Structure of a GDP:AlF4 complex of the SRP GTPases Ffh and FtsY, and identification of a peripheral nucleotide interaction site

Common Mechanism of Activated Catalysis in P-loop Fold Nucleoside Triphosphatases-United in Diversity

To clarify the obscure hydrolysis mechanism of ubiquitous P-loop-fold nucleoside triphosphatases (Walker NTPases), we analysed the structures of 3136 catalytic sites with bound Mg-NTP complexes or their analogues. Our results are presented in two articles; here, in the second of them, we elucidated whether the Walker A and Walker B sequence motifs-common to all P-loop NTPases-could be directly involved in catalysis. We found that the hydrogen bonds (H-bonds) between the strictly conserved, Mg-coordinating Ser/Thr of the Walker A motif ([Ser/Thr]WA) and aspartate of the Walker B motif (AspWB) are particularly short (even as short as 2.4 ångströms) in the structures with bound transition state (TS) analogues. Given that a short H-bond implies parity in the pKa values of the H-bond partners, we suggest that, in response to the interactions of a P-loop NTPase with its cognate activating partner, a proton relocates from [Ser/Thr]WA to AspWB. The resulting anionic [Ser/Thr]WA alkoxide withdraws a proton from the catalytic water molecule, and the nascent hydroxyl attacks the gamma phosphate of NTP. When the gamma-phosphate breaks away, the trapped proton at AspWB passes by the Grotthuss relay via [Ser/Thr]WA to beta-phosphate and compensates for its developing negative charge that is thought to be responsible for the activation barrier of hydrolysis.

Structural and Functional Impact of SRP54 Mutations Causing Severe Congenital Neutropenia

The SRP54 GTPase is a key component of co-translational protein targeting by the signal recognition particle (SRP). Point mutations in SRP54 have been recently shown to lead to a form of severe congenital neutropenia displaying symptoms overlapping with those of Shwachman-Diamond syndrome. The phenotype includes severe neutropenia, exocrine pancreatic deficiency, and neurodevelopmental as well as skeletal disorders. Using a combination of X-ray crystallography, hydrogen-deuterium exchange coupled to mass spectrometry and complementary biochemical and biophysical methods, we reveal extensive structural defects in three disease-causing SRP54 variants resulting in critical protein destabilization. GTP binding is mostly abolished as a consequence of an altered GTPase core. The mutations located in conserved sequence fingerprints of SRP54 eliminate targeting complex formation with the SRP receptor as demonstrated in yeast and human cells. These specific defects critically influence the entire SRP pathway, thereby causing this life-threatening disease.

Structure, dynamics and interactions of large SRP variants

Co-translational protein targeting to membranes relies on the signal recognition particle (SRP) system consisting of a cytosolic ribonucleoprotein complex and its membrane-associated receptor. SRP recognizes N-terminal cleavable signals or signal anchor sequences, retards translation, and delivers ribosome-nascent chain complexes (RNCs) to vacant translocation channels in the target membrane. While our mechanistic understanding is well advanced for the small bacterial systems it lags behind for the large bacterial, archaeal and eukaryotic SRP variants including an Alu and an S domain. Here we describe recent advances on structural and functional insights in domain architecture, particle dynamics and interplay with RNCs and translocon and GTP-dependent regulation of co-translational protein targeting stimulated by SRP RNA.

Structure of the quaternary complex between SRP, SR, and translocon bound to the translating ribosome

During co-translational protein targeting, the signal recognition particle (SRP) binds to the translating ribosome displaying the signal sequence to deliver it to the SRP receptor (SR) on the membrane, where the signal peptide is transferred to the translocon. Using electron cryo-microscopy, we have determined the structure of a quaternary complex of the translating Escherichia coli ribosome, the SRP–SR in the ‘activated' state and the translocon. Our structure, supported by biochemical experiments, reveals that the SRP RNA adopts a kinked and untwisted conformation to allow repositioning of the ‘activated' SRP–SR complex on the ribosome. In addition, we observe the translocon positioned through interactions with the SR in the vicinity of the ribosome exit tunnel where the signal sequence is extending beyond its hydrophobic binding groove of the SRP M domain towards the translocon. Our study provides new insights into the mechanism of signal sequence transfer from the SRP to the translocon.

