A mutation in the second largest subunit of TFIIIC increases a rate-limiting step in transcription by RNA polymerase III.

In previous studies, we have shown that the PCF1-1 mutation of Saccharomyces cerevisiae suppresses the negative effect of a tRNA gene A block promoter mutation in vivo and increases the transcription of a variety of RNA polymerase III genes in vitro. Here, we report that PCF1 encodes the second largest subunit of transcription factor IIIC (TFIIIC) and that the PCF1-1 mutation causes an amino acid substitution in a novel protein structural motif, a tetratricopeptide repeat, in this subunit. In agreement with the nature of the mutation, in vitro transcription studies with crude extracts indicate that PCF1-1 facilitates the rate-limiting step in transcription, namely, the recruitment of TFIIIB to the template. Additionally, biochemical fractionation of wild-type and mutant cell extracts shows that PCF1-1 increases the amount of the 70-kDa TFIIIB subunit detectable by Western (immunoblot) analysis in purified TFIIIB fractions and the transcription activity of a TFIIIB" fraction containing the 90-kDa subunit of this factor. We suggest that the effect of PCF1-1 on TFIIIB activity in vitro is a consequence of its increased rate of recruitment in vivo.

A tetratricopeptide repeat mutation in yeast transcription factor IIIC131 (TFIIIC131) facilitates recruitment of TFIIB-related factor TFIIIB70.

Transcription factor IIIC (TFIIIC) plays an important role in assembling the initiation factor TFIIIB on genes transcribed by RNA polymerase III (Pol III). In Saccharomyces cerevisiae, assembly of the TFIIIB complex by promoter-bound TFIIIC is thought to be initiated by its tetratricopeptide repeat (TPR)-containing subunit, TFIIIC131, which interacts directly with the TFIIB-related factor, TFIIIB70/Brf1. In this work, we have identified 10 dominant mutations in TFIIIC131 that increase Pol III gene transcription. All of these mutations are found within a discrete 53-amino-acid region of the protein encompassing TPR2. Biochemical studies of one of the mutations (PCF1-2) show that the increase in transcription is due to an increase in the recruitment of TFIIIB70 to TFIIC-DNA. The PCF1-2 mutation does not affect the affinity of TFIIIC for DNA, and the differential in both transcription and TFIIIB complex assembly is observed at saturating levels of TFIIIB70. This indicates that mutant and wild-type TFIIIC-DNA complexes have the same affinity for TFIIIB70 and suggests that the increased recruitment of this factor is achieved by a nonequilibrium binding mechanism. A novel mechanism of activation in which the TPR mutations facilitate a conformational change in TFIIIC that is required for TFIIIB70 binding is proposed. The implications of this model for the regulation of processes involving TPR proteins are discussed.

Preparation of cyanogen bromide fragments of MM, NN, and MN glycoproteins (glycophorins) from human erythrocyte membranes of single donors.

A procedure is described for the preparation of three cyanogen bromide fragments of the MM, NN, or MN glycoprotein (glycophorin) of the human erythrocyte membranes, from erythrocytes of single donors. The fragments are obtained in pure form and excellent yields by employing procedures which include proteolytic inhibitors during membrane processing, thorough delipidation of the glycoprotein, and CNBr cleavage conditions which lead to quantitative fragmentation without loss of carbohydrates. A phenol-urea extraction resolves the two glycopeptide fragments from the carbohydrate-free fragment. The two glycopeptides are further purified by Bio-Gel P-6 and P-100 chromatography. The three fragments include the amino terminal 8 residue glycopeptide, a large glycopeptide form the middle of the molecule which bears the Asn-linked oligosaccharide and 8--9 O-glycosidically linked units, and a carboxyl terminal, carbohydrate-free, approx. 50 residue fragment. Their amino acid and carbohydrate composition, and size, are in close agreement with the sequence data of Tomita, M., Furthmayr, H. and Marchesi, V.T. (Biochemistry (1978), 17, 4756--4770). The fragments represent three well delineated portions of the glycoprotein molecule.

Amino acid and carbohydrate structural variants of glycoprotein products (M-N glycoproteins) of the M-N allelic locus.

