Role of acidic amino acid residues of PsaD subunit on limiting the affinity of photosystem I for ferredoxin

The PsaD subunit of photosystem I is one of the central polypeptides for the interaction with ferredoxin, its acidic electron acceptor. In the cyanobacterium Synechocystis 6803, this role is partly performed by a sequence extending approximately from histidine 97 to arginine 119, close to the C-terminus. In the present work, acidic amino acids D100, E105, and E109 are shown to moderate the affinity of Photosystem I for ferredoxin. Most single replacements of these residues by neutral amino acids increased the affinity for ferredoxin, resulting in a dissociation constant as low as 0.015 microM for the E105Q mutant (wild-type K(D) = 0.4 microM). This is the first report on the limitation of photosystem I affinity for ferredoxin due to acidic amino acids from PsaD subunit. It highlights the occurrence of a negative control on the binding during the formation of transient complexes between electron carriers.     

Molecular characterization and overexpression of the petF gene from Synechococcus elongatus: Evidence for a second site of electrostatic interaction between ferredoxin and the PS I-D subunit

The petF gene from the cyanobacterium Synechococcus elongatus was isolated using the same gene from Synechocystis sp. PCC 6803 as a heterologous probe. The deduced primary sequence of the isolated single copy petF gene is identical to the primary sequence determined from the protein. Wild-type ferredoxin and a E93-95/Q93-95 mutant were overexpressed in E. coli and purified. Both types of ferredoxins are photoreduced by Photosystem I and can be cross-linked to the PsaD subunit of PS I, although with reduced affinity in case of the E93-95/Q93-95 mutant. These data indicate that the acidic patch of amino acids Glu94-95 of ferredoxin is most likely neither essential for the interaction of ferredoxin with PS I nor the only site of electrostatic contact with the PS I-D subunit. In contrast, NADP⁺photoreduction assays show drastically reduced rates in the presence of the E93-95/Q93-95 mutant ferredoxin, indicating that these residues play a crucial role in the interaction of ferredoxin with ferredoxin-NADP⁺reductase.     

The PsaD subunit of photosystem I. Mutations in the basic domain reduce the level of PsaD in the membranes.

The PsaD subunit of photosystem I (PSI) is a peripheral protein that provides a docking site for ferredoxin and interacts with the PsaB, PsaC, and PsaL subunits of PSI. We used site-directed mutagenesis to determine the function of a basic region in PsaD of the cyanobacterium Synechocystis sp. PCC 6803. We generated five mutant strains in which one or more charged residues were altered. Western blotting showed that replacement of lysine (Lys)-74 with glutamine or glutamic acid led to a substantial decrease in the level of PsaD in the membranes. The mutant PSI complexes showed reduced NADP+ photoreduction activity mediated by ferredoxin; the decrease in activity correlated with the reduced level of PsaD. Using protein synthesis inhibitors we showed that the degradation rates of the mutant and wild-type PsaD were similar, indicating a defect in the assembly of the mutant protein. Treatment of the mutant PSI complexes with a different concentration of NaI showed that the mutations decreased affinity between PsaD and the transmembrane components of PSI. With glutaraldehyde, the mutant and wild-type PsaD proteins could be cross-linked with PsaC, but the PsaD-PsaL cross-linked product was reduced drastically when arginine-72, Lys-74, and Lys-76 were mutated simultaneously. These studies demonstrate that the basic residues in the central region of PsaD, especially Lys-74, are crucial in the assembly of PsaD into the PSI complex.     

