Linkage between substrate recognition and catalysis during cleavage of sarcin/ricin loop RNA by restrictocin

Restrictocin is a site-specific endoribonuclease that inactivates ribosomes by cleaving the sarcin/ricin loop (SRL) of 23S-28S rRNA. Here we present a kinetic and thermodynamic analysis of the SRL cleavage reaction based on monitoring the cleavage of RNA oligonucleotides (2-27-mers). Restrictocin binds to a 27-mer SRL model substrate (designated wild-type SRL) via electrostatic interactions to form a nonspecific ground state complex E:S. At pH 6.7, physical steps govern the reaction rate: the wild-type substrate reacts at a partially diffusion-limited rate, and a faster-reacting SRL, containing a 3'-sulfur atom at the scissile phosphate, reacts at a fully diffusion-limited rate (k2/K1/2 = 1.1 x 10(9) M-1 s-1). At pH 7.4, the chemical step apparently limits the SRL cleavage rate. After the nonspecific binding step, restrictocin recognizes the SRL structure, which imparts 4.3 kcal/mol transition state stabilization relative to a single-stranded RNA. The two conserved SRL modules, bulged-G motif and GAGA tetraloop, contribute at least 2.4 and 1.9 kcal/mol, respectively, to the recognition. These findings suggest a model of SRL recognition in which restrictocin contacts the GAGA tetraloop and the bulged guanosine of the bulged-G motif to progress from the nonspecific ground state complex (E:S) to the higher-energy-specific complex (E.S) en route to the chemical transition state. Comparison of restrictocin with other ribonucleases revealed that restrictocin exhibits a 10(3)-10(6)-fold smaller ribonuclease activity against single-stranded RNA than do the restrictocin homologues, non-structure-specific ribonucleases T1 and U2. Together, these findings show how structural features of the SRL substrate facilitate catalysis and provide a mechanism for distinguishing between cognate and noncognate RNA.     

Crystal structures of restrictocin-inhibitor complexes with implications for RNA recognition and base flipping.

The cytotoxin sarcin disrupts elongation factor binding and protein synthesis by specifically cleaving one phosphodiester bond in ribosomes. To elucidate the molecular basis of toxin action, we determined three cocrystal structures of the sarcin homolog restrictocin bound to different analogs that mimic the target sarcin/ricin loop (SRL) structure of the rat 28S rRNA. In these structures, restrictocin contacts the bulged-G motif and an unfolded form of the tetraloop of the SRL RNA. In one structure, toxin loops guide selection of the target site by contacting the base critical for recognition (G4319) and the surrounding S-shaped backbone. In another structure, base flipping of the tetraloop enables cleavage by placing the target nucleotide in the active site with the nucleophile nearly inline for attack on the scissile bond. These structures provide the first views of how a site-specific protein endonuclease recognizes and cleaves a folded RNA substrate.     

The common and the distinctive features of the bulged-G motif based on a 1.04 A resolution RNA structure

Bulged-G motifs are ubiquitous internal RNA loops that provide specific recognition sites for proteins and RNAs. To establish the common and distinctive features of the motif we determined the structures of three variants and compared them with related structures. The variants are 27-nt mimics of the sarcin/ricin loop (SRL) from Escherichia coli 23S ribosomal RNA that is an essential part of the binding site for elongation factors (EFs). The wild-type SRL has now been determined at 1.04 Å resolution, supplementing data obtained before at 1.11 Å and allowing the first calculation of coordinate error for an RNA motif. The other two structures, having a viable (C2658U^•G2663A) or a lethal mutation (C2658G^• G2663C), were determined at 1.75 and 2.25 Å resolution, respectively. Comparisons reveal that bulged-G motifs have a common hydration and geometry, with flexible junctions at flanking structural elements. Six conserved nucleotides preserve the fold of the motif; the remaining seven to nine vary in sequence and alter contacts in both grooves. Differences between accessible functional groups of the lethal mutation and those of the viable mutation and wild-type SRL may account for the impaired elongation factor binding to ribosomes with the C2658G^•G2663C mutation and may underlie the lethal phenotype.     

