Role of conserved residues in structure and stability: tryptophans of human serum retinol-binding protein, a model for the lipocalin superfamily

Serum retinol binding protein (RBP) is a member of the lipocalin family, proteins with up-and-down beta-barrel folds, low levels of sequence identity, and diverse functions. Although tryptophan 24 of RBP is highly conserved among lipocalins, it does not play a direct role in activity. To determine if Trp24 and other conserved residues have roles in stability and/or folding, we investigated the effects of conservative substitutions for the four tryptophans and some adjacent residues on the structure, stability, and spectroscopic properties of apo-RBP. Crystal structures of recombinant human apo-RBP and of a mutant with substitutions for tryptophans 67 and 91 at 1.7 A and 2.0 A resolution, respectively, as well as stability measurements, indicate that these relatively exposed tryptophans have little influence on structure or stability. Although Trp105 is largely buried in the wall of the beta-barrel, it can be replaced with minor effects on stability to thermal and chemical unfolding. In contrast, substitutions of three different amino acids for Trp24 or replacement of Arg139, a conserved residue that interacts with Trp24, lead to similar large losses in stability and lower yields of native protein generated by in vitro folding. The results and the coordinated nature of natural substitutions at these sites support the idea that conserved residues in functionally divergent homologs have roles in stabilizing the native relative to misfolded structures. They also establish conditions for studies of the kinetics of folding and unfolding by identifying spectroscopic signals for monitoring the formation of different substructures.

TIMP-3 is a potent inhibitor of aggrecanase 1 (ADAM-TS4) and aggrecanase 2 (ADAM-TS5)

The proteoglycan aggrecan is an important major component of cartilage matrix that gives articular cartilage the ability to withstand compression. Increased breakdown of aggrecan is associated with the development of arthritis and is considered to be catalyzed by aggrecanases, members of the ADAM-TS family of metalloproteinases. Four endogenous tissue inhibitors of metalloproteinases (TIMPs) regulate the activities of functional matrix metalloproteinases (MMPs), enzymes that degrade most components of connective tissue, but no endogenous factors responsible for the regulation of aggrecanases have been found. We show here that the N-terminal inhibitory domain of TIMP-3, a member of the TIMP family that has functional properties distinct from other TIMPs, is a strong inhibitor of human aggrecanases 1 and 2, with K(i) values in the subnanomolar range. This truncated inhibitor, which lacks the C-terminal domain that is responsible for interactions with molecules other than active metalloproteinases, is produced at high yield by bacterial expression and folding from inclusion bodies. This provides a starting point for developing a biologically available aggrecanase inhibitor suitable for the treatment of arthritis.

Specificity and mechanism of metal ion activation in UDP-galactose:beta -galactoside-alpha -1,3-galactosyltransferase

UDP-galactose:beta-galactosyl-alpha1,3-galactosyltransferase (alpha3GT) catalyzes the synthesis of galactosyl-alpha-1,3-beta-galactosyl structures in mammalian glycoconjugates. In humans the gene for alpha3GT is inactivated, and its product, the alpha-Gal epitope, is the target of a large fraction of natural antibodies. alpha3GT is a member of a family of metal-dependent-retaining glycosyltransferases that includes the histo blood group A and B enzymes. Mn(2+) activates the catalytic domain of alpha3GT (alpha3GTcd), but the affinity reported for this ion is very low relative to physiological levels. Enzyme activity over a wide range of metal ion concentrations indicates a dependence on Mn(2+) binding to two sites. At physiological metal ion concentrations, Zn(2+) gives higher levels of activity and may be the natural cofactor. To determine the role of the cation, metal activation was perturbed by substituting Co(2+) and Zn(2+) for Mn(2+) and by mutagenesis of a conserved D(149)VD(151) sequence motif that is considered to act in cation binding in many glycosyltransferases. The aspartates of this motif were found to be essential for activity, and the kinetic properties of a Val(150) to Ala mutant with reduced activity were determined. The results indicate that the cofactor is involved in binding UDP-galactose and has a crucial influence on catalytic efficiency for galactose transfer and for the low endogenous UDP-galactose hydrolase activity. It may therefore interact with one or more phosphates of UDP-galactose in the Michaelis complex and in the transition state for cleavage of the UDP to galactose bond. The DXD motif conserved in many glycosyltransferases appears to have a key role in metal-mediated donor substrate binding and phosphate-sugar bond cleavage.

