysis which can be inhibited by synthetic signal peptides. These properties mimic those of mammalian SRP and its receptor, suggesting that the E. coli Ffh/4.5S ribonucleoprotein and FtsY have functions in protein targeting that are similar to those of their mammalian counterparts.

Structure of a GDP:AlF4 Complex of the SRP GTPases Ffh and FtsY, and Identification of a Peripheral Nucleotide Interaction Site

“…Biochemical studies of the effects of SRP RNA on various components and on the formation and behavior of the Ffh:FtsY targeting complex have shown that SRP RNA, when bound to Ffh, enhances the association and GTPase activity of Ffh and FtsY, 45 and increases dramatically the rate at which GTP-loaded SRP and SR associate and dissociate. 5 The proximity of an external nucleotide-binding pocket to the active center of FtsY and to both nucleotides within the shared catalytic chamber suggests the existence of a direct regulatory interaction between SRP RNA and the active center of the heterodimeric GTPase.…”

The signal recognition particle receptor of Escherichia coli (FtsY) has a nucleotide exchange factor built into the GTPase domain

“…8). In the presence of GTP, Ffh͞4.5S RNA forms a complex with FtsY, leading to a reciprocal stimulation of both GTPases (9)(10)(11). This observation led to the proposal that Ffh and FtsY are GTPase-activating proteins (GAPs) for each other (11).…”

Biochemical characterization of the presecretory protein translocation machinery of Escherichia coli

“…Furthermore, the Ffh/4.5 S RNA complex, having an SRP-like function, was found to bind to FtsY in a GTP-dependent manner [46]. This interaction caused the stimulation of GTP hydrolysis, which signal peptides inhibited [46]. These properties resemble those of the eukaryotic system, suggesting that the E. coli system functions in protein targeting in a similar manner to that of eukaryotic counterparts.…”

“…It has been reported that FIh is important for protein translocation in vivo [45]. Furthermore, the Ffh/4.5 S RNA complex, having an SRP-like function, was found to bind to FtsY in a GTP-dependent manner [46]. This interaction caused the stimulation of GTP hydrolysis, which signal peptides inhibited [46].…”