Multiple functions for the C terminus of the PsaD subunit in the cyanobacterial photosystem I complex

PsaD subunit of Synechocystis sp PCC 6803 photosystem I (PSI) plays a critical role in the stability of the complex and is part of the docking site for ferredoxin (Fd). In the present study we describe major physiological and biochemical effects resulting from mutations in the accessible C-terminal end of the protein. Four basic residues were mutated: R111, K117, K131, and K135, and a large 36-amino acid deletion was generated at the C terminus. PSI from R111C mutant has a 5-fold decreased affinity for Fd, comparable with the effect of the C terminus deletion, and NADP+ is photoreduced with a 2-fold decreased rate, without consequence on cell growth. The K117A mutation has no effect on the affinity for Fd, but decreases the stability of PsaE subunit, a loss of stability also observed in R111C and the deletion mutants. The double mutation K131A/K135A does not change Fd binding and reduction, but decreases the overall stability of PSI and impairs the cell growth at temperatures above 30 degrees C. Three mutants, R111C, K117A, and the C-terminal deleted exhibit a higher content of the trimeric form of PSI, in apparent relation to the removal of solvent accessible positive charges. Various regions in the C terminus of cyanobacterial PsaD thus are involved in Fd strong binding, PSI stability, and accumulation of trimeric PSI.     

Structural features and assembly of the soluble overexpressed PsaD subunit of photosystem I

PsaD is a peripheral protein on the reducing side of photosystem I (PS I). We expressed the psaD gene from the thermophilic cyanobacterium Mastigocladus laminosus in Escherichia coli and obtained a soluble protein with a polyhistidine tag at the carboxyl terminus. The soluble PsaD protein was purified by Ni-affinity chromatography and had a mass of 16716 Da by MALDI-TOF. The N-terminal amino acid sequence of the overexpressed PsaD matched the N-terminal sequence of the native PsaD from M. laminosus. The soluble PsaD could assemble into the PsaD-less PS I. As determined by isothermal titration calorimetry, PsaD bound to PS I with 1.0 binding site per PS I, the binding constant of 7.7x10(6) M-1, and the enthalpy change of -93.6 kJ mol-1. This is the first time that the binding constant and binding heat have been determined in the assembly of any photosynthetic membrane protein. To identify the surface-exposed domains, purified PS I complexes and overexpressed PsaD were treated with N-hydroxysuccinimidobiotin (NHS-biotin) and biotin-maleimide, and the biotinylated residues were mapped. The Cys66, Lys21, Arg118 and Arg119 residues were exposed on the surface of soluble PsaD whereas the Lys129 and Lys131 residues were not exposed on the surface. Consistent with the X-ray crystallographic studies on PS I, circular dichroism spectroscopy revealed that PsaD contains a small proportion of alpha-helical conformation.     

The assembly of the PsaD subunit into the membranal photosystem I complex occurs via an exchange mechanism.

PsaD is a peripheral stromal-facing subunit of photosystem I (PSI), a multisubunit complex of the thylakoid membranes. PsaD plays a major role in both the function and assembly of PSI. Past studies with radiolabeled PsaD indicated that PsaD is able to assemble in vitro specifically into the PSI complex. To unravel the mechanism by which this assembly takes place, the following steps were taken. (i) Mature PsaD of spinach and PsaD of the prokaryotic caynobacterium Mastigocladus laminosus, both bearing a six-histidine tag at their C-termini, were overexpressed in Escherichia coli and purified to homogeneity. (ii) The purified recombinant protein was introduced into the isolated PSI complex. (iii) Following incubation, the PsaD that assembled into PSI was separated from the nonassembled PsaD by a sucrose gradient. Differential Western blot analysis was used to determine whether the native and the recombinant PsaD were present as free or assembled proteins of the PSI complex. Antibodies that can recognize only the recombinant PsaD (anti-his) or both the native and recombinant PsaD (anti-PsaD) were used. The findings indicated that an exchange mechanism enables the assembly of a newly introduced PsaD into PSI. The latter replaces the PsaD subunit that is present in situ within the complex. In vivo studies that followed the assembly of PsaD in Chlamydomonas reinhardtii cells supported this in vitro-characterized exchange mechanism. In C. reinhardtii, in the absence of synthesis and assembly of new PSI complexes, newly synthesized PsaD assembled into pre-existing PSI complexes.     