Membrane proteins are inserted co-transnationally through the association between ribosome, the signal recognition particle and its receptor, and the membrane-bound translocon. Here the authors present a cryo-EM reconstruction of this quaternary complex in the activated state and propose a model for signal sequence transfer to the translocon.

The structural basis of FtsY recruitment and GTPase activation by SRP RNA

The universally conserved signal recognition particle (SRP) system mediates the targeting of membrane proteins to the translocon in a multistep process controlled by GTP hydrolysis. Here we present the 2.6 Å crystal structure of the GTPase domains of the E. coli SRP protein (Ffh) and its receptor (FtsY) in complex with the tetraloop and the distal region of SRP-RNA, trapped in the activated state in presence of GDP:AlF4. The structure reveals the atomic details of FtsY recruitment and, together with biochemical experiments, pinpoints G83 as the key RNA residue that stimulates GTP hydrolysis. Insertion of G83 into the FtsY active site orients a single glutamate residue provided by Ffh (E277), triggering GTP hydrolysis and complex disassembly at the end of the targeting cycle. The complete conservation of the key residues of the SRP-RNA and the SRP protein implies that the suggested chemical mechanism of GTPase activation is applicable across all kingdoms.

Is there a twist in the Escherichia coli signal recognition particle pathway?

Integral membrane proteins (IMPs) are usually synthesized by membrane-bound ribosomes, and this process requires proper localization of ribosomes and IMP-encoding transcripts. However, the underlying molecular mechanism of the pathway has not yet been fully established in vivo. The prevailing hypothesis is that ribosomes and transcripts are delivered to the membrane together during IMP translation by the signal recognition particle (SRP) and its receptor. Here, I discuss an alternative hypothesis that posits that ribosomes and transcripts are targeted separately. Ribosome targeting to the membrane might be mediated by the SRP receptor, rather than by SRP, and IMP-encoding transcripts might be targeted to the membrane in a translation-independent manner. According to this scenario, the SRP executes its essential function on the membrane at a later stage of the targeting pathway.

Structural basis for the molecular evolution of SRP-GTPase activation by protein

Small G proteins have key roles in signal transduction pathways. They are switched from the signaling 'on' to the non-signaling 'off' state when GTPase-activating proteins (GAPs) provide a catalytic residue. The ancient signal recognition particle (SRP)-type GTPases form GTP-dependent homo- and heterodimers and deviate from the canonical switch paradigm in that no GAPs have been identified. Here we show that the YlxH protein activates the SRP-GTPase FlhF. The crystal structure of the Bacillus subtilis FlhF-effector complex revealed that the effector does not contribute a catalytic residue but positions the catalytic machinery already present in SRP-GTPases. We provide a general concept that might also apply to the RNA-driven activation of the universally conserved, co-translational protein-targeting machinery comprising the SRP-GTPases Ffh and FtsY. Our study exemplifies the evolutionary transition from RNA- to protein-driven activation in SRP-GTPases and suggests that the current view on SRP-mediated protein targeting is incomplete.

Structure-function relationships of the G domain, a canonical switch motif

GTP-binding (G) proteins constitute a class of P-loop (phosphate-binding loop) proteins that work as molecular switches between the GDP-bound OFF and the GTP-bound ON state. The common principle is the 160-180-residue G domain with an α,β topology that is responsible for nucleotide-dependent conformational changes and drives many biological functions. Although the G domain uses a universally conserved switching mechanism, its structure, function, and GTPase reaction are modified for many different pathways and processes.

Direct visualization reveals dynamics of a transient intermediate during protein assembly

Interactions between proteins underlie numerous biological functions. Theoretical work suggests that protein interactions initiate with formation of transient intermediates that subsequently relax to specific, stable complexes. However, the nature and roles of these transient intermediates have remained elusive. Here, we characterized the global structure, dynamics, and stability of a transient, on-pathway intermediate during complex assembly between the Signal Recognition Particle (SRP) and its receptor. We show that this intermediate has overlapping but distinct interaction interfaces from that of the final complex, and it is stabilized by long-range electrostatic interactions. A wide distribution of conformations is explored by the intermediate; this distribution becomes more restricted in the final complex and is further regulated by the cargo of SRP. These results suggest a funnel-shaped energy landscape for protein interactions, and they provide a framework for understanding the role of transient intermediates in protein assembly and biological regulation.