Crystal structures of apo- and holo-bovine alpha-lactalbumin at 2. 2-A resolution reveal an effect of calcium on inter-lobe interactions

High affinity binding of Ca(2+) to alpha-lactalbumin (LA) stabilizes the native structure and is required for the efficient generation of native protein with correct disulfide bonds from the reduced denatured state. A progressive increase in affinity of LA conformers for Ca(2+) as they develop increasingly native structures can account for the tendency of the apo form to assume a molten globule state and the large acceleration of folding by Ca(2+). To investigate the effect of calcium on structure of bovine LA, x-ray structures have been determined for crystals of the apo and holo forms at 2.2-A resolution. In both crystal forms, which were grown at high ionic strength, the protein is in a similar global native conformation consisting of alpha-helical and beta-subdomains separated by a cleft. Even though alternative cations and Ca(2+) liganding solvent molecules are absent, removal of Ca(2+) has only minor effects on the structure of the metal-binding site and a structural change was observed in the cleft on the opposite face of the molecule adjoining Tyr(103) of the helical lobe and Gln(54) of the beta-lobe. Changes include increased separation of the lobes, loss of a buried solvent molecule near the Ca(2+)-binding site, and the replacement of inter- and intra-lobe H-bonds of Tyr(103) by interactions with new immobilized water molecules. The more open cleft structure in the apo protein appears to be an effect of calcium binding transmitted via a change in orientation of helix H3 relative to the beta-lobe to the inter-lobe interface. Calcium is well known to promote the folding of LA. The results from the comparison of apo and holo structures of LA provide high resolution structural evidence that the acceleration of folding by Ca(2+) is mediated by an effect on interactions between the two subdomains.

TIMP-3 binds to sulfated glycosaminoglycans of the extracellular matrix

Of the four known tissue inhibitors of metalloproteinases (TIMPs), TIMP-3 is distinguished by its tighter binding to the extracellular matrix. The present results show that glycosaminoglycans such as heparin, heparan sulfate, chondroitin sulfates A, B, and C, and sulfated compounds such as suramin and pentosan efficiently extract TIMP-3 from the postpartum rat uterus. Enzymatic treatment by heparinase III or chondroitinase ABC also releases TIMP-3, but neither one alone gives complete release. Confocal microscopy shows colocalization of heparan sulfate and TIMP-3 in the endometrium subjacent to the lumen of the uterus. Immunostaining of TIMP-3 is lost upon digestion of tissue sections with heparinase III and chondroitinase ABC. The N-terminal domain of human TIMP-3 was expressed and found to bind to heparin with affinity similar to that of full-length mouse TIMP-3. The A and B beta-strands of the N-terminal domain of TIMP-3 contain two potential heparin-binding sequences rich in lysine and arginine; these strands should form a double track on the outer surface of TIMP-3. Synthetic peptides corresponding to segments of these two strands compete for heparin in the DNase II binding assay. TIMP-3 binding may be important for the cellular regulation of activity of the matrix metalloproteinases.

Tissue inhibitors of metalloproteinases: evolution, structure and function

The matrix metalloproteinases (MMPs) play a key role in the normal physiology of connective tissue during development, morphogenesis and wound healing, but their unregulated activity has been implicated in numerous disease processes including arthritis, tumor cell metastasis and atherosclerosis. An important mechanism for the regulation of the activity of MMPs is via binding to a family of homologous proteins referred to as the tissue inhibitors of metalloproteinases (TIMP-1 to TIMP-4). The two-domain TIMPs are of relatively small size, yet have been found to exhibit several biochemical and physiological/biological functions, including inhibition of active MMPs, proMMP activation, cell growth promotion, matrix binding, inhibition of angiogenesis and the induction of apoptosis. Mutations in TIMP-3 are the cause of Sorsby's fundus dystrophy in humans, a disease that results in early onset macular degeneration. This review highlights the evolution of TIMPs, the recently elucidated high-resolution structures of TIMPs and their complexes with metalloproteinases, and the results of mutational and other studies of structure-function relationships that have enhanced our understanding of the mechanism and specificity of the inhibition of MMPs by TIMPs. Several intriguing questions, such as the basis of the multiple biological functions of TIMPs, the kinetics of TIMP-MMP interactions and the differences in binding in some TIMP-metalloproteinase pairs are discussed which, though not fully resolved, serve to illustrate the kind of issues that are important for a full understanding of the interactions between families of molecules.

