Selective proteolysis of the beta 2 subunit of Serratia marcescens tryptophan synthase.

Mild digestion of Serratia marcescens tryptophan synthase β₂ subunit produces a modified β₂ subunit (nicked β₂). The nicked β₂ subunit remains essentially intact and is immunochemically reactive with native β₂ subunit antiserum. Denaturation of the nicked β₂ subunit yields two principal peptide fragments whose minimum molecular weights are 29,500 and 13,400. Loss of enzyme activity is associated with the selective proteolysis. The enzyme cofactor pyridoxal phosphate binding site is on the larger fragment. Following separation of the fragments by urea-gel chromatography, the separated peptides retain immunological cross-reactivity with native β₂ subunit antiserum. These fragments apparently represent two domains that comprise the native Holo β₂ subunit. The immunochemical data suggest that these fragments, when isolated, can assume some tertiary structure and that they may exist as such prior to β monomer or β₂ dimer assembly. The folded fragments may represent intermediates in the biosynthesis of the β₂ subunit as has been suggested for the E. coli enzyme (A. Högberg-Raibaud and M. E. Goldberg, 1977, Proc. Nat. Acad. Sci. USA74, 442; Biochemistry16, 4014).     

Deglycosylation of ovine pituitary lutropin subunits: Effects on subunit interaction and hormone activity

Brief exposure of the isolated α and β subunits of ovine lutropin to anhydrous liquid HF resulted in effective but incomplete removal of the oligosaccharide moiety. Fucose and hexoses were completely eliminated while hexosamine content was considerably reduced. The partially deglycosylated subunits (pDGα and pDGβ) retained their capability to recognize the native counterparts as well as each other. Both partially deglycosylated subunits retained full activity in specific radioimmunoassays. The pDGα + native β as well as native α + pDGβ recombinants showed full receptor binding activity, but the former had approximately 60% less in vitro bioactivity. The recombinant of native α + pDGβ showed full bioactivity in vitro. The receptor binding and biological activities of pDGα + pDGβ were comparable to that of deglycosylated lutropin. These two derivatives antagonized the action of intact lutropin as assessed by steroidogenesis in dispersed rat Leydig cells in vitro. The results suggest an important role for the oligosaccharide moiety in the expression of full hormone function.     

Selective cleavage at lysine of the 50 kDa-20 kDa connector loop segment of skeletal myosin S-1 by endoproteinase Arg-C

The reaction of endoproteinase Arg-C on the skeletal myosin head heavy chain was investigated through characterization of peptides and amino acid sequence analysis. The protease splits exclusively the 50 kDa-20 kDa junction at the lysine cluster spanning residues 639-641 and does not affect any other protease-sensitive region of the entire myosin heavy chain. The sensitivity of the cleavage to actin and nucleotide binding makes this protease a very specific conformational probe of S-1. The nicked S-1 derivative, containing an intact NH2-terminal 75 kDa fragment, may serve as a tool for gaining further insights into the domain structure and function of the myosin head.     

Intrinsic and extrinsic fluorescence probes of subunit interactions in ovine lutropin.

The fluorescence of 1,8-anilinonaphthalene sulfonate (ANS) was enhanced in the presence of ovine lutropin (oLH). Fluorescence titration curves were sigmoidal with 50% saturation between 200 and 500 μm. Exclusion chromatography experiments indicated that the hormone self-associates to form dimers in the presence of excess ANS. By contrast, the isolated a and β subunits of oLH caused a much smaller enhancement of the fluorescence of ANS and did not self-associate in its presence. Dissociation of the intact hormone into its subunits was accompanied by 1) a loss in the ability to enhance ANS fluorescence, 2) the appearance of a negative differential absorption spectrum whose magnitude indicated the increased solvent-exposure of at least two tyrosines, and 3) a loss in conformational rigidity as evidenced by a decrease in polarization (P) of tyrosyl fluorescence from ~0.17 to ~0.13. Similar rates of dissociation were obtained by all three measurements and the first order rate constant at pH 3.6 and 37 °C under conditions of low ionic strength was k = 0.18 min^?1; at high ionic strength, e.g., 0.5 m KC1, dissociation was incomplete even after prolonged incubation. Acid-dissociated subunits recombined readily in 0.5 m acetate buffer, pH 5.3, and the recovery of the intrinsic absorption and fluorescence properties as well as the ability to enhance ANS fluorescence ranged between 70 and 90%. Titration of the isolated α and β subunits with acid or GdmCl had little or no effect on P, suggesting that residual secondary or tertiary structure is either absent, very stable, or its disruption does not alter the rigidity of the tyrosyl environment. The relatively high P for oLH-β (0.17) suggests a conformation which is rigid compared with oLH-α (0.13). P for both subunits decreased smoothly with increasing temperature between 20 and 70 °C. By contrast, oLH exhibited a thermal transition near 50 °C characterized by a drop in P from a value near that of β to a value near that of a as the subunits dissociated. Because α has more tyrosines with a higher average quantum yield, its fluorescence would be expected to dominate that of the hormone or of an equimolar mixture of subunits. Thus, most of the conformation changes which accompany dissociation and recombination appear to occur in the α subunit.     