Electrostatic interactions guide the active site face of a structure-specific ribonuclease to its RNA substrate

Restrictocin, a member of the alpha-sarcin family of site-specific endoribonucleases, uses electrostatic interactions to bind to the ribosome and to RNA oligonucleotides, including the minimal specific substrate, the sarcin/ricin loop (SRL) of 23S-28S rRNA. Restrictocin binds to the SRL by forming a ground-state E:S complex that is stabilized predominantly by Coulomb interactions and depends on neither the sequence nor structure of the RNA, suggesting a nonspecific complex. The 22 cationic residues of restrictocin are dispersed throughout this protein surface, complicating a priori identification of a Coulomb interacting surface. Structural studies have identified an enzyme-substrate interface, which is expected to overlap with the electrostatic E:S interface. Here, we identified restrictocin residues that contribute to binding in the E:S complex by determining the salt dependence [partial differential log(k 2/ K 1/2)/ partial differential log[KCl]] of cleavage of the minimal SRL substrate for eight point mutants within the protein designed to disrupt contacts in the crystallographically defined interface. Relative to the wild-type salt dependence of -4.1, a subset of the mutants clustering near the active site shows significant changes in salt dependence, with differences of magnitude being >or=0.4. This same subset was identified using calculated salt dependencies for each mutant derived from solutions to the nonlinear Poisson-Boltzmann equation. Our findings support a mechanism in which specific residues on the active site face of restrictocin (primarily K110, K111, and K113) contribute to formation of the E:S complex, thereby positioning the SRL substrate for site-specific cleavage. The same restrictocin residues are expected to facilitate targeting of the SRL on the surface of the ribosome.     

Functional analysis of peptide motif for RNA microhelix binding suggests new family of RNA-binding domains.

RNA microhelices that recreate the acceptor stems of transfer RNAs are charged with specific amino acids. Here we identify a two-helix pair in alanyl-tRNA synthetase that is required for RNA microhelix binding. A single point mutation at an absolutely conserved residue in this motif selectively disrupts RNA binding without perturbation of the catalytic site. These results, and findings of similar motifs in the proximity of the active sites of other tRNA synthetases, suggest that two-helix pairs are widespread and provide a structural framework important for contacts with bound RNA substrates.     

Evidence for induced fit in bacterial RNase P RNA-mediated cleavage

RNase P with its catalytic RNA subunit is involved in the processing of a number of RNA precursors with different structures. However, precursor tRNAs are the most abundant substrates for RNase P. Available data suggest that a tRNA is folded into its characteristic structure already at the precursor state and that RNase P recognizes this structure. The tRNA D-/T-loop domain (TSL-region) is suggested to interact with the specificity domain of RNase P RNA while residues in the catalytic domain interact with the cleavage site. Here, we have studied the consequences of a productive interaction between the TSL-region and its binding site (TBS) in the specificity domain using tRNA precursors and various hairpin-loop model substrates. The different substrates were analyzed with respect to cleavage site recognition, ground-state binding, cleavage as a function of the concentration of Mg(2+) and the rate of cleavage under conditions where chemistry is suggested to be rate limiting using wild-type Escherichia coli RNase P RNA, M1 RNA, and M1 RNA variants with structural changes in the TBS-region. On the basis of our data, we conclude that a productive TSL/TBS interaction results in a conformational change in the M1 RNA substrate complex that has an effect on catalysis. Moreover, it is likely that this conformational change comprises positioning of chemical groups (and Mg(2+)) at and in the vicinity of the cleavage site. Hence, our findings are consistent with an induced-fit mechanism in RNase P RNA-mediated cleavage.     

Engineering the substrate specificity of Staphylococcus aureus Sortase A. The beta6/beta7 loop from SrtB confers NPQTN recognition to SrtA

The Staphylococcus aureus transpeptidase Sortase A (SrtA) anchors virulence and colonization-associated surface proteins to the cell wall. SrtA selectively recognizes a C-terminal LPXTG motif, whereas the related transpeptidase Sortase B (SrtB) recognizes a C-terminal NPQTN motif. In both enzymes, cleavage occurs after the conserved threonine, followed by amide bond formation between threonine and the pentaglycine cross-bridge of cell wall peptidoglycan. Genetic and biochemical studies strongly suggest that SrtA and SrtB exhibit exquisite specificity for their recognition motifs. To better understand the origins of substrate specificity within these two isoforms, we used sequence and structural analysis to predict residues and domains likely to be involved in conferring substrate specificity. Mutational analyses and domain swapping experiments were conducted to test their function in substrate recognition and specificity. Marked changes in the specificity profile of SrtA were obtained by replacing the beta6/beta7 loop in SrtA with the corresponding domain from SrtB. The chimeric beta6/beta7 loop swap enzyme (SrtLS) conferred the ability to acylate NPQTN-containing substrates, with a k(cat)/K(m)(app) of 0.0062 +/- 0.003 m(-1) s(-1). This enzyme was unable to perform the transpeptidation stage of the reaction, suggesting that additional domains are required for transpeptidation to occur. The overall catalytic specificity profile (k(cat)/K(m)(app)(NPQTN)/k(cat)/K(m)(app)(LPETG)) of SrtLS was altered 700,000-fold from SrtA. These results indicate that the beta6/beta7 loop is an important site for substrate recognition in sortases.     