NMR structure of tissue inhibitor of metalloproteinases-1 implicates localized induced fit in recognition of matrix metalloproteinases

A high quality solution structure of the matrix metalloproteinase inhibitory N-terminal domain of recombinant human tissue inhibitor of metalloproteinases-1 (N-TIMP-1) has been determined. For the rigidly packed residues, the average RMSD to the mean structure is 0. 57 A for the backbone atoms and 1.00 A for all heavy atoms. Comparison of the solution structure of free N-TIMP-1 with the crystal structure of TIMP-1 bound to the catalytic domain of MMP-3 ( Gomis-R]uth et al., 1997 ) shows that the structural core of the beta barrel flanked by helices is nearly unchanged by the association with MMP-3, evident from a backbone RMSD of 1.15 A. However, clear differences in the conformation of the MMP-binding ridge of free and MMP-bound TIMP-1 suggest induced fit throughout the ridge. The MMP-dependent conformational changes in the ridge include a dramatic bending of AB loop residues Glu28 through Leu34, moderate hinge bending of the CD-loop about residues Ala65 and Cys70, and modest bending of the Cys1 through Pro6 segment. A large number of interresidue Nuclear Overhauser enhancements (NOEs) augmented by stereospecific assignments, torsion restraints, and dipolar couplings (an average of 18 non-trivial restraints per residue) engender confidence in these structural inferences. A tight cluster of three lysine residues and one arginine residue atop beta-strands A and B, and identical among TIMP sequences, form the heart of a highly conserved electropositive patch that may interact with anionic components of the extracellular matrix.

Role of a conserved acidic cluster in bovine beta1,4 galactosyltransferase-1 probed by mutagenesis of a bacterially expressed recombinant enzyme

The truncated catalytic domain of bovine beta1,4 galactosyltransferase-1 was expressed as inclusion bodies in E.coli and folded to generate 10-15 mg of active enzyme per liter of bacterial culture after extraction and purification under denaturing conditions. Mutations were introduced to investigate the roles of Trp312, Asp318, and Asp320, components of a highly conserved region of sequence in all known beta4GT-1 homologues that includes a cluster of acidic residues. Near and far UV CD spectra of the mutants indicate that the substitutions did not perturb the secondary and tertiary structure of beta4GT-1, and steady state kinetic studies indicate only minor effects on the response to an essential metal cofactor. However substitutions for the two aspartyl residues result in a reduction in catalytic efficiency of a magnitude that suggests they are important for catalysis. It seems possible that this anionic center may act in stabilizing a carbocation formed from the galactose component of the donor substrate in the transition state, reflecting a common reaction mechanism for beta-galactosyltransferase reactions.

Stability, activity and flexibility in alpha-lactalbumin

alpha-Lactalbumins and the type-c lysozymes are homologues with similar folds that differ in function and stability. To determine if the lower stability of alpha-lactalbumin results from specific substitutions required for its adaptation to a new function, the effects of lysozyme-based and other substitutions on thermal stability were determined. Unblocking the upper cleft in alpha-lactalbumin by replacing Tyr103 with Ala, perturbs stability and structure but Pro, which also generates an open cleft, is compatible with normal structure and activity. These effects appear to reflect alternative enthalpic and entropic forms of structural stabilization by Tyr and Pro. Of 23 mutations, only three, which involve substitutions for residues in flexible substructures adjacent to the functional site, increase stability. Two are lysozyme-based substitutions for Leu110, a component of a region with alternative helix and loop conformations, and one is Asn for Lys114, a residue whose microenvironment changes when alpha-lactalbumin interacts with its target enzyme. While all substitutions for Leu110 perturb activity, a Lys114 to Asn mutation increases T(m) by more than 10 degrees C and reduces activity, but two other destabilizing substitutions do not affect activity. It is proposed that increased stability and reduced activity in Lys114Asn result from reduced flexibility in the functional site of alpha-lactalbumin.