Major glycoprotein of MgM, MM Miltenberger III (MiIII), and M-N erythrocyte membranes from individual donors were cleaved with CNBr and their amino-terminal octapeptides were examined with respect to amino acid and carbohydrate composition. The amino-terminal octapeptides from the heterozygous MgM donor were resolved into two types, A and A'. MgM A was identical to octapeptide A from MM glycoproteins in carbohydrate and amino acid compositions. MgM A' exhibited amino acid composition similar to NN peptide A except for a single substitution of an Asx for a Thr and, as a result, was not glycosylated. MM(MiIII) octapeptide A was identical to M peptide A in amino acid composition, but differed in carbohydrate content. This glycopeptide contained three O-glycosidically linked carbohydrate units, one of which contained GlcNAc bound to a core of NeuAc, Gal, and GalNAc. About two such units were also present in the CNBr glycopeptide B of the glycoprotein, and on the basis of studies with alkaline borohydride and alkaline sulfite degradations, these units are believed to have the following structure: (formula see text) The Mg is an allelomorph of the M-N locus, likely evolved from a single base substitution in the N gene. The resulting single amino acid substitution effects the posttranslational carbohydration of neighboring Ser and Thr residues. The MM(MiIII) appears to be a product of the M gene that undergoes sequences of posttranslational glycosylations different from those of the M-N glycoproteins.

Interactions between the tetratricopeptide repeat-containing transcription factor TFIIIC131 and its ligand, TFIIIB70. Evidence for a conformational change in the complex.

In the transcription of tRNA and 5 S genes by RNA polymerase III, recruitment of the transcription factor (TF)IIIB is mediated by the promoter-bound assembly factor TFIIIC. A critical limiting step in this process is the interaction between the tetratricopeptide repeat (TPR)-containing subunit of TFIIIC (TFIIIC131) and the TFIIB-related factor Brf1p/TFIIIB70. To facilitate biochemical studies of this interaction, we expressed a fragment of TFIIIC131, TFIIIC131-(1-580), that includes the minimal TFIIIB70 interaction domain defined by two-hybrid studies together with adjacent sequences, up to the end of TPR9, implicated in the assembly reaction. TFIIIC131-(1-580) interacts with TFIIIB70 in solution and inhibits the formation of TFIIIB70.TFIIIC.DNA complexes. In a coupled equilibrium binding assay, the formation of TFIIIC131-(1-580).TFIIIB70 complexes was adequately described by a single-site binding model and yielded an apparent equilibrium dissociation constant of 334 +/- 23 nm. CD spectroscopy and limited proteolysis experiments defined a well structured and largely protease-resistant core in TFIIIC131-(1-580) comprising part of the hydrophilic amino terminus, TPR1-5, the intervening non-TPR region, and TPR6-8. CD spectra showed that trifluoroethanol induced significant alpha-helical structure in TFIIIC131-(1-580). A more modest monovalent ion-dependent CD difference was observed in mixtures of TFIIIC131-(1-580) and TFIIIB70, suggesting that formation of the binary complex may proceed with the acquisition of alpha-helicity.

PCF4 encodes an RNA polymerase III transcription factor with homology to TFIIB.

A dominant mutation in the PCF4 gene of S. cerevisiae was isolated as a suppressor of a tRNA gene A block promoter mutation. In vitro studies indicate that PCF4 is a stoichiometrically-required RNA polymerase III (pol III) transcription initiation factor. We show that the PCF4-1 mutation increases the number of transcriptionally competent preinitiation complexes by affecting a limiting activity in yeast cell extracts that is squelched by excess TFIIIC. The PCF4 gene encodes a TFIIB homolog whose size, biochemical, and genetic properties are consistent with those of the 70 kd subunit of TFIIIB. The TFIIB homology of PCF4 suggests a means for determining the polymerase specificity of a gene.

O-linked oligosaccharides of glycophorins A and B in erythrocytes of two individuals with the Tn polyagglutinability syndrome.

Tn polyagglutinability syndrome is an acquired condition where erythrocytes express Tn neo-antigen and become susceptible to hemagglutination by the naturally occurring anti-Tn present in normal sera. Early studies had indicated that O-linked N-acetyl galactosamine was the sole serologic Tn determinant, but more recently O-linked NeuNAc alpha 2, 6GalNAc also has been implicated as a Tn antigen (sialosyl-Tn). However, none of these studies were performed on purified glycoproteins. In this report we examine oligosaccharides of glycophorins A and B purified from Tn erythrocytes of two affected individuals to establish how N- and O-linked saccharides differ from normal. Analysis of carbohydrate composition and treatment with N-glycanase showed that the Asn-linked unit of glycophorin A was not affected. O-linked oligosaccharides were obtained by beta-elimination in the presence of tritiated sodium borohydride. The reduced radiolabeled products were fractionated by Bio-Gel P-2 chromatography, and their structures were investigated by comparison with standards, by monosaccharide quantification, and by neuraminidases of known specificities. The results show that Tn glycophorins from both donors contain intact and truncated forms of trisaccharide and tetrasaccharide NeuNAc alpha 2,3Gal beta 1,3GalNAc and NeuNAc alpha 2,3Gal beta 1,3- (NeuNAc alpha 2,6)GalNAc usually present in glycophorins A and B. The truncated forms include the protein O-linked monosaccharide, GalNAc and disaccharide, NeuNAc alpha 2,6GalNAc (major isomer). The presence of intact glycans in the total population of Tn erythrocytes was confirmed by their susceptibility to T activation after treatment with neuraminidase. The proportion of the four species was not identical in glycophorins of these two donors but, in both, the truncated units predominated and the amount of the disaccharide was approximately one half of that of the monosaccharide. The data are consistent with alterations in UDPGal:GalNAc beta 1,3galactosyl transferase that may have multiple molecular origins and with induction of a specific GalNAc protein alpha 2,6 sialosyl transferase in Tn hematopoietic precursor cells. The molecular basis for these alterations awaits further study.