Classical swine fever virus glycoprotein E rns is an endoribonuclease with an unusual base specificity

The glycoprotein E(rns) of pestiviruses is a virion-associated and -secreted RNase that is involved in virulence. The requirements at the cleavage site in heteropolymeric RNA substrates were studied for E(rns). Limited digestion of heteropolymeric RNA substrates indicated a cleavage 5' of uridine residues irrespective of the preceding nucleotide (Np/U). To further study specificity radiolabeled RNA, molecules of 45 to 56 nucleotides in length were synthesized that contained no or a single Np/U cleavage site. Cleavage was only observed in substrates containing an ApU, CpU, GpU, or UpU dinucleotide and occurred in two steps, an initial NpU-specific and a consecutive unspecific degradation. The NpU-specific cleavage was resistant to 7 M urea while the second-order cleavage was sensitive to denaturation. Kinetic analyses revealed that E(rns) is a highly active endoribonuclease (k(cat)/K(m) = 2 x 10(6) to 10 x 10(6) M(-1) s(-1)) with a strong affinity to NpU containing single-stranded RNA substrates (K(m) = 85 to 260 nM).     

Kinetic analysis of precursor M1 RNA molecules for exploring substrate specificity of the N-terminal catalytic half of RNase E

To gain insight into the mechanism by which the sequence at the rne-dependent site of substrate RNA affects the substrate specificity of Escherichia coli RNase E, we performed kinetic analysis of the cleavage of precursor M1 RNA molecules containing various sequences at the rne-dependent site by the N-terminal catalytic half of RNase E (NTH-RNase E). NTH-RNase E displayed higher K(m) and k(cat) values for more specific substrates. The retention of single strandedness at the rne-dependent site was essential for cleavage efficiency. Moreover, the loss of single-strandedness was accompanied by a decrease in both the K(m) and k(cat) values.     

Structural insight into the mechanism of double-stranded RNA processing by ribonuclease III

Members of the ribonuclease III (RNase III) family are double-stranded RNA (dsRNA) specific endoribonucleases characterized by a signature motif in their active centers and a two-base 3' overhang in their products. While Dicer, which produces small interfering RNAs, is currently the focus of intense interest, the structurally simpler bacterial RNase III serves as a paradigm for the entire family. Here, we present the crystal structure of an RNase III-product complex, the first catalytic complex observed for the family. A 7 residue linker within the protein facilitates induced fit in protein-RNA recognition. A pattern of protein-RNA interactions, defined by four RNA binding motifs in RNase III and three protein-interacting boxes in dsRNA, is responsible for substrate specificity, while conserved amino acid residues and divalent cations are responsible for scissile-bond cleavage. The structure reveals a wealth of information about the mechanism of RNA hydrolysis that can be extrapolated to other RNase III family members.     

Secondary structures common to chloroplast mRNA 3'-untranslated regions direct cleavage by CSP41, an endoribonuclease belonging to the short chain dehydrogenase/reductase superfamily

CSP41 (chloroplast stem-loop-binding protein of 41 kDa), a chloroplast endonuclease belonging to the SDR superfamily, preferentially cleaves stem-loop-containing RNAs in vitro. This potentially directs it to the 3'-ends of mature chloroplast mRNAs, which generally possess such structures. To understand the basis for this discrimination, the RNA elements directing CSP41 cleavage of petD RNA in vitro were dissected. Substrates containing fully base-paired stem-loops were optimal substrates, whereas deletion of part of the stem-loop decreased activity by 100-fold, and deletion of the distal arm of the stem-loop abolished cleavage, even in substrates containing the primary CSP41 cleavage site. Competition assays showed that the decrease in activity resulted from decreased affinity for the RNA by CSP41. Mutations of the residues at the scissile bond and mutations and deletions at the terminal loop of the stem had a moderate effect on activity but no effect on cleavage site specificity, suggesting that CSP41 has no sequence specificity. Titration of ethidium bromide into the assay decreased activity to a basal level of approximately 18%, and introduction of a single base bulge into either arm of the stem-loop decreased cleavage at the primary cleavage site by up to 70%. This suggests that changing the structure of the helical stem has a mild effect on activity. Deletion analysis of CSP41 suggests that the specificity domain lies in the first 73 amino acids of the protein, a domain that also contains a putative dehydrogenaselike mononucleotide binding motif. These results are consistent with a broad role for CSP41 in the degradation of stem-loop-containing mRNAs.     