The ferredoxin docking site of photosystem I

The reaction center of photosystem I (PSI) reduces soluble ferredoxin on the stromal side of the photosynthetic membranes of cyanobacteria and chloroplasts. The X-ray structure of PSI from the cyanobacterium Synechococcus elongatus has been recently established at a 2.5 A resolution [Nature 411 (2001) 909]. The kinetics of ferredoxin photoreduction has been studied in recent years in many mutants of the stromal subunits PsaC, PsaD and PsaE of PSI. We discuss the ferredoxin docking site of PSI using the X-ray structure and the effects brought by the PSI mutations to the ferredoxin affinity.     

Organization of Photosystem I Polypeptides (A Structural Interaction between the PsaD and PsaL Subunits)

The wild-type, PsaD-less, and PsaL-less strains of the cyanobacterium Synechocystis sp. PCC 6803 were used to study subunit interactions in photosystem I (PSI). When the membranes of a PsaD-less strain were solubilized with Triton X-100 and PSI was purified using ion-exchange chromatography and sucrose-gradient ultracentrifugation, the PsaL subunit was substantially removed from the core of PSI, whereas other subunits, such as PsaE and PsaF, were quantitatively retained during purification. When the wild-type PSI was exposed to increasing concentrations of NaI, the PsaE, PsaD, and PsaC subunits were gradually removed, whereas PsaF, PsaL, PsaK, and PsaJ resisted removal by up to 3 M NaI. The absence of PsaL enhanced the accessibility of PsaD to removal by NaI. Treatment of the wild-type PSI complexes with glutaraldehyde at 4[deg] C resulted in a 29-kD cross-linked product between PsaD and PsaL. The formation of such cross-linked species was independent of PSI concentrations, suggesting an intracomplex cross-linking between PsaD and PsaL. Taken together, these results demonstrate a structural interaction between PsaD and PsaL that plays a role in their association with the PSI core.     

An insight into the assembly and organization of photosystem I complex in the thylakoid membranes of the thermophilic cyanobacterium, Mastigocladus laminosus

The present study characterizes the assembly and organization of Photosystem I (PSI) complex, and its individual subunits into the thylakoid membranes of the thermophilic cyanobacterium, Mastigocladus laminosus. PSI is a multiprotein complex that contains peripheral as well as integral subunits. Hence, it serves as a suitable model system for understanding the formation and organization of membrane protein complexes. In the present study, two peripheral cytosol facing subunits of PSI, namely, PsaD and PsaE were overexpressed in E. coli and used for assembly studies. The gene encoding PsaK, an integral membrane spanning subunit of PSI, was cloned and the deduced amino acid sequence revealed PsaK to have two transmembrane alpha-helices. The characterization of the in vitro assembly of the peripheral subunits, PsaD and PsaE, as well as of the integral subunit, PsaK, was performed by incubating each subunit with thylakoids isolated from Mastigocladus laminosus. All three subunits studied were found to assemble into the thylakoids in a spontaneous mechanism, showing no requirement for cytosolic factors or NTP's (nucleotide 5'-triphosphate). Nevertheless, further characterization of the assembly of PsaK revealed its membrane integration to be most efficient at 55 degrees C. The associations and protein-protein interactions between different subunits within the assembled PSI complex were directly quantified by measurements performed using the BIACORE technology. The preliminary results indicated the existence of specific interaction between PsaD and PsaE, and revealed a very high binding affinity between PsaD and the PSI electron acceptor ferridoxin (Kd = 5.8 x 10(-11) M). PsaE has exhibited a much lower binding affinity for ferridoxin (Kd = 3.1 x 10(-5) M), thereby supporting the possibility of PsaE being one of the subunits responsible for the dissociation of ferridoxin from the PSI complex.     

The PsaC subunit of photosystem I provides an essential lysine residue for fast electron transfer to ferredoxin.