Engineering of selective TIMPs

Differences in proteinase susceptibility between free TIMP-1 and the TIMP-1-MMP-3 complex and mutagenesis studies suggested that the residues around the disulfide bond between Cys1 and Cys70 in TIMP-1 may interact with MMPs. The crystal structure of the complex between TIMP-1 and the catalytic domain of MMP-3 has revealed that the alpha-amino group of Cys1 bidentately chelates the catalytic zinc of MMP-3 and the Thr2 side chain occupies the S1' pocket. Generation of the N-terminal domain of TIMP-1 (N-TIMP-1) variants with 15 different amino acid substitutions for Thr2 has indicated that the nature of the side chain of residue 2 has a major effect on the affinity of N-TIMP-1 for three different MMPs (MMPs-1, -2 and -3). The results also demonstrate that the mode of binding of N-TIMP-1 residue 2 differs from the binding of the P1' residue of a peptide substrate.

Rapid collapse and slow structural reorganisation during the refolding of bovine alpha-lactalbumin

The refolding of bovine alpha-lactalbumin (BLA) from its chemically denatured state in 6 M GuHCl has been investigated by a variety of complementary biophysical approaches. CD experiments indicate that the species formed in the early stages of refolding of the apo-protein have at least 85 % of the alpha-helical content of the native state, and kinetic NMR experiments show that they possess near-native compactness. Hydrogen exchange measurements using mass spectrometry and NMR indicate that persistent structure in these transient species is located predominantly in the alpha-domain of the native protein and is similar to that present in the partially folded A-state formed by the protein at low pH. The extent of the exchange protection is, however, small, and there is no evidence for the existence of well-defined discrete kinetic intermediates of the type populated in the refolding of the structurally homologous c-type lysozymes. Rather, both mass spectrometric and NMR data indicate that the rate-determining transition from the compact partially structured (molten globule) species to the native state is highly cooperative. The data show that folding in the presence of Ca2+ is similar to that in its absence, although the rate is increased by more than two orders of magnitude. Sequential mixing experiments monitored by fluorescence spectroscopy indicate that this slower folding is not the result of the accumulation of kinetically trapped species. Rather, the data are consistent with a model in which binding of Ca2+ stabilizes native-like contacts in the partially folded species and reduces the barriers for the conversion of the protein to its native state. Taken together the results indicate that folding of BLA, in the presence of its four disulphide bonds, corresponds to one of the limiting cases of protein folding in which rapid collapse to a globule with a native-like fold is followed by a search for native-like side-chain contacts that enable efficient conversion to the close packed native structure.

Residue 2 of TIMP-1 is a major determinant of affinity and specificity for matrix metalloproteinases but effects of substitutions do not correlate with those of the corresponding P1' residue of substrate

The unregulated activities of matrix metalloproteinases (MMPs) are implicated in disease processes including arthritis and tumor cell invasion and metastasis. MMP activities are controlled by four homologous endogenous protein inhibitors, tissue inhibitors of metalloproteinases (TIMPs), yet different TIMPs show little specificity for individual MMPs. The large interaction interface in the TIMP-1.MMP-3 complex includes a contiguous region of TIMP-1 around the disulfide bond between Cys1 and Cys70 that inserts into the active site of MMP-3. The effects of fifteen different substitutions for threonine 2 of this region reveal that this residue makes a large contribution to the stability of complexes with MMPs and has a dominant influence on the specificity for different MMPs. The size, charge, and hydrophobicity of residue 2 are key factors in the specificity of TIMP. Threonine 2 of TIMP-1 interacts with the S1' specificity pocket of MMP-3, which is a key to substrate specificity, but the structural requirements in TIMP-1 residue 2 for MMP binding differ greatly from those for the corresponding residue of a peptide substrate. These results demonstrate that TIMP variants with substitutions for Thr2 represent suitable starting points for generating more targeted TIMPs for investigation and for intervention in MMP-related diseases.

Structural evidence for the presence of a secondary calcium binding site in human alpha-lactalbumin

The high-resolution X-ray crystal structure of human alpha-lactalbumin (at 1.8 A) in the presence of an elevated level of calcium reveals a new secondary calcium binding site, 7.9 A away from the primary calcium binding site known in all alpha-lactalbumin structures so far. The new calcium binding site is different from the zinc and sulfate binding sites [Ren, J., et al. (1993) J. Biol. Chem. 268, 19292-19298] but shares common features with the manganese binding site as described by Gerkin [Gerkin, T. A. (1984) Biochemistry 23, 4688-4697]. The proximity of the manganese and calcium binding region and the location of the functional site on one side of the charged surface of the alpha-lactalbumin molecule suggest that these binding sites might play a role in the formation of the lactose synthase complex.