Biochemical and mutagenic analysis of I-CreII reveals distinct but important roles for both the H-N-H and GIY-YIG motifs

Homing endonucleases typically contain one of four conserved catalytic motifs, and other elements that confer tight DNA binding. I-CreII, which catalyzes homing of the Cr.psbA4 intron, is unusual in containing two potential catalytic motifs, H-N-H and GIY-YIG. Previously, we showed that cleavage by I-CreII leaves ends (2-nt 3′ overhangs) that are characteristic of GIY-YIG endonucleases, yet it has a relaxed metal requirement like H-N-H enzymes. Here we show that I-CreII can bind DNA without an added metal ion, and that it binds as a monomer, akin to GIY-YIG enzymes. Moreover, cleavage of supercoiled DNA, and estimates of strand-specific cleavage rates, suggest that I-CreII uses a sequential cleavage mechanism. Alanine substitution of a number of residues in the GIY-YIG motif, however, did not block cleavage activity, although DNA binding was substantially reduced in several variants. Substitution of conserved histidines in the H-N-H motif resulted in variants that did not promote DNA cleavage, but retained high-affinity DNA binding—thus identifying it as the catalytic motif. Unlike the non-specific H-N-H colicins, however; substitution of the conserved asparagine substantially reduced DNA binding (though not the ability to promote cleavage). These results indicate that, in I-CreII, two catalytic motifs have evolved to play important roles in specific DNA binding. The data also indicate that only the H-N-H motif has retained catalytic ability.     

Covariance analysis of RNA recognition motifs identifies functionally linked amino acids.

The RNA recognition motif (RRM) is one of the most common eukaryotic protein motifs. RRM sequences form a conserved globular structure known as the RNA-binding domain (RBD) or the ribonucleoprotein domain. Many proteins that contain RRM sequences bind RNA in a sequence-specific manner. To investigate the basis for the RNA-binding specificity of RRMs, we subjected 330 aligned RRM sequences to covariance analysis. The analysis revealed a single network of covariant amino acid pairs comprising the buried core of the RBD and a surface patch. Structural studies have implicated a subset of these residues in RNA binding. The covariance linkages identify a larger set of amino acid residues, including some not directly in contact with bound RNA, that may influence RNA-binding specificity.     

Cyclin B and Cyclin A Confer Different Substrate Recognition Properties on CDK2

The transitions of the cell cycle are regulated by the cyclin dependent protein kinases(CDKs). The cyclins activate their respective CDKs and confer substrate recognitionproperties. We report the structure of phospho-CDK2/cyclin B and show that cyclin Bconfers M phase-like properties on CDK2, the kinase that is usually associated with S phase.Cyclin B produces an almost identical activated conformation of CDK2 as that produced bycyclin A. There are differences between cyclin A and cyclin B at the recruitment site, whichin cyclin A is used to recruit substrates containing an RXL motif. Because of sequencedifferences this site in cyclin B binds RXL motifs more weakly than in cyclin A. Despitesimilarity in kinase structures, phospho-CDK2/cyclin B phosphorylates substrates, such asnuclear lamin and a model peptide derived from p107, at sequences SPXX that differ fromthe canonical CDK2/cyclin A substrate recognition motif, SPXK. CDK2/cyclin Bphosphorylation at these non-canonical sites is not dependent on the presence of a RXLrecruitment motif. The p107 peptide contained two SP motifs each followed by a noncanonicalsequence of which only one site (Ser640) is phosphorylated by pCDK2/cyclin Awhile two sites are phosphorylated by pCDK2/cyclin B. The second site is too close to theRXL motif to allow the cyclin A recruitment site to be effective, as previous work has shownthat there must be at least 16 residues between the catalytic site serine and the RXL motif.Thus the cyclins A and B in addition to their role in promoting the activatory conformationalswitch in CDK2, also provide differential substrate specificity.     