The crystal structure of the signal recognition particle in complex with its receptor

Cotranslational targeting of membrane and secretory proteins is mediated by the universally conserved signal recognition particle (SRP). Together with its receptor (SR), SRP mediates the guanine triphosphate (GTP)-dependent delivery of translating ribosomes bearing signal sequences to translocons on the target membrane. Here, we present the crystal structure of the SRP:SR complex at 3.9 angstrom resolution and biochemical data revealing that the activated SRP:SR guanine triphosphatase (GTPase) complex binds the distal end of the SRP hairpin RNA where GTP hydrolysis is stimulated. Combined with previous findings, these results suggest that the SRP:SR GTPase complex initially assembles at the tetraloop end of the SRP RNA and then relocalizes to the opposite end of the RNA. This rearrangement provides a mechanism for coupling GTP hydrolysis to the handover of cargo to the translocon.

Early targeting events during membrane protein biogenesis in Escherichia coli

All living cells have co-translational pathways for targeting membrane proteins. Co-translation pathways for secretory proteins also exist but mostly in eukaryotes. Unlike secretory proteins, the biosynthetic pathway of most membrane proteins is conserved through evolution and these proteins are usually synthesized by membrane-bound ribosomes. Translation on the membrane requires that both the ribosomes and the mRNAs be properly localized. Theoretically, this can be achieved by several means. (i) The current view is that the targeting of cytosolic mRNA-ribosome-nascent chain complexes (RNCs) to the membrane is initiated by information in the emerging hydrophobic nascent polypeptides. (ii) The alternative model suggests that ribosomes may be targeted to the membrane also constitutively, whereas the appropriate mRNAs may be carried on small ribosomal subunits or targeted by other cellular factors to the membrane-bound ribosomes. Importantly, the available experimental data do not rule out the possibility that cells may also utilize both pathways in parallel. In any case, it is well documented that a major player in the targeting pathway is the signal recognition particle (SRP) system composed of the SRP and its receptor (SR). Although the functional core of the SRP system is evolutionarily conserved, its composition and biological practice come with different flavors in various organisms. This review is dedicated mainly to the Escherichia (E.) coli SRP, where the biochemical and structural properties of components of the SRP system have been relatively characterized, yielding essential information about various aspects of the pathway. In addition, several cellular interactions of the SRP and its receptor have been described in E. coli, providing insights into their spatial function. Collectively, these in vitro studies have led to the current view of the targeting pathway [see (i) above]. Interestingly, however, in vivo studies of the role of the SRP and its receptor, with emphasis on the temporal progress of the pathway, elicited an alternative hypothesis [see (ii) above]. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Transient tether between the SRP RNA and SRP receptor ensures efficient cargo delivery during cotranslational protein targeting

Kinetic control of macromolecular interactions plays key roles in biological regulation. An example of such control occurs in cotranslational protein targeting by the signal recognition particle (SRP), during which the SRP RNA and the cargo both accelerate complex assembly between the SRP and SRP receptor FtsY 10(2)-fold. The molecular mechanism underlying these rate accelerations was unclear. Here we show that a highly conserved basic residue, Lys399, on the lateral surface of FtsY provides a novel RNA tetraloop receptor to mediate the SRP RNA- and cargo-induced acceleration of SRP-FtsY complex assembly. We propose that the SRP RNA, by using its tetraloop to interact with FtsY-Lys399, provides a transient tether to stabilize the early stage and transition state of complex formation; this accelerates the assembly of a stable SRP-FtsY complex and allows the loading of cargo to be efficiently coupled to its membrane delivery. The use of a transient tether to increase the lifetime of collisional intermediates and reduce the dimension of diffusional search represents a novel and effective mechanism to accelerate macromolecular interactions.

Protein targeting by the signal recognition particle

Protein targeting by the signal recognition particle (SRP) is universally conserved and starts with the recognition of a signal sequence in the context of a translating ribosome. SRP54 and FtsY, two multidomain proteins with guanosine triphosphatase (GTPase) activity, are the central elements of the SRP system. They have to coordinate the presence of a signal sequence with the presence of a vacant translocation channel in the membrane. For coordination the two GTPases form a unique, nearly symmetric heterodimeric complex in which the activation of GTP hydrolysis plays a key role for membrane insertion of substrate proteins. Recent results are integrated in an updated perception of the order of events in SRP-mediated protein targeting.