PsaC is the stromal subunit of photosystem I (PSI) which binds the two terminal electron acceptors FA and FB. This subunit resembles 2[4Fe-4S] bacterial ferredoxins but contains two additional sequences: an internal loop and a C-terminal extension. To gain new insights into the function of the internal loop, we used an in vivo degenerate oligonucleotide-directed mutagenesis approach for analysing this region in the green alga Chlamydomonas reinhardtii. Analysis of several psaC mutants affected in PSI function or assembly revealed that K35 is a main interaction site between PsaC and ferredoxin (Fd) and that it plays a key role in the electrostatic interaction between Fd and PSI. This is based upon the observation that the mutations K35T, K35D and K35E drastically affect electron transfer from PSI to Fd, as measured by flash-absorption spectroscopy, whereas the K35R change has no effect on Fd reduction. Chemical cross-linking experiments show that Fd interacts not only with PsaD and PsaE, but also with the PsaC subunit of PSI. Replacement of K35 by T, D, E or R abolishes Fd cross-linking to PsaC, and cross-linking to PsaD and PsaE is reduced in the K35T, K35D and K35E mutants. In contrast, replacement of any other lysine of PsaC does not alter the cross-linking pattern, thus indicating that K35 is an interaction site between PsaC and its redox partner Fd.     

Molecular interactions between photosystem I and ferredoxin: an integrated energy frustration and experimental model

The stromal domain (PsaC, PsaD, and PsaE) of photosystem I (PSI) reduces transiently bound ferredoxin (Fd) or flavodoxin. Experimental structures exist for all of these protein partners individually, but no experimental structure of the PSI/Fd or PSI/flavodoxin complexes is presently available. Molecular models of Fd docked onto the stromal domain of the cyanobacterial PSI site are constructed here utilizing X‐ray and NMR structures of PSI and Fd, respectively. Predictions of potential protein‐protein interaction regions are based on experimental site‐directed mutagenesis and cross‐linking studies to guide rigid body docking calculations of Fd into PSI, complemented by energy landscape theory to bring together regions of high energetic frustration on each of the interacting proteins. The results identify two regions of high localized frustration on the surface of Fd that contain negatively charged Asp and Glu residues. This study predicts that these regions interact predominantly with regions of high localized frustration on the PsaC, PsaD, and PsaE chains of PSI, which include several residues predicted by previous experimental studies. Copyright © 2014 John Wiley & Sons, Ltd.     

Phosphorylation of the insulin receptor by casein kinase I

Insulin receptor was examined as a substrate for the multipotential protein kinase casein kinase I. Casein kinase I phosphorylated partially purified insulin receptor from human placenta as shown by immunoprecipitation of the complex with antiserum to the insulin receptor. Analysis of the phosphorylated complex by polyacrylamide gel electrophoresis under nonreducing conditions showed a major phosphorylated band at the position of the alpha 2 beta 2 complex. When the phosphorylated receptor was analyzed on polyacrylamide gels under reducing conditions, two phosphorylated bands, Mr 95,000 and Mr 135,000, were observed which corresponded to the alpha and beta subunits. The majority of the phosphate was associated with the beta subunit with minor phosphorylation of the alpha subunit. Phosphoamino acid analysis revealed that casein kinase I phosphorylated only seryl residues. The autophosphorylated alpha 2 beta 2 receptor purified by affinity chromatography on immobilized O-phosphotyrosyl binding antibody was also a substrate for casein kinase I. Reduction of the phosphorylated alpha 2 beta 2 receptor indicated that casein kinase I incorporated phosphate into seryl residues only in the beta subunit.     