Crystal structure of a phospholipase D family member

The first crystal structure of a phospholipase D (PLD) family member has been determined at 2.0 A resolution. The PLD superfamily is defined by a common sequence motif, HxK(x)4D(x)6GSxN, and includes enzymes involved in signal transduction, lipid biosynthesis, endonucleases and open reading frames in pathogenic viruses and bacteria. The crystal structure suggests that residues from two sequence motifs form a single active site. A histidine residue from one motif acts as a nucleophile in the catalytic mechanism, forming a phosphoenzyme intermediate, whereas a histidine residue from the other motif appears to function as a general acid in the cleavage of the phosphodiester bond. The structure suggests that the conserved lysine residues are involved in phosphate binding. Large-scale genomic sequencing revealed that there are many PLD family members. Our results suggest that all of these proteins may possess a common structure and catalytic mechanism.     

Structural analysis of multifunctional peptide motifs in human bifunctional tRNA synthetase: identification of RNA-binding residues and functional implications for tandem repeats

Human bifunctional glutamyl-prolyl-tRNA synthetase (EPRS) contains three tandem repeats linking the two catalytic domains. These repeated motifs have been shown to be involved in protein-protein and protein-nucleic acid interactions. The single copy of the homologous motifs has also been found in several different aminoacyl-tRNA synthetases. The solution structure of repeat 1 (EPRS-R1) and the secondary structure of the whole appended domain containing three repeated motifs in EPRS (EPRS-R123) was determined by nuclear magnetic resonance (NMR) spectroscopy. EPRS-R1 consists of two helices (residues 679-699 and 702-721) arranged in a helix-turn-helix, which is similar to other RNA binding proteins and the j-domain of DnaJ, and EPRS-R123 is composed of three helix-turn-helix motifs linked by an unstructured loop. When tRNA is bound to the appended domain, chemical shifts of several residues in each repeat are perturbed. However, the perturbed residues in each repeat are not the same although they are in the same binding surface, suggesting that each repeat in the appended domain is dynamically arranged to maximize contacts with tRNA. The affinity of tRNA to the three-repeated motif was much higher than to the single motif. These results indicate that each of the repeated motifs has a weak intrinsic affinity for tRNA, but the repetition of the motifs may be required to enhance binding affinity. Thus, the results of this work gave information on the RNA-binding mode of the multifunctional peptide motif attached to different ARSs and the functional reason for the repetition of this motif.     

Specificity determinants of recruitment peptides bound to phospho-CDK2/cyclin A

Progression through S phase of the eukaryotic cell cycle is regulated by the action of the cyclin dependent protein kinase 2 (CDK2) in association with cyclin A. CDK2/cyclin A phosphorylates numerous substrates. Substrate specificity often employs a dual recognition strategy in which the sequence flanking the phospho-acceptor site (Ser.Pro.X.Arg/Lys) is recognized by CDK2, while the cyclin A component of the complex contains a hydrophobic site that binds Arg/Lys.X.Leu ("RXL" or "KXL") substrate recruitment motifs. To determine additional sequence specificity motifs around the RXL sequence, we have performed X-ray crystallographic studies at 2.3 A resolution and isothermal calorimetry measurements on complexes of phospho-CDK2/cyclin A with a recruitment peptide derived from E2F1 and with shorter 11-mer peptides from p53, pRb, p27, E2F1, and p107. The results show that the cyclin recruitment site accommodates a second hydrophobic residue either immediately C-terminal or next adjacent to the leucine of the "RXL" motif and that this site makes important contributions to the recruitment peptide recognition. The arginine of the RXL motif contacts a glutamate, Glu220, on the cyclin. In those substrates that contain a KXL motif, no ionic interactions are observed with the lysine. The sequences N-terminal to the "RXL" motif of the individual peptides show no conservation, but nevertheless make common contacts to the cyclin through main chain interactions. Thus, the recruitment site is able to recognize diverse but conformationally constrained target sequences. The observations have implications for the further identification of physiological substrates of CDK2/cyclin A and the design of specific inhibitors.     