The amino acid sequence of the rabbit lutropin beta subunit

The amino acid sequence of the beta subunit of rabbit lutropin (lLH) has been determined. The amino terminus of about 97% of the beta subunit has a two amino acid extension (pyro-Glu-Pro) compared to other lutropin beta sequences. Overlapping peptides from trypsin and chymotrypsin digestions of the performic acid-oxidized beta subunit and trypsin digestion of the S-aminoethylated cysteine beta subunit were isolated by chromatography on TSK Fractogel 40F and high-pressure liquid chromatography (HPLC). Sequencing was by a combination of the dansyl-Edman method and the direct Edman method. Amide placements were established by HPLC analysis of the PTH amino acid derivatives. The proposed sequence of lLH subunit is: This sequence is highly homologous to the other known lutropin beta subunits, especially rat and pig lutropin beta (91%). Partial cleavage of the peptide bond between Asp-79 and Pro-80 was observed during cyanogen bromide treatment. Rabbit thyrotropin and thyrotropin beta subunit copurified with lLH and lLH except at a final chromatography on Sephadex G-100.     

Further characterization of the ovine lutropin alpha and beta subunits prepared by the salt precipitation method.

The subunits of ovine lutropin prepared by acid dissociation and salt precipitation were characterized by end group analysis, tryptic peptide mapping, SDS gel electrophoresis and biological activity. No evidence of internal peptide cleavage was found in the alpha subunit. The subunits possessed low activity. The alpha and beta subunits recombined effectively to generate a complex that had full receptor binding activity and in vitro biological activity. The recombinants of subunits prepared by countercurrent distribution showed only 50% activity in both assays. The salt precipitation method alpha subunit could be completely reduced and reoxidized in the absence of denaturants. The reoxidized alpha subunit combines with the native beta subunit generating full activity. However, this recombined hormone tends to lose activity with time, suggesting that the reoxidation may not fully restore the native structur of the reduced alpha subunit. The native lutropin alpha subunit effectively combined with follitropin beta subunit generating complete follitropin activity.     

Effects of beta subunit acylation on lutropin receptor site binding.

The effects of various modifications on the beta subunit of lutropin have been studied using the binding characteristics of the reconstituted hormone in the rat testicular radioligand assay. Conditions for iodinating lutropin and lutropin derivatives were determined which resulted in 15 per cent specific binding when tested immediately and retention of 6 to 7 per cent specific binding even after storage for 6 months. Acetimidinyl, acetyl, and carbamyl derivatives of the beta subunit were prepared and combined with unmodified alpha subunit to form reconstituted lutropin. Modification of the beta subunit was shown to have no effect on the time course of binding to testicular receptors or, with one exception, on the extent of receptor saturation. Very high concentrations of lutropin reconstituted with acetylated beta subunit showed an anomalous binding behavior. Scatchard plots of the binding data support the view that the native hormone has a unique receptor affinity which is irreversibly disrupted by separation of subunits and that derivatization of the beta subunit does not alter this parameter further. These data also suggest that there are no significant differences in the amino groups modified on the beta subunit. Competition and preincubation tests for receptor sites that reacted only with modified lutropin and not with the native hormone were negative.     

Expression of alpha subunit and luteinizing hormone beta genes in the ovine anterior pituitary. Estradiol suppresses accumulation of mRNAS for both alpha subunit and luteinizing hormone beta

Bovine cDNA clones containing coding sequences for growth hormone, prolactin, alpha subunit, and luteinizing hormone beta (LH beta) have been used to quantitate their respective mRNA concentrations in anterior pituitary glands isolated from ovariectomized ewes, or from ovariectomized ewes treated for three weeks with estradiol. Concentrations of mRNAs for prolactin or growth hormone remained unchanged in either physiological state. In contrast, treatment with estradiol resulted in a 98% decrease of mRNA for LH beta, relative to untreated animals. This change in mRNA was associated with a similar decrease in the concentrations of pituitary and serum LH. Administration of estradiol also led to a reduction (86%) of alpha subunit mRNA. These results suggest that estrogen regulates the expression of the genes encoding both the alpha and LH beta subunit prior to translation. Furthermore, the pronounced effect of estradiol on the concentrations of mRNAs for alpha subunit and LH beta suggest that the assembly of mature glycoprotein hormones may not be limited solely by the rate of accumulation of the beta subunit.     

