The oncofetal protein, 5T4, is a suitable target for antibody-guided anti-cancer chemotherapy with calicheamicin

The oncofetal protein, 5T4, is a tumor-associated protein displayed on the cell membrane of various carcinomas. This molecule is a promising target for anti-tumor vaccine development and for targeted therapy with staphylococcus exotoxin. The potential use of 5T4 as a target for antibody-guided chemotherapy has not been demonstrated. We report oncolytic efficacy and selectivity in vitro and in vivo with immuno-conjugates of calicheamicin (CM) and the anti-5T4 antibody, H8. CM is a potent cytotoxic drug that causes double strand breaks in DNA. Conjugates of CM and H8 were constructed with acid-labile as well as acid-stabile linkers. In vitro, when applied to monolayers of 5T4(+) cells, CM-conjugates targeting 5T4 were consistently more toxic than either free drug or a non-binding control CM-conjugate. This difference was less pronounced on 5T4-deficient cells. In vivo, four 5T4-positive subcutaneous tumor models were treated with conjugates. Efficacy was demonstrated by reduction of tumor growth relative to controls treated with drug vehicle. To evidence selectivity, the efficacy of the anti-5T4 conjugates was compared to the efficacy of H8, a mixture of H8 and calicheamicin, calicheamicin alone or calicheamicin conjugated to the anti-CD33 antibody, hP67.6. In addition, the efficacy and selectivity of an acid-labile conjugate of H8 was evaluated in an orthotopic model for 5T4(+) lung cancer. Increased survival following treatment was used as a parameter of efficacy. Calicheamicin conjugates of H8 were effective and selective in all the examined tumor models. Differences in efficacy between the acid-labile and acid-stabile conjugates depended on the investigated tumor model.

Restriction in the conformational flexibility of apoproteins in the presence of organic cosolvents: a consequence of the formation of "native-like conformation"

The influence of n-propanol on the overall alpha-helical conformation of beta-globin, apocytochrome C, and the functional domain of streptococcal M49 protein (pepM49) and its consequence on the proteolysis of the respective proteins has been investigated. A significant amount of alpha-helical conformation is induced into these proteins at pH 6.0 and 4 degrees C in the presence of relatively low concentrations of n-propanol. The induction of alpha-helical conformation into the proteins increased as a function of the propanol concentration, the maximum induction occurring around 30% n-propanol. In the case of alpha-globin, the fluorescence of its tryptophyl residues also increased as a function of n-propanol concentration, the midpoint of this transition being around 20% n-propanol. Furthermore, concomitant with the induction of helical conformation into these proteins, the proteolysis of their polypeptide chain by V8 protease also gets restricted. The alpha-helical conformation induced into alpha- and beta-globin by n-propanol decreased as the temperature is raised from 4 to 24 degrees C. In contrast, the alpha-helical conformation of both alpha- and beta-chain (i.e., globin with noncovalently bound heme) did not exhibit such a sensitivity to this change in temperature. However, distinct differences exist between the n-propanol induced "alpha-helical conformation" of globins and the "alpha-helical conformation" of alpha- and beta-chains. A cross-correlation of the n-propanol induced increase in the fluorescence of beta-globin with the corresponding increase in the alpha-helical conformation of the polypeptide chain suggested that the fluorescence increase represents a structural change of the protein that is secondary to the induction of the alpha-helical conformation into the protein (i.e., an integration of the helical conformation induced to the segments of the polypeptide chain to influence the microenvironment of the tryptophyl residues). Presumably, the fluorescence increase is a consequence of the packing of the helical segments of globin to generate a "native-like structure." The induction of alpha-helical conformation into these proteins in the presence of n-propanol and the consequent generation of "native-like conformation" is not unique to n-propanol. Trifluoroethanol, another helix-inducing organic solvent, also behaves in the same fashion as n-propanol. However, in contrast to the proteins described above, n-propanol could neither induce an alpha-helical conformation into performic acid oxidized RNAse-A nor restrict its proteolysis by proteases.(ABSTRACT TRUNCATED AT 400 WORDS)