Expression of human pro-matrix metalloproteinase 3 that lacks the N-terminal 34 residues in Escherichia coli: autoactivation and interaction with tissue inhibitor of metalloproteinase 1 (TIMP-1)

Human pro-matrix metalloproteinase 3 (proMMP-3) lacking the N-terminal 34 amino acids and the C-terminal hemopexin-like domain was expressed in E. coli and used to investigate the process of proenzyme activation and its interaction with an endogenous inhibitor TIMP-1 during activation. The truncated precursor was purified from the E. coli extract in the presence of 5mM EGTA. The active 23.5 kDa form was generated simply by exposure to Ca2+ and Zn2+ but not either by Ca2+ alone or by Zn2+ alone. The rate of MMP-3(deltaC) formation was concentration dependent, indicating that autoactivation is a bimolecular reaction. The truncated precursor was able to interact with the N-terminal domain of TIMP-1 without losing the 48 residue-long propeptide. However, upon a longer incubation, the propeptide was slowly processed, indicating that the association of the N-terminally truncated proMMP-3 with TIMP-1 is weaker than that of the fully activated MMP-3 and TIMP-1. These results indicate that the expression of MMP activities is regulated by endogenous inhibitor TIMPs during their activation processes which provide an additional control mechanism of extracellular matrix breakdown.

Mechanism of inhibition of the human matrix metalloproteinase stromelysin-1 by TIMP-1

Matrix metalloproteinases (MMPs) are zinc endopeptidases that are required for the degradation of extracellular matrix components during normal embryo development, morphogenesis and tissue remodelling. Their proteolytic activities are precisely regulated by endogenous tissue inhibitors of metalloproteinases (TIMPs). Disruption of this balance results in diseases such as arthritis, atherosclerosis, tumour growth and metastasis. Here we report the crystal structure of an MMP-TIMP complex formed between the catalytic domain of human stromelysin-1 (MMP-3) and human TIMP-1. TIMP-1, a 184-residue protein, has the shape of an elongated, contiguous wedge. With its long edge, consisting of five different chain regions, it occupies the entire length of the active-site cleft of MMP-3. The central disulphide-linked segments Cys 1-Thr 2-Cys 3-Val 4 and Ser 68-Val 69 bind to either side of the catalytic zinc. Cys 1 bidentally coordinates this zinc, and the Thr-2 side chain extends into the large specificity pocket of MMP-3. This unusual architecture of the interface between MMP-3 and TIMP-1 suggests new possibilities for designing TIMP variants and synthetic MMP inhibitors with potential therapeutic applications.

Mutational study of the amino-terminal domain of human tissue inhibitor of metalloproteinases 1 (TIMP-1) locates an inhibitory region for matrix metalloproteinases

A bacterial expression system for the inhibitory N-terminal domain of human tissue inhibitor of metalloproteinases 1 (N-TIMP-1) (Huang, W., Suzuki, K., Nagase, H., Arumugam, S., Van Doren, S. R., and Brew, K. (1996) FEBS Lett. 384, 155-161) has been used to produce 20 single- and double-site mutants that probe the roles of different residues in its inhibitory action on metalloproteinases. Mutations that produce the largest increases in the Ki for a C-terminally truncated form of stromelysin 1, MMP-3(DeltaC), but do not disturb the conformation involve substitutions of residues that are located in a ridge that is centered around the disulfide bond between Cys1 and Cys70. Specific residues that have a large influence on activity include Cys1, Thr2, Met66, Val69, and Cys70. Of the mutations introduced, the greatest functional disturbances, reflected in Ki increases of 2-4 orders of magnitude, are generated by changes that disrupt the Cys1-Cys70 disulfide bond and by substitution of Ala for Thr2. Most mutations that perturb the interaction with MMP-3 have parallel effects on the affinity of N-TIMP-1 for MMP-1 (interstitial collagenase) and MMP-2 (gelatinase A). However, the Thr2 to Ala mutation produces an inhibitor that is 17-fold more effective against MMP-3 than MMP-1, suggesting that it is feasible to engineer TIMP-1 variants that are more specifically targeted to selected matrix metalloproteinases. The reactive site identified by these studies is a structurally constrained but elongated region of TIMP that can fit the matrix metalloproteinase substrate-binding site.