Characterization of cleavage products in selected human lutropin preparations. A protease-sensitive site in human lutropin beta subunit

Low molecular weight fragments derived from the beta subunit of human lutropin have been frequently observed. These fragments are detected by polyacrylamide gel electrophoresis in sodium dodecyl sulfate following reduction of the disulfide bonds. A sample of human lutropin was identified that had a major portion of its beta subunit showing this proteolytic nick. Over 83% of the subunit was nicked based on reduction, carboxymethylation, and isolation of the low molecular weight fragments. This preparation had 53% of the activity of an intact human lutropin (radioligand assay). The proteolytic nick in the subunit was shown by N-terminal sequencing of the C-terminal fragments to be derived from three clips in a hexapeptide region (residues 44-49) characterized by hydrophobic alkyl side chains. Specific clips were on the amino side of Leu-45 (8%), Val-48 (45%) and Leu-49 (47%). Thus the proteolytic activity, presumably derived from the pituitary during processing, has a substrate specificity reminiscent of the bacterial protease, thermolysin.     

The primary structure of ovine interstitial cell stimulating hormone. IV: Disulfide bridges of the beta subunit.

The disulfide linkage of the 12 half-cystine residues in the beta subunit of ovine interstitial cell stimulating hormone has been investigated by enzymic and partial acid hydrolysis of the intact molecule. Results indicate that the disulfide bridges are formed by residues 9-38, 23-72, 26-110, 34-90, 57-88, and 93-100.     

A carboxyl-terminal sequence in the lutropin beta subunit contributes to the sorting of lutropin to the regulated pathway

Although synthesized in the same pituitary gonadotropes, the secretion profiles of lutropin (LH) and follitropin (FSH) differ. LH is secreted through a regulated pathway and associated with a bolus release at mid-estrous cycle. In contrast, the majority of FSH is secreted constitutively with an incremental increase until ovulation. Both share an identicalalpha subunit, and thus thebeta subunit contains determinants for sorting into the regulated pathway. Previously, we demonstrated that a hydrophobic carboxyl-terminal heptapeptide of the LHbeta subunit (Leu-Ser-Gly-Leu-Leu-Phe-Leu), not found in the FSHbeta subunit, influences the intracellular behavior of the LH dimer. To test the hypothesis that the peptide contributes to differential sorting, we monitored the fates of LH and LHDeltaT (LHbeta subunit lacking the carboxyl-terminal seven amino acids) dimers in the rat somatotrope-derived GH(3) cell line in which both the regulated and constitutive secretory pathways operate. Pulse-chase labeling demonstrated that the LHDeltaT dimer was diverted to the constitutive pathway, resulting in a significant decrease in the corresponding intracellular pool. Forskolin stimulated LH dimer release 3-fold, which was accompanied by a parallel decrease of intracellular LH; only marginal forskolin stimulation of LHDeltaT was seen. Immunofluorescence after cycloheximide treatment demonstrated decreased retention of LHDeltaT compared with LH, consistent with increased constitutive secretion of LHDeltaT. We also demonstrated that fusing the heptapeptide to the carboxyl terminus of the FSHbeta subunit resulted in an increased regulated secretion of this FSH analog compared with wild-type FSH. These data are the first to identify a novel structural determinant responsible for the sorting of a member of the glycoprotein hormone family into the regulated secretory pathway.     

Amphibian lutropin from the bullfrog Rana catesbeiana. Complete amino acid sequence of the beta subunit.

The amino acid sequence of lutropin (LH) beta subunit of an amphibian, the bullfrog Rana catesbeiana, has been determined. The primary structure was determined by sequencing the intact protein (residues 1-44) and peptides originated by cyanogen bromide cleavage and lysyl endopeptidase digestion. 12 cysteine residues are conserved in the bullfrog and mammalian LH beta subunit. One sugar-chain-binding site at Asn-8 is also conserved in the bullfrog and in all mammals except humans. This glycoprotein is composed of 112 amino acid residues with a molecular mass of 12675 Da, considering the six cystine bridges and excepting the sugar chain. The bullfrog beta subunit has approximately 50% sequence identity with that of mammals and with the fish gonadotropin beta subunit, and about 40% with bullfrog follicle-stimulating hormone beta subunit.     