Helix formation in enzymically ligated peptides as a driving force for the synthetic reaction: example of alpha-globin semisynthetic reaction

The alpha-globin semisynthetic reaction, namely, the ligation of the complementary fragments of alpha-globin, alpha 1-30 and alpha 31-141, in the presence of 30% l-propanol that is catalyzed by V8 protease is distinct as compared with the previously studied protease-catalyzed splicing of the discontinuity sites of the fragment complementing systems [Sahni et al. (1989) Biochemistry 28, 5456]. The complementary fragments of alpha-globin do not exhibit noncovalent interaction between them even in the presence of l-propanol, the organic cosolvent used to facilitate the alpha-globin semisynthetic reaction. Besides, a significant portion of the fragment alpha 31-141 does not contribute to the protease-catalyzed splicing reaction. Alpha 1-30 and alpha 31-40 are ligated by V8 protease to yield alpha 1-40 in much the same way as the splicing of alpha 1-30 with either alpha 31-141 or alpha 31-47 to yield alpha-globin or alpha 1-47, respectively. An equimolar mixture of alpha 1-30 and alpha 31-40 does not show any 'complexation' in the presence of 30% l-propanol, the medium used for the synthetic reaction. The splicing junction, i.e., Glu30-Arg31 peptide bond, is located in the middle of the B-helix (residues 20-35) of the parent protein. Most of the residues from the A-helix of the protein could also be deleted from segment alpha 1-30 without influencing the V8 protease-catalyzed splicing reaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Heptad motifs within the distal subdomain of the coiled-coil rod region of M protein from rheumatic fever and nephritis associated serotypes of group A streptococci are distinct from each other: nucleotide sequence of the M57 gene and relation of the deduced amino acid sequence to other M proteins

Streptococcal M protein, a dimeric alpha helical coiled-coil molecule, is an antigenically variable virulence factor on the surface of the bacteria. Our recent conformational analysis of the complete sequence of the M6 protein led us to propose a basic model for the M protein consisting of an extended central coiled-coil rod domain flanked by a variable N-terminal and a conserved C-terminal end domains. The central coiled-coil rod domain of M protein, which constitutes the major part of the M molecule, is made up of repeating heptads of the generalized sequence a-b-c-d-e-f-g, wherein "a" and "d" are predominantly apolar residues. Based on the differences in the heptad pattern of apolar residues and internal sequence homology, the central coiled-coil rod domain of M protein could be further divided into three subdomains I, II, and III. The streptococcal sequelae rheumatic fever (RF) and acute glomerulonephritis (AGN) have been known to be associated with distinct serotypes. Consistent with this, we observed that the AGN associated M49 protein exhibits a heptad motif that is distinct from the RF associated M5 and M6 proteins. Asn and Leu predominated in the "a" and "d" positions, respectively, in subdomain I of the M5 and M6 proteins, whereas apolar residues predominated in both these positions in the M49 protein. To establish whether the heptad motif of M49 is unique to this protein, or is a general characteristic of nephritis-associated serotypes, the amino acid sequence of M57, another nephritis-associated serotype, has now been examined. The gene encoding M57 was amplified by PCR, cloned into pUC19 vector, and sequenced. The C-terminal half of M57 is highly homologous to other M proteins (conserved region). In contrast, its N-terminal half (variable region) revealed no significant homology with any of the M proteins. Heptad periodicity analysis of the M57 sequence revealed that the basic design principles, consisting of distinct domains observed in the M6 protein, are also conserved in the M57 molecule. However, the heptad motif within the coiled-coil subdomain I of M57 was distinct from M5 and M6 but similar to M49. Similar analyses of the heptad characteristics within the reported sequences of M1, M12, and M24 proteins further confirmed the conservation of the overall architectural design of sequentially distinct M proteins.(ABSTRACT TRUNCATED AT 400 WORDS)

The amino-terminal region of group A streptococcal M protein determines its molecular state of assembly and function