Ferredoxin-NADP+ reductase. Kinetics of electron transfer, transient intermediates, and catalytic activities studied by flash-absorption spectroscopy with isolated photosystem I and ferredoxin

The electron transfer cascade from photosystem I to NADP+ was studied at physiological pH by flash-absorption spectroscopy in a Synechocystis PCC6803 reconstituted system comprised of purified photosystem I, ferredoxin, and ferredoxin-NADP+ reductase. Experiments were conducted with a 34-kDa ferredoxin-NADP+ reductase homologous to the chloroplast enzyme and a 38-kDa N-terminal extended form. Small differences in kinetic and catalytic properties were found for these two forms, although the largest one has a 3-fold decreased affinity for ferredoxin. The dissociation rate of reduced ferredoxin from photosystem I (800 s(-1)) and the redox potential of the first reduction of ferredoxin-NADP+ reductase (-380 mV) were determined. In the absence of NADP+, differential absorption spectra support the existence of a high affinity complex between oxidized ferredoxin and semireduced ferredoxin-NADP+ reductase. An effective rate of 140-170 s(-1) was also measured for the second reduction of ferredoxin-NADP+ reductase, this process having a rate constant similar to that of the first reduction. In the presence of NADP+, the second-order rate constant for the first reduction of ferredoxin-NADP+ reductase was 20% slower than in its absence, in line with the existence of ternary complexes (ferredoxin-NADP+ reductase)-NADP+-ferredoxin. A single catalytic turnover was monitored, with 50% NADP+ being reduced in 8-10 ms using 1.6 microM photosystem I. In conditions of multiple turnover, we determined initial rates of 360-410 electrons per s and per ferredox-in-NADP+ reductase for the reoxidation of 3.5 microM photoreduced ferredoxin. Identical rates were found with photosystem I lacking the PsaE subunit and wild type photosystem I. This suggests that, in contrast with previous proposals, the PsaE subunit is not involved in NADP+ photoreduction.     

NMR study of the electron transfer complex of plant ferredoxin and sulfite reductase: mapping the interaction sites of ferredoxin

Plant ferredoxin serves as the physiological electron donor for sulfite reductase, which catalyzes the reduction of sulfite to sulfide. Ferredoxin and sulfite reductase form an electrostatically stabilized 1:1 complex for the intermolecular electron transfer. The protein-protein interaction between these proteins from maize leaves was analyzed by nuclear magnetic resonance spectroscopy. Chemical shift perturbation and cross-saturation experiments successfully mapped the location of two major interaction sites of ferredoxin: region 1 including Glu-29, Glu-30, and Asp-34 and region 2 including Glu-92, Glu-93, and Glu-94. The importance of these two acidic patches for interaction with sulfite reductase was confirmed by site-specific mutation of acidic ferredoxin residues in regions 1 and 2, separately and in combination, by which the ability of mutant ferredoxins to transfer electrons and bind to sulfite reductase was additively lowered. Taken together, this study gives a clear illustration of the molecular interaction between ferredoxin and sulfite reductase. We also present data showing that this interaction surface of ferredoxin significantly differs from that when ferredoxin-NADP(+) reductase is the interaction partner.     

Assembly of protein subunits within the stromal ridge of photosystem I. Structural changes between unbound and sequentially PS I-bound polypeptides and correlated changes of the magnetic properties of the terminal iron sulfur clusters