It takes two to tango: regulation of G proteins by dimerization

Guanine nucleotide-binding (G) proteins, which cycle between a GDP- and a GTP-bound conformation, are conventionally regulated by GTPase-activating proteins (GAPs) and guanine nucleotide-exchange factors (GEFs), and function by interacting with effector proteins in the GTP-bound 'on' state. Here we present another class of G proteins that are regulated by homodimerization, which we would categorize as G proteins activated by nucleotide-dependent dimerization (GADs). This class includes proteins such as signal recognition particle (SRP), dynamin, septins and the newly discovered Roco protein Leu-rich repeat kinase 2 (LRRK2). We propose that the juxtaposition of the G domains of two monomers across the GTP-binding sites activates the biological function of these proteins and the GTPase reaction.

SRP RNA controls a conformational switch regulating the SRP-SRP receptor interaction

The interaction of the signal-recognition particle (SRP) with its receptor (SR) mediates co-translational protein targeting to the membrane. SRP and SR interact via their homologous core GTPase domains and N-terminal four-helix bundles (N domains). SRP-SR complex formation is slow unless catalyzed by SRP's essential RNA component. We show that truncation of the first helix of the N domain (helix N1) of both proteins dramatically accelerates their interaction. SRP and SR with helix N1 truncations interact at nearly the RNA-catalyzed rate in the absence of RNA. NMR spectroscopy and analysis of GTPase activity show that helix N1 truncation in SR mimics the conformational switch caused by complex formation. These results demonstrate that the N-terminal helices of SRP and SR are autoinhibitory for complex formation in the absence of SRP RNA, suggesting a mechanism for RNA-mediated coordination of the SRP-SR interaction.

Multiple conformational switches in a GTPase complex control co-translational protein targeting

The "GTPase switch" paradigm, in which a GTPase switches between an active, GTP-bound state and an inactive, GDP-bound state through the recruitment of nucleotide exchange factors (GEFs) or GTPase activating proteins (GAPs), has been used to interpret the regulatory mechanism of many GTPases. A notable exception to this paradigm is provided by two GTPases in the signal recognition particle (SRP) and the SRP receptor (SR) that control the co-translational targeting of proteins to cellular membranes. Instead of the classical "GTPase switch," both the SRP and SR undergo a series of discrete conformational rearrangements during their interaction with one another, culminating in their reciprocal GTPase activation. Here, we show that this series of rearrangements during SRP-SR binding and activation provide important control points to drive and regulate protein targeting. Using real-time fluorescence, we showed that the cargo for SRP--ribosomes translating nascent polypeptides with signal sequences--accelerates SRP.SR complex assembly over 100-fold, thereby driving rapid delivery of cargo to the membrane. A series of subsequent rearrangements in the SRP x SR GTPase complex provide important driving forces to unload the cargo during late stages of protein targeting. Further, the cargo delays GTPase activation in the SRP.SR complex by 8-12 fold, creating an important time window that could further improve the efficiency and fidelity of protein targeting. Thus, the SRP and SR GTPases, without recruiting external regulatory factors, constitute a self-sufficient system that provides exquisite spatial and temporal control of a complex cellular process.

Structures of the signal recognition particle receptor from the archaeon Pyrococcus furiosus: implications for the targeting step at the membrane

In all organisms, a ribonucleoprotein called the signal recognition particle (SRP) and its receptor (SR) target nascent proteins from the ribosome to the translocon for secretion or membrane insertion. We present the first X-ray structures of an archeal FtsY, the receptor from the hyper-thermophile Pyrococcus furiosus (Pfu), in its free and GDP•magnesium-bound forms. The highly charged N-terminal domain of Pfu-FtsY is distinguished by a long N-terminal helix. The basic charges on the surface of this helix are likely to regulate interactions at the membrane. A peripheral GDP bound near a regulatory motif could indicate a site of interaction between the receptor and ribosomal or SRP RNAs. Small angle X-ray scattering and analytical ultracentrifugation indicate that the crystal structure of Pfu-FtsY correlates well with the average conformation in solution. Based on previous structures of two sub-complexes, we propose a model of the core of archeal and eukaryotic SRP•SR targeting complexes.