Group A streptococcal M protein, a major virulence factor, is an alpha-helical coiled-coil dimer on the surface of the bacteria. Limited proteolysis of type 57 streptococcus with pepsin released two fragments of the M57 molecule, with apparent molecular weights of 32,000 and 27,000 on SDS-PAGE. However, on gel filtration under nondenaturing conditions, each of these proteins eluted as two distinct molecular forms. The two forms corresponded to their dimeric and monomeric state as compared to the gel filtration characteristics of known dimeric coiled-coil proteins. The results of sedimentation equilibrium measurements were consistent with this, but further indicated that the "dimeric form" consisted of a dimer in rapid equilibrium with its monomer, whereas the "monomeric form" does not dimerize. The monomeric form was the predominant species for the 27 kD species, whereas the dimeric form predominated for the 32 kD species. Sequence analysis revealed the 27 kD species to be a truncated derivative of the 32 kD PepM57 species, lacking the N-terminal nonheptad region of the M57 molecule. These data strongly suggested that the N-terminal nonheptad region of PepM57 is important in determining the molecular state of the molecule. Consistent with this, PepM49, another nephritis-associated serotype, which lacks the nonheptad N-terminal region, also eluted as a monomer on gel filtration under nondenaturing conditions. Furthermore, removal of the N-terminal nonheptad segment of the dimeric PepM6 protein converted it into a monomeric form. The dimeric molecular form of both the 32 kD PepM57 and the 27 kD PepM57 did not represent a stable state of assembly, and were susceptible to conversion to the corresponding monomeric molecular forms by simple treatments, such as lyophilization. The 27 kD PepM57 exhibited a greater propensity than the 32 kD species to exist in the monomeric form. The 32 kD species contained the opsonic epitope of the M57 molecule, whereas the 27 kD species lacked the same. This is consistent with the previous reports on the importance of the N-terminal region of M protein for its opsonic activity. Together, these results strongly suggest that, in addition to its importance for the biological function, the N-terminal region of the M protein plays a dominant role in determining the molecular state of the M molecule, as well as its stability.

Domain structure and molecular flexibility of streptococcal M protein in situ probed by limited proteolysis

Serologically distinct group A streptococcal M proteins, the antiphagocytic determinants of the bacteria, have a highly repetitive sequence and exhibit a heptad periodicity characteristic of alpha-helical coiled-coil proteins. Based on the differences in the pattern of hepatad periodicity, the coiled-coil region of the complete M molecule has been divided into three distinct domains: I, II, and III. Domains I and II together constitute the variable part of M protein, whereas domain III is conserved among serotypes. Pepsin treatment of the M5, M6, and M24 streptococci results in a preferential cleavage of their M molecules between the predicted domains II and III, releasing biologically active fragments of the respective M proteins. Thus, a pepsin cleavage site at the junction of their variable and conserved regions is conserved in the M5, M6, and M24 proteins. In contrast, in the case of the M49 streptococci, the primary site of pepsin cleavage was observed to be within the conserved region of the M49 molecule, rather than at the junction of its variable and conserved regions. Despite containing part of the conserved region, the PepM49 protein is significantly smaller than the pepsin fragments of the M5, M6, and M24 proteins, which contain only the variable regions. However, in addition to the major PepM49 species, the pepsin digest of the type-49 streptococci also contained a smaller fragment, PepM49/a, as a minor component. Its formation was extremely sensitive to the pH of pepsin digestion. PepM49/a, which retains both the propensity to attain an alpha-helical conformation and the opsonic antibody epitope of the M49 molecule, contains only domains I and II like the other PepM proteins. Thus, as in the M5, M6, and M24 proteins, a pepsin cleavage site at the junction of the variable and conserved regions is indeed present in the M49 molecule, but is much less accessible relative to the other serotypes. Thus, the pepsin cleavage sites in the M protein correlate quite well with the boundaries of structurally distinct domains reflected by the predictive analysis. These sites apparently represent the flexible/hinge regions of the molecule. PepM49/a is the least repetitive and the shortest of the M protein pepsin fragments isolated so far. These results suggest that the flexibility of the interdomain regions in M protein may be dependent on the molecular size of their variable domains.(ABSTRACT TRUNCATED AT 400 WORDS)