The X-ray structure of Photosystem I (PS I) from Synechococcus elongatus was recently solved at 2.5A resolution (PDB entry 1JB0). It provides a structural model for the stromal subunits PsaC, PsaD and PsaE, which comprise the "stromal ridge" of PS I. In a separate set of studies the three-dimensional solution structures of the unbound, recombinant PsaC (PDB entry 1K0T) and PsaE (PDB entries 1PSF, 1QP2 and 1GXI) subunits were solved by NMR. The PsaC subunit of PS I is a small (9.3 kDa) protein that harbors binding sites for two [4Fe-4S] clusters F(A) and F(B), which are the terminal electron acceptors in PS I. Comparison of the PsaC structure in solution with that in the X-ray structure of PS I reveals significant differences between them which are summarized and evaluated here. Changes in the magnetic properties of [4Fe-4S] centers F(A) and F(B) are related to changes in the protein structure of PsaC, and they are further influenced by the presence of PsaD. Based on experimental evidence, three assembly stages are analyzed: PsaC(free), PsaC(only), PsaC(PS I). Unbound, recombinant PsaD, studied by NMR, has only a few elements of secondary structure and no stable three-dimensional structure in solution. When PsaD is bound in PS I, it has a well-defined three-dimensional structure. For PsaE the three-dimensional structure is very similar in solution and in the PS I-bound form, with the exception of two loop regions. We suggest that the changes in the structures of PsaC and PsaD are caused by the sequential formation of multiple networks of contacts between the polypeptides of the stromal ridge and between those polypeptides and the PsaA/PsaB core polypeptides. The three-dimensional structure of the C(2)-symmetric F(X)-binding loops on PsaA and PsaB were also analyzed and found to be significantly different from the binding sites of other proteins that contain interpolypeptide [4Fe-4S] clusters. The aim of this work is to relate contact information to structural changes in the proteins and to propose a model for the assembly of the stromal ridge of PS I based on this analysis.     

PsaD is required for the stable binding of PsaC to the photosystem I core protein of Synechococcus sp. PCC 6301.

The psaC gene product from Synechococcus sp. PCC 7002 and the psaD gene product from Nostoc sp. PCC 8009 were synthesized in Escherichia coli and purified to homogeneity. Incubation of the PsaC apoprotein with the Synechoccus sp. PCC 6301 photosystem I core protein in the presence of FeCl3, Na2S, and beta-mercaptoethanol resulted in a time-dependent transition in the flash-induced absorption change from a 1.2-ms, P700+ FX- back-reaction to a long-lived, P700+ [FA/FB]- back-reaction. ESR studies showed that FB and FA were photoreduced about equally at 19 K, and while the resonances were shifted upfield, they remained as broad as in the free PsaC holoprotein. When the reconstituted complex was purified in a sucrose gradient containing 0.1% Triton X-100, most of the optical absorption transient reverted to that characteristic of the P700+ FX- back-reaction. Addition of purified PsaD to the incubation mixture led to a greater extent of recovery of electron flow to FA/FB for any given concentration of PsaC. ESR studies showed that FA, rather than FB, became the preferred electron acceptor at 19 K; moreover, the resonances moved upfield and sharpened to become nearly identical with those of a control photosystem I complex. When the sample was purified in a sucrose gradient containing 0.1% Triton X-100, the long-lived P700+ [FA/FB]- optical transient remained stable. Analysis by denaturing polyacrylamide gel electrophoresis showed that the PsaC and PsaD proteins had rebound to the photosystem I core. The data indicate that although PsaC can bind loosely, the presence of PsaD leads to a stable, isolatable photosystem I complex which is spectroscopically indistinguishable from the native complex. Since a PsaC1 fusion protein which contains an amino-terminal extension of five amino acids (MEHSM...) does not bind in the absence of PsaD [Zhao, J., et al. (1990) FEBS Lett. 276, 175-180], the N-terminus of the PsaC protein could provide a site of interaction with the photosystem I core. We propose that the binding of PsaC to the PsaA/PsaB heterodimer is potentiated by insertion of the FA/FB clusters into PsaC, and stabilized by the presence of PsaD.     