Structures of SRP54 and SRP19, the two proteins that organize the ribonucleic core of the signal recognition particle from Pyrococcus furiosus

In all organisms the Signal Recognition Particle (SRP), binds to signal sequences of proteins destined for secretion or membrane insertion as they emerge from translating ribosomes. In Archaea and Eucarya, the conserved ribonucleoproteic core is composed of two proteins, the accessory protein SRP19, the essential GTPase SRP54, and an evolutionarily conserved and essential SRP RNA. Through the GTP-dependent interaction between the SRP and its cognate receptor SR, ribosomes harboring nascent polypeptidic chains destined for secretion are dynamically transferred to the protein translocation apparatus at the membrane. We present here high-resolution X-ray structures of SRP54 and SRP19, the two RNA binding components forming the core of the signal recognition particle from the hyper-thermophilic archaeon Pyrococcus furiosus (Pfu). The 2.5 Å resolution structure of free Pfu-SRP54 is the first showing the complete domain organization of a GDP bound full-length SRP54 subunit. In its ras-like GTPase domain, GDP is found tightly associated with the protein. The flexible linker that separates the GTPase core from the hydrophobic signal sequence binding M domain, adopts a purely α-helical structure and acts as an articulated arm allowing the M domain to explore multiple regions as it scans for signal peptides as they emerge from the ribosomal tunnel. This linker is structurally coupled to the GTPase catalytic site and likely to propagate conformational changes occurring in the M domain through the SRP RNA upon signal sequence binding. Two different 1.8 Å resolution crystal structures of free Pfu-SRP19 reveal a compact, rigid and well-folded protein even in absence of its obligate SRP RNA partner. Comparison with other SRP19•SRP RNA structures suggests the rearrangement of a disordered loop upon binding with the RNA through a reciprocal induced-fit mechanism and supports the idea that SRP19 acts as a molecular scaffold and a chaperone, assisting the SRP RNA in adopting the conformation required for its optimal interaction with the essential subunit SRP54, and proper assembly of a functional SRP.

Structure of the Roc-COR domain tandem of C. tepidum, a prokaryotic homologue of the human LRRK2 Parkinson kinase

Ras of complex proteins (Roc) belongs to the superfamily of Ras-related small G-proteins that always occurs in tandem with the C-terminal of Roc (COR) domain. This Roc-COR tandem is found in the bacterial and eukaryotic world. Its most prominent member is the leucine-rich repeat kinase LRRK2, which is mutated and activated in Parkinson patients. Here, we investigated biochemically and structurally the Roco protein from Chlorobium tepidum. We show that Roc is highly homologous to Ras, whereas the COR domain is a dimerisation device. The juxtaposition of the G-domains and mutational analysis suggest that the Roc GTPase reaction is stimulated and/or regulated by dimerisation in a nucleotide-dependent manner. The region most conserved between bacteria and man is the interface between Roc and COR, where single-point Parkinson mutations of the Roc and COR domains are in close proximity. The analogous mutations in C. tepidum Roc-COR decrease the GTPase reaction rate, most likely due to a modification of the interaction between the Roc and COR domains.

Demonstration of a multistep mechanism for assembly of the SRP x SRP receptor complex: implications for the catalytic role of SRP RNA

Two GTPases in the signal recognition particle (SRP) and its receptor (SR) control the delivery of newly synthesized proteins to the endoplasmic reticulum or plasma membrane. During the protein targeting reaction, the 4.5S SRP RNA accelerates the association between the two GTPases by 400-fold. Using fluorescence resonance energy transfer, we demonstrate here that formation of a stable SRP x SR complex involves two distinct steps: a fast initial association between SRP and SR to form a GTP-independent early complex and then a GTP-dependent conformational rearrangement to form the stable final complex. We also found that the 4.5S SRP RNA significantly stabilizes the early GTP-independent intermediate. Furthermore, mutational analyses show that there is a strong correlation between the ability of the mutant SRP RNAs to stabilize the early intermediate and their ability to accelerate SRP x SR complex formation. We propose that the SRP RNA, by stabilizing the early intermediate, can give this transient intermediate a longer life time and therefore a higher probability to rearrange to the stable final complex. This provides a coherent model that explains how the 4.5S RNA exerts its catalytic role in SRP x SR complex assembly.