Purification and characterization of an alpha-D-glucuronidase from a thermophilic fungus, Thermoascus aurantiacus

An alpha-D-glucuronidase was purified from the culture filtrates of Thermoascus aurantiacus. A simple colorimetric method for its assay is reported. The enzyme is a single polypeptide chain with a molecular weight of 118,000. It acts optimally at pH 4.5. It shows maximum activity at 65 degrees C. The t 1/2 at 70 degrees C was 40 min. It specifically cleaved the alpha-(1----2) linkage between 4-O-methyl-alpha-D-glucuronic acid and the xylose residue in xylan and several glucurono-xylooligosaccharides.

Degradation of larchwood xylan by enzymes of a thermophilic fungus, Thermoascus aurantiacus

Proteins from the culture filtrates of Thermoascus aurantiacus grown on paper were found to hydrolyze larchwood xylan completely to form xylose and 4-O-methyl-alpha-D-glucuronic acid. Partial hydrolysis of xylan by a xylanase purified from the culture filtrates resulted in the formation of neutral xylooligosaccharides of dp from 2 to 6 and acidic xylooligosaccharides of dp from 5 to 8. Each of these acidic sugars contained a single molecule of 4-O-methyl-alpha-D-glucuronic acid as a branch. Extensive hydrolysis of these oligosaccharides or xylan by xylanase led to the isolation of xylose, xylobiose, and an aldotetrauronic acid as terminal products. The structure of the aldotetrauronic acid was established by NMR as (2(2)-O-alpha-D,4-O-methyl-alpha-D-glucurono)-xylotriose. A beta-glucosidase, also purified from the culture filtrates, hydrolyzed xylan and the neutral or the acidic xylooligosaccharides from the nonreducing end to release only xylose. Neither xylanase nor beta-glucosidase hydrolyzed the beta-(1----4) linkage between the xylose carrying the branch and the adjacent xylose residue on each side.

Purification of xylanase, beta-glucosidase, endocellulase, and exocellulase from a thermophilic fungus, Thermoascus aurantiacus

A strain of thermophilic fungus, Thermoascus aurantiacus, was isolated from local soil. From the culture filtrates of the organism grown on blotting paper, a xylanase, beta-glucosidase, exocellulase, and endocellulase were obtained in large amounts in highly purified form by employing ion-exchange and gel-permeation chromatography. The xylanase was crystallized. The xylanase and endocellulase were stable at 70 degrees C for 8 h, whereas the beta-glucosidase and exocellulase were less stable at 70 degrees C.

Influence of ions on cyclization of the amino terminal glutamine residues of tryptic peptides of streptococcal PepM49 protein. Resolution of cyclized peptides by HPLC and characterization by mass spectrometry

RPHPLC of the tryptic digest of lysine blocked group A streptococcal PepM49 protein (DHP-PepM49) consistently yielded, among others, two pairs of peptides which were well resolved, eluted in tandem, and had identical amino acid compositions. In each pair, the earlier eluting peptide was readily amenable to sequencing and yielded an amino-terminal glutamine whereas the later eluting peptide could not be sequenced. Mass spectral analysis revealed that each of these pairs of peptides differed in mass corresponding to the loss of ammonia. These data suggested that the later eluting peptide in each pair is a result of cyclization of the amino-terminal glutamine residue to pyroglutamic acid, which apparently leads to an increase in the hydrophobicity of the peptide. A kinetic analysis of the tryptic digestion of the DHP-PepM49 protein revealed that the cyclized form of the peptides were essentially absent during the initial time and increased with time of incubation, with a concomitant decrease in the uncyclized form. In 0.2 M ammonium bicarbonate at 37 degrees, nearly 44% conversion of the glutaminyl peptides to the pyroglutamyl peptides was observed in 24 h. This conversion was accelerated in sodium phosphate buffer relative to that in ammonium bicarbonate whereas it had a significantly lower rate in ammonium acetate buffer. The conversion was also temperature dependent, with essentially no cyclization at 0 degree, in all the three buffers. Thus, an extended digestion at 0 degree or a brief digestion at 37 degrees in ammonium acetate was found to be a suitable condition for limiting the cyclization of amino-terminal glutamine residues of PepM49 peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Complete amino acid sequence of streptococcal PepM49 protein, a nephritis-associated serotype. Conserved conformational design among sequentially distinct M protein serotypes