Residue Glu-91 of Chlamydomonas reinhardtii ferredoxin is essential for electron transfer to ferredoxin-thioredoxin reductase

The [2Fe-2S] soluble ferredoxin from Chlamydomonas reinhardtii was mutated by site directed mutagenesis, using PCR and the expression plasmid pET-Fd as a template. The recombinant mutated proteins were purified to homogeneity and tested in the activation of NADP-malate dehydrogenase, a light dependent reaction in which ferredoxin thioredoxin reductase (FTR) and thioredoxin are involved. The mutation of residue Glu-91 (E92 in spinach, E94 in Anabaena) alone, either to Gln (E91Q) or to Lys (E91K), was found to completely abolish the reaction of the enzyme light activation. On the other hand, the mutants (E92Q) or (E92K) were as efficient as the wild type ferredoxin in this reaction whereas the double mutants (E91Q/E92Q) or (E91K/E92K) had no activity. In addition, a triple mutant (D25A/E28Q/E29Q) was also found to be inactive for this redox dependent light activation. All these mutations had much weaker effects on the ferredoxin/ferredoxin NADP reductase interaction as measured by the cytochrome c reduction assay. These results indicate that there is a recognition site for FTR in the C terminus part of ferredoxin, but also that a core of negatively charged residues in the α1 helix of ferredoxin might be important in the general process of light activation.     

High-level expression and purification of human xylosyltransferase I in High Five insect cells as biochemically active form

Human xylosyltransferase I (XT-I) catalyzes the transfer of xylose from UDP-xylose to consensus serine residues of proteoglycan core proteins. Expression of a soluble form of recombinant histidine-tagged XT-I (rXT-I-HIS) was accomplished at a high level with High Five/pCG255-1 insect cells in suspension culture. The recombinant protein was purified to homogeneity by a combination of heparin affinity chromatography and metal (Ni(2+)) chelate affinity chromatography. Using the modern technique of perfusion chromatography, a rapid procedure for purification of the rXT-I-HIS from insect cell culture supernatant was developed. The purified, biologically active enzyme was homogeneous on SDS-PAGE, was detected with anti-XT-I-antibodies, and had the expected tryptic fragment mass spectrum. N-terminal amino acid sequencing demonstrated that the N-terminal signal sequence of the expressed protein was quantitatively cleaved. The total yield of the enzyme after purification was 18% and resulted in a specific XT-I activity of 7.9mU/mg. The K(m) of the enzyme for recombinant [Val(36),Val(38)](delta1),[Gly(92),Ile(94)](delta2)bikunin was 0.8microM. About 5mg purified enzyme could be obtained from 1L cell culture supernatant. The availability of substantial quantities of active, homogeneous enzyme will be of help in future biochemical and biophysical characterization of XT-I and for the development of a immunological XT-I assay.     

Mapping of the vitronectin-binding site on the urokinase receptor: involvement of a coherent receptor interface consisting of residues from both domain I and the flanking interdomain linker region

The urokinase-type plasminogen activator receptor (uPAR) has been implicated as a modulator of several biochemical processes that are active during tumor invasion and metastasis, e.g. extracellular proteolysis, cell adhesion, and cell motility. The structural basis for the high affinity interaction between the urokinase-type plasminogen activator (uPA) and uPAR, which focuses cell surface-associated plasminogen activation in vivo, is now thoroughly characterized by site-directed mutagenesis studies and x-ray crystallography. In contrast, the structural basis for the interaction between uPAR and the extracellular matrix protein vitronectin, which is involved in the regulation of cell adhesion and motility, remains to be clarified. In this study, we have identified the functional epitope on uPAR that is responsible for its interaction with the full-length, extended form of vitronectin by using a comprehensive alanine-scanning library of purified single-site uPAR mutants (244 positions tested). Interestingly, the five residues identified as "hot spots" for vitronectin binding form a contiguous epitope consisting of two exposed loops connecting the central fourstranded beta-sheet in uPAR domain I (Trp(32), Arg(58), and Ile(63)) as well as a proximal region of the flexible linker peptide connecting uPAR domains I and II (Arg(91) and Tyr(92)). This binding topology provides the molecular basis for the observation that uPAR can form a ternary complex with uPA and vitronectin. Furthermore, it raises the intriguing possibility that the canonical receptor and inhibitor for uPA (uPAR and PAI-1) may have reached a convergent solution for binding to the somatomedin B domain of vitronectin.