Involvement of a region near valine-69 of tissue inhibitor of metalloproteinases (TIMP)-1 in the interaction with matrix metalloproteinase 3 (stromelysin 1)

Tissue inhibitors of metalloproteinases (TIMPs) inhibit matrix metalloproteinases (MMPs) by forming a 1:1 stoichiometric complex, but the inhibition mechanism of these inhibitors is not known. Here we have investigated the reactive site of TIMP-1 by its proteinase susceptibility before and after forming a complex with MMP-3 (stromelysin 1). When TIMP-1 was allowed to react with human neutrophil elastase, its inhibitory activity was destroyed. This resulted from cleavage of the Val69-Cys70 bond. However, cleavage of this bond by neutrophil elastase was prevented when TIMP-1 formed a complex with the catalytic domain of MMP-3, and full TIMP-1 activity was restored after dissociation of the complex at pH 3.0 in the presence of EDTA. These results indicate that the region around Val69 closely associates with an active MMP. The three-dimensional structure of the N-terminal domain of TIMP-2 elucidated by NMR studies [Williamson, Martorell, Carr, Murphy, Docherty, Freedman and Feeney (1994) Biochemistry 33, 11745-11759] reveals that Val69 and Cys70 form part of an extended ridge that also includes the N-terminal section of the inhibitor. This region is probably involved in the interaction with the catalytic domains of MMPs.

Protein folding monitored at individual residues during a two-dimensional NMR experiment

An approach is described to monitor directly at the level of individual residues the formation of structure during protein folding. A two-dimensional heteronuclear nuclear magnetic resonance (NMR) spectrum was recorded after the rapid initiation of the refolding of a protein labeled with nitrogen-15. The intensities and line shapes of the cross peaks in the spectrum reflected the kinetic time course of the folding events that occurred during the spectral accumulation. The method was used to demonstrate the cooperative nature of the acquisition of the native main chain fold of apo bovine alpha-lactalbumin. The general approach, however, should be applicable to the investigation of a wide range of chemical reactions.

Functional site in alpha-lactalbumin encompasses a region corresponding to a subsite in lysozyme and parts of two adjacent flexible substructures

Aromatic cluster 1 of alpha-lactalbumin (LA), a substructure adjacent to the cleft, is important for its interaction with galactosyltransferase (GT) and effects on glucose binding in the lactose synthase complex [Grobler, J. A., Wang, M., Pike, A. K., & Brew, K. (1994) J. Biol. Chem. 269, 5106-5114]. The full extent of the functional region in LA has been probed by mutagenesis of residues that are near aromatic cluster 1 or within the cleft that corresponds to the active site in the homologous type c lysozymes. The conserved residues Val42, Gln54, and Ile59, which correspond to residues of lysozyme that act in substrate binding in subsites C to E, together with residues adjacent to aromatic cluster 1, were found to be not required for activity. In contrast, replacing Leu110, a component of the region corresponding to lysozyme subsite F, with His or Glu greatly reduces the affinity of LA for GT while the introduction of Arg lowers the synergism of LA and glucose binding to GT and also reduces the affinity of LA for GT. Substitutions for Ala106, which is adjacent to Leu110 in the structure, also perturb activity. The region of the cleft corresponding to subsite F is important for function in LA as well as in lysozyme since other components of this subsite, His32 and Phe31, are also crucial for LA activity. The qualitatively different effects of various substitutions for Leu110 may be mediated by their influence on His32 or by changes in the structure of the lactose synthase complex.

Crystal structures of guinea-pig, goat and bovine alpha-lactalbumin highlight the enhanced conformational flexibility of regions that are significant for its action in lactose synthase

BACKGROUND

The regulation of milk lactose biosynthesis is highly dependent on the action of a specifier protein, alpha-lactalbumin (LA). Together with a glycosyltransferase, LA forms the enzyme complex lactose synthase. LA promotes the binding of glucose to the complex and facilitates the biosynthesis of lactose. To gain further insight into the molecular basis of LA function in lactose synthase we have determined the structures of three species variants of LA.