The complete amino acid sequence of PepM49, a peptic fragment of the group A streptococcal type 49 M protein, the antiphagocytic cell surface molecule of the bacteria, is described. This fragment retains the opsonic antibody epitope of the native molecule. The sequence of PepM49, as determined by automated Edman degradations of the uncleaved molecule, and its tryptic and chymotryptic peptides, consists of a total of 143 residues (Mr = 17,187). PepM49, a nephritis-associated M protein serotype, exhibits significant internal homology in its sequence. However, identical sequence repeats of the kind seen in the rheumatic fever-associated serotypes M5, M6, and M24, are absent in PepM49. PepM49 exhibits varying degrees of homology with the M5, M6, and M24 proteins, which is consistent with the existence of variable and conserved regions in the M protein molecule. Predictive analysis as well as CD measurements revealed a high propensity of the PepM49 molecule to assume an alpha-helical conformation. Furthermore, a heptad periodicity of the nonpolar residues, a characteristic of alpha-helical coiled-coil proteins, extends over the entire length of the PepM49 protein. The differences in the nonpolar residue distribution divide the PepM49 sequence into three distinct domains, similar to those seen earlier in the M5 and M6 proteins. Together, these studies establish a conserved conformational design for the sequentially diverse M protein serotypes. However, the pattern of heptad periodicity in the PepM49 protein is quite distinct from that present in the PepM5 and M6 proteins, suggesting distinct differences in structural features among conformationally similar M protein serotypes. This may have relevance to the pathological differences associated with these M protein serotypes.

Difference in the structural features of streptococcal M proteins from nephritogenic and rheumatogenic serotypes

The association of only certain M protein serotypes of group A streptococci with acute glomerulonephritis is very well recognized. Structural information on the M protein, a dimeric alpha-helical coiled-coil molecule, has come so far from three rheumatogenic serotypes, 5, 6, and 24. However, M proteins from the nephritogenic serotypes have not been well characterized. In the present study, we have isolated a biologically active 20,000 Mr pepsin fragment of type 49 M protein (PepM49), a nephritogenic serotype, and purified it to homogeneity using DEAE-Sephadex and gel filtration. The amino acid composition of PepM49 is similar to those of the rheumatogenic M protein serotypes PepM5, PepM6, and PepM24. However, the sequence of the NH2-terminal 60 residues of PepM49 shows little homology to any of these M protein serotypes, although the latter have significant homology among themselves. Nevertheless, PepM49 exhibits a strong heptad periodicity in its nonpolar residues, suggesting its overall conformational similarity with the other M molecules. During the course of the present studies, Moravek et al. (17) reported the NH2-terminal sequence of another M protein serotype, PepM1, which also does not exhibit much homology with the PepM5, PepM6, and PepM24 proteins. Our analysis of this sequence revealed that the PepM1 protein also exhibits a heptad periodicity of the nonpolar amino acids. A closer examination has revealed that the pattern of heptad periodicity in PepM49 and PepM1 proteins is more regular and more similar to each other than has been previously seen for the PepM5, PepM6, and PepM24 proteins. PepM1 is also a nephritogenic serotype. Taken together, these findings indicate an underlying conservation of the tertiary structure of the various M protein serotypes, despite the complexity in their antigenic variation and suggest that the nephritogenic M protein serotypes M1 and M49 may be further apart evolutionarily from the rheumatogenic serotypes 5, 6, and 24. The distinct differences in the structural features of the PepM1 and PepM49 proteins relative to the PepM5, PepM6, and PepM24 proteins are also suggestive of a correlation with the earlier broader classification of the group A streptococci into rheumatogenic and nephritogenic serotypes.