X-ray structures of the signal recognition particle receptor reveal targeting cycle intermediates

The signal recognition particle (SRP) and its conjugate receptor (SR) mediate cotranslational targeting of a subclass of proteins destined for secretion to the endoplasmic reticulum membrane in eukaryotes or to the plasma membrane in prokaryotes. Conserved active site residues in the GTPase domains of both SRP and SR mediate discrete conformational changes during formation and dissociation of the SRP.SR complex. Here, we describe structures of the prokaryotic SR, FtsY, as an apo protein and in two different complexes with a non-hydrolysable GTP analog (GMPPNP). These structures reveal intermediate conformations of FtsY containing GMPPNP and explain how the conserved active site residues position the nucleotide into a non-catalytic conformation. The basis for the lower specificity of binding of nucleotide in FtsY prior to heterodimerization with the SRP conjugate Ffh is also shown. We propose that these structural changes represent discrete conformational states assumed by FtsY during targeting complex formation and dissociation.

X-ray structure of the T. aquaticus FtsY:GDP complex suggests functional roles for the C-terminal helix of the SRP GTPases

FtsY and Ffh are structurally similar prokaryotic Signal Recognition Particle GTPases that play an essential role in the Signal Recognition Particle (SRP)-mediated cotranslational targeting of proteins to the membrane. The two GTPases assemble in a GTP-dependent manner to form a heterodimeric SRP targeting complex. We report here the 2.1 A X-ray structure of FtsY from T. aquaticus bound to GDP. The structure of the monomeric protein reveals, unexpectedly, canonical binding interactions for GDP. A comparison of the structures of the monomeric and complexed FtsY NG GTPase domain suggests that it undergoes a conformational change similar to that of Ffh NG during the assembly of the symmetric heterodimeric complex. However, in contrast to Ffh, in which the C-terminal helix shifts independently of the other subdomains, the C-terminal helix and N domain of T. aquaticus FtsY together behave as a rigid body during assembly, suggesting distinct mechanisms by which the interactions of the NG domain "module" are regulated in the context of the two SRP GTPases.

Structure of the GMPPNP-stabilized NG domain complex of the SRP GTPases Ffh and FtsY

Ffh and FtsY are GTPase components of the signal recognition particle co-translational targeting complex that assemble during the SRP cycle to form a GTP-dependent and pseudo twofold symmetric heterodimer. Previously the SRP GTPase heterodimer has been stabilized and purified for crystallographic studies using both the non-hydrolysable GTP analog GMPPCP and the pseudo-transition state analog GDP:AlF4, revealing in both cases a buried nucleotide pair that bridges and forms a key element of the heterodimer interface. A complex of Ffh and FtsY from Thermus aquaticus formed in the presence of the analog GMPPNP could not be obtained, however. The origin of this failure was previously unclear, and it was thought to have arisen from either instability of the analog, or, alternatively, from differences in its interactions within the tightly conscribed composite active site chamber of the complex. Using insights gained from the previous structure determinations, we have now determined the structure of the SRP GTPase targeting heterodimer stabilized by the non-hydrolysable GTP analog GMPPNP. The structure demonstrates how the different GTP analogs are accommodated within the active site chamber despite slight differences in the geometry of the phosphate chain. It also reveals a K+ coordination site at the highly conserved DARGG loop at the N/G interdomain interface.

SRP RNA provides the physiologically essential GTPase activation function in cotranslational protein targeting

The signal recognition particle (SRP) cotranslationally targets proteins to cell membranes by coordinated binding and release of ribosome-associated nascent polypeptides and a membrane-associated SRP receptor. GTP uptake and hydrolysis by the SRP-receptor complex govern this targeting cycle. Because no GTPase-activating proteins (GAPs) are known for the SRP and SRP receptor GTPases, however, it has been unclear whether and how GTP hydrolysis is stimulated during protein trafficking in vivo. Using both biochemical and genetic experiments, we show here that SRP RNA enhances GTPase activity of the SRP-receptor complex above a critical threshold required for cell viability. Furthermore, this stimulation is a property of the SRP RNA tetraloop. SRP RNA tetraloop mutants that confer defective growth phenotypes can assemble into SRP-receptor complexes, but fail to stimulate GTP hydrolysis in these complexes in vitro. Tethered hydroxyl radical probing data reveal that specific positioning of the RNA tetraloop within the SRP-receptor complex is required to stimulate GTPase activity to a level sufficient to support cell growth. These results explain why no external GAP is needed and why the phylogenetically conserved SRP RNA tetraloop is required in vivo.