RESULTS

The crystal structures of guinea-pig, goat and a recombinant from of bovine LA have been determined using molecular replacement techniques. Overall, the structures are very similar and reflect their high degree of amino acid sequence identity (66-94%). Nonetheless, the structures show that a portion of the molecule (residues 105-110), known to be important for function, exhibits a variety of distinct conformers. This region lies adjacent to two residues (Phe31 and His32) that have been implicated in monosaccharide binding by lactose synthase and its conformation has significant effects on the environments of these functional groups. The crystal structures also demonstrate that residues currently implicated in LA's modulatory properties are located in a region of the structure that has relatively high thermal parameters and is therefore probably flexible in vivo.

CONCLUSIONS

LA's proposed interaction site for the catalytic component of the lactose synthase complex is primarily located in the flexible C-terminal portion of the molecule. This general observation implies that conformational adjustments may be important for the formation and function of lactose synthase.

Mutational analysis of the catalytic subunit of muscle protein phosphatase-1

A mutational analysis of rabbit skeletal muscle protein phosphatase-1 was performed by site-directed mutagenesis of the recombinant protein expressed in Escherichia coli. The selection of the sites to be mutated was based on sequence alignments which showed the existence of a number of invariant residues when eukaroytic Ser/Thr protein phosphatases were compared with bacteriophage phosphatases and adenosinetetraphosphatase [Barton et al. (1995) Eur. J. Biochem. 220, 225-237]. In other studies, it had been shown that PP1 is a metalloprotein [Chu et al. (1996) J. Biol. Chem. 271, 2574-2577], and in this study, we have largely focused on invariant histidine and aspartate residues which may be involved in metal binding. The residues which were mutated were H66, H125, H173, H248, D64, D71, D92, D95, N124, and R96E. The results showed that mutation of H66, H248, D64, and D92 resulted in severe loss of catalytic function. Mutation of D95, N124, and R96 also led to loss of function, while attempts to mutate H125 and H173 led to production of insoluble, inactive proteins. The results of the mutational analysis are consistent with the involvement of conserved His and Asp residues in metal binding, and are discussed in the context of the recently described crystal structure of PP1 [Goldberg et al. (1995) Nature, 376, 745-753], which reveals that PP1 possesses a bimetallic center at the active site. The behavior of the D95, R96, and N124 mutants supports a catalytic mechanism involving nucleophilic attack by a hydroxide ion with H125 functioning as a proton donor to the leaving alcohol group.

Folding and characterization of the amino-terminal domain of human tissue inhibitor of metalloproteinases-1 (TIMP-1) expressed at high yield in E. coli

Methods are described for producing an active amino-terminal domain of tissue inhibitor of metalloproteinases-1 (N-TIMP-1) from inactive protein expressed as inclusion bodies in E. coli. Yields exceed 20 mg per litre of bacterial culture. Activity measurements, CD spectroscopy and NMR spectroscopy of the 15N-labeled protein show that it is fully active, homogeneous in conformation and suitable for high-resolution structural analysis. The affinity of N-TIMP-1 for matrix metalloproteinases 1, 2 and 3 is 6-8-fold less than that of the recombinant full-length protein, indicating that deletion of the C-terminal domain reduces the free energy of interaction by < 10%.

Spectroscopic characterization by photodiode array detection of human urinary and amniotic protein HC subpopulations fractionated by anion-exchange and size-exclusion high-performance liquid chromatography

A procedure for spectroscopic characterization and partial fractionation of human protein HC populations by high-performance liquid chromatography-photodiode array ultraviolet-visible detection is reported. Human protein HC from urine or amniotic fluid fractionated by anion-exchange HPLC in a protein Pak DEAE 5PW appeared to be heterogeneous as judged by the asymmetric elution pattern, consisting of a continuous irregular broad peak with several shoulders distributed along the whole chromatogram. Selected fractions containing shoulders were rechromatographed and finally six symmetrical homogeneous peaks with different retention times were obtained from each protein HC preparation. The direct automatic absorption spectra analyses at each peak maximum, indicated that all of the homogeneous peaks seemed to be protein HC, all of them associated to the same chromophore although with different stoichiometry ratios. Isoelectric focusing showed that each peak was composed of a limited number of subpopulations of protein HC with different isoelectric points. Size microheterogeneity has been also demonstrated in both urinary and amniotic protein HC preparations by a combination of size-exclusion HPLC on a TSK 3000 SW6 column and photodiode array detection. Partial fractionation of human albumin on an analytical anion-exchange Mono-Q PC 1.6/5 column, has allowed the identification of heterogeneous chromophore-containing populations displaying significant absorption in the visible region in resemblance to that of protein HC.