Studies on substrate recognition by the budding yeast separase

Sister chromatid cohesion is resolved at anaphase onset when separase, a site-specific protease, cleaves the Scc1 subunit of the chromosomal cohesin complex that is responsible for holding sister chromatids together. This mechanism to initiate anaphase is conserved in eukaryotes from budding yeast to man. Budding yeast separase recognizes and cleaves two conserved peptide motifs within Scc1. In addition, separase cleaves a similar motif in the kinetochore and spindle protein Slk19. Separase may cleave further substrate proteins to orchestrate multiple cellular events that take place during anaphase. To investigate substrate recognition by budding yeast separase we analyzed the sequence requirements at one of the Scc1 cleavage site motifs by systematic mutagenesis. We derived a cleavage site consensus motif (not(FKRWY))(ACFHILMPVWY)(DE)X(AGSV)R/X. This motif is found in 1,139 of 5,889 predicted yeast proteins. We analyzed 28 candidate proteins containing this motif as well as 35 proteins that contain a core (DE)XXR motif. We could so far not confirm new separase substrates, but we have uncovered other forms of mitotic regulation of some of the proteins. We studied whether determinants other than the cleavage site motif mediate separase-substrate interaction. When the separase active site was occupied with a peptide inhibitor covering the cleavage site motif, separase still efficiently interacted with its substrate Scc1. This suggests that separase recognizes both a cleavage site consensus sequence as well as features outside the cleavage site.     

The serine protease and RNA-stimulated nucleoside triphosphatase and RNA helicase functional domains of dengue virus type 2 NS3 converge within a region of 20 amino acids

NS3 protein of dengue virus type 2 has a serine protease domain within the N-terminal 180 residues. NS2B is required for NS3 to form an active protease involved in processing of the viral polyprotein precursor. The region carboxy terminal to the protease domain has conserved motifs present in several viral RNA-stimulated nucleoside triphosphatase (NTPase)/RNA helicases. To define the functional domains of protease and NTPase/RNA helicase activities of NS3, full-length and amino-terminal deletion mutants of NS3 were expressed in Escherichia coli and purified. Deletion of 160 N-terminal residues of NS3 (as in NS3del.2) had no detrimental effect on the basal and RNA-stimulated NTPase as well as RNA helicase activities. However, mutagenesis of the conserved P-loop motif of the RNA helicase domain (K199E) resulted in loss of ATPase activity. The RNA-stimulated NTPase activity was significantly affected by deletion of 20 amino acid residues from the N terminus or by substitutions of the cluster of basic residues, 184RKRK-->QNGN, of NS3del.2, although both mutant proteins retained the conserved RNA helicase motifs. Furthermore, the minimal NS3 protease domain, required for cleavage of the 2B-3 site, was precisely defined to be 167 residues, using the in vitro processing of NS2B-NS3 precursors. Our results reveal that the functional domains required for serine protease and RNA-stimulated NTPase activities map within the region between amino acid residues 160 and 180 of NS3 protein and that a novel motif, the cluster of basic residues 184RKRK, plays an important role for the RNA-stimulated NTPase activity.     

Genetic complementation in yeast reveals functional similarities between the catalytic subunits of mammalian signal peptidase complex

Type I signal peptidases (SPs) comprise a family of structurally related enzymes that cleave signal peptides from precursor proteins following their transport out of the cytoplasmic space in eukaryotic and prokaryotic cells. One such enzyme, the mitochondrial inner membrane peptidase, has two catalytic subunits, which recognize distinct cleavage site motifs in their signal peptide substrates. The only other known type I SP with two catalytic subunits is the signal peptidase complex (SPC) in the mammalian endoplasmic reticulum. Here, we tested the hypothesis that, as with inner membrane peptidase catalytic subunits, SPC catalytic subunits exhibit nonoverlapping substrate specificity. We constructed two yeast strains without endogenous SP, one expressing canine SPC18 and the other expressing a truncation of canine SPC21 (SPC21 Delta N), which lacks 24 N-terminal residues that prevent expression of SPC21 in yeast. By monitoring a variety of soluble and membrane-bound substrates, we find that, in contrast to the tested hypothesis, SPC catalytic subunits exhibit overlapping substrate specificity. SPC18 and SPC21 Delta N do, however, cleave some substrates with different efficiencies, although no pattern for this behavior could be discerned. In light of the functional similarities between SPC proteins, we developed a membrane protein fragmentation assay to monitor the position of the catalytic sites relative to the surface of the endoplasmic reticulum membrane. Using this assay, our results suggest that the active sites of SPC18 and SPC21 Delta N are located 4-11 A above the membrane surface. These data, thus, support a model that SPC18 and SPC21 are functionally and structurally similar to each other.