Protein inactivation in mycobacteria by controlled proteolysis and its application to deplete the beta subunit of RNA polymerase

Using a component of the Escherichia coli protein degradation machinery, we have established a system to regulate protein stability in mycobacteria. A protein tag derived from the E. coli SsrA degradation signal did not affect several reporter proteins in wild-type Mycobacterium smegmatis or Mycobacterium tuberculosis. Expression of the adaptor protein SspB, which recognizes this modified tag and helps deliver tagged proteins to the protease ClpXP, strongly decreased the activities and protein levels of different reporters. This inactivation did not occur when the function of ClpX was inhibited. Using this system, we constructed a conditional M. smegmatis knockdown mutant in which addition of anhydrotetracycline (atc) caused depletion of the beta subunit of RNA polymerase, RpoB. The impact of atc on this mutant was dose-dependent. Very low amounts of atc did not prevent growth but increased sensitivity to an antibiotic that inactivates RpoB. Intermediate amounts of RpoB knockdown resulted in bacteriostasis and a more substantial depletion led to a decrease in viability by up to 99%. These studies identify SspB-mediated proteolysis as an efficient approach to conditionally inactivate essential proteins in mycobacteria. They further demonstrate that depletion of RpoB by ～ 93% is sufficient to cause death of M. smegmatis.     

Studies on prolactin. Selective reduction of the disulfide bonds of the ovine hormone.

Methods for selective reduction of the disulfide bonds in ovine prolactin are reported. Cleavage of all three disulfide bonds abolishes biological activity and denatures the hormone. Reduction-carbamidomethylation of one or two of the disulfide bridges does not diminish the biological activities in the pigeon crop-sac and mouse mammary gland bioassays. When compared to the native hormone, monomers of these two partially reduced-carbamidomethylated derivatives also show only modest changes in properties measured by exclusion chromatography, circular dichroism, and immunological cross-reactivities. However, cleavage of cystine-4--11 and cystine-191--199, followed by carbamidomethylation, destroys the biological activity of this derivative in a teleost fish bioassay (Gillichthys urinary bladder). In contrast, reduction of cystine-4--11 actually increased the teleost potency of this derivative compared to the intact hormone. Since teleost prolactin appears to lack a homologue to the cystine-4--11 disulfide bond in the amino-terminal loop of the ovine hormone, selective reduction of this bond in ovine prolactin may produce a derivative whose properties more closely resemble the fish hormone.     

On the optical activity of ionized tyrosyl residues in ovine lutropin.

The effect of alkali on the circular dichroic (CD) spectra of ovine lutropin and its subunits has been studied. Mild alkaline pH induces the appearance of a new optically active band in the 250-nm region of the spectra of lutropin without any detectable alteration in the secondary structure of the protein. This change is reversible and can be correlated with ionization of 2--3 exposed tyrosyl residues in the intact hormone. In a previous report from this laboratory it was concluded that the three exposed tyrosyl residues are located in the alpha subunit, in positions 21, 92 and 93 [Burleigh, B.D., Liu, W.-K, and Ward, D.M. (1976) J. Biol. Chem. 251, 308--315]. Nitration of these residues lowers the pH at which the intensity of the 250-nm band is maximal. The importance of the tyrosyl residues of lutropin alpha (as opposed to those of lutropin beta) is also supported by the similarity of the effect of alkali on the CD spectra of lutropin and lutropin alpha. Further evidence for this involvement was also obtained by a comparison of the alkali-induced changes of refolded lutropin (alpha + beta recombinant) and the product obtained by recombination of des-(92--96)-lutropin alpha (obtained from carboxypeptidase treatment of the alpha-subunit) and lutropin beta. The results indicate that removal of tyrosines alpha 92 and alpha 93 results in a decrease of the intensity of the 235-nm band of ovine lutropin (at pH7.5) as well as that of the 250-nm band observed under alkaline conditions. It is therefore concluded that the 250-nm band observed in alkaline solutions of lutropin arises (at least partially) from the red shift produced in the short-wavelength optically active band of tyrosines alpha 21, alpha 92, and alpha 93 upon ionization.     

Identification of subunit contact sites on the alpha-subunit of lutropin.

Peptides corresponding to the entire sequence of the alpha-subunit of the human glycoprotein hormones were synthesized by using standard solid-phase procedures. Purified peptides were incubated in the presence of alpha- and beta-subunits of bovine lutropin, and subunit recombination was monitored by difference spectroscopy, reverse-phase high-pressure liquid chromatography, and gel filtration chromatography. Although the binding of alpha-peptides to either subunit could not be detected by these techniques, it was possible to demonstrate that some peptides could inhibit the recombination of alpha- and beta-subunits. Specifically, alpha-peptide 33-58 allowed only 0-11% of subunit recombination in 24 h (38-56% after 48 h), while alpha-peptide 51-65 allowed 10-60% of subunits to recombine in 24 h (65-94% in 48 h). Peptides 1-15, 11-27, 22-39, 61-78, and 73-92 of the alpha-subunit could not inhibit subunit recombination at any time or at any concentration tested. The data suggest that at least a portion of the alpha-subunit contact site has been identified, and results are discussed in terms of protein structure assessment tools.