Collagen alterations in the corpus cavernosum of men with sexual dysfunction.

Previous studies have noted the abundance of collagen in human erectile tissues and the association of altered collagen content with erectile dysfunction. We investigated these notions by studying the collagen characteristics of biopsies from the corpus cavernosum of men who required surgical correction of their sexual dysfunction. Histologic analysis revealed abundant collagen within the erectile tissues. With the exception of patients with Peyronie's disease and priapism, only mild alterations in collagen architecture were noted in the remainder of the patients. Biochemical quantitation confirmed the histologic study. The mean collagen content represented 47% of total protein in most patients. The proportion rose to 68% and 73% in the patients with Peyronie's disease and priapism, respectively. No statistical difference in collagen content was noted in all the patients studied. Immunohistochemistry revealed collagen types I and IV to predominate in the corpus cavernosum, with type III making up the minority. There were no qualitative changes in collagen ratios with age and disease. We conclude that though collagen is a major component of the penis, there are no changes in its histologic characteristics that can be correlated to senescence or to the etiology of erectile dysfunction.

Properties of heart fibroblasts of adult rats in culture.

In the heart of the adult rat, fibroblasts are mainly responsible for the synthesis and deposition of the collagenous matrix. Because these cells in vitro may serve as an important model system for studies of collagen metabolism in heart tissue, we have cultured and characterized rat-heart fibroblasts from young adult and old animals. Conditions included use of media of different compositions with and without addition of ascorbate. Cells used were either cultured directly from fresh tissues or thawed previously frozen cells. Cultured cells were studied with respect to growth properties, morphology and ultrastructure and patterns of collagen. Heart fibroblasts generally resembled fibroblasts cultured from other tissues, but were more like skeletal muscle fibroblasts in that they deposited, in addition to type I collagen, type IV collagen and laminin. The fibroblasts showed a typical appearance in phase-contrast microscopy and electron microscopy. In the case of cells grown with added ascorbate, aligned collagen fibrils in the extracellular matrix showed a periodicity typical of type I collagen. The deposition of type I collagen occurred only in medium supplemented with ascorbate, and in that circumstance increased as a function of time past confluence; this was independent of the age of the animal from which the cells were obtained or of other changes of medium composition studied. Immunofluorescence studies with specific antibodies revealed that the cells deposited types I and IV collagens, laminin and fibronectin. In contrast to the case of type I collagen, the deposition of type IV collagen occurred in cells grown either with or without ascorbate.(ABSTRACT TRUNCATED AT 250 WORDS)

The chimpanzee M blood-group antigen is a variant of the human M-N glycoproteins.

Chimpanzee erythrocytes express strong M but weak, occasional N blood-group activity, as detected by anti-M and anti-N reagents. We have found that the M activity is carried by a major membrane glycoprotein that is similar but not identical to the human MM glycoprotein (glycophorin A). We have isolated and characterized this glycoprotein from erythrocyte membranes of four individual chimpanzees. The purified glycoproteins strongly inhibited agglutination of M cells by rabbit anti-human M sera and only weakly inhibited the agglutination of N cells by rabbit anti-human N sera. They also displayed medium-to-strong inhibitory activity against chimpanzee iso- and crossimmune antisera tested with chimpanzee erythrocytes of various V-A-B-D and Wc specificities, which are known as chimpanzee extensions of the human type M-N system and the Miltenberger counterpart, respectively. Each glycoprotein was cleaved with CNBr into three fragments, whose size, solubility, and composition were analogous to those obtained by similar treatment of the human M-N antigens. The amino-terminal fragment was found to be a glycooctapeptide whose amino acid composition and partial sequence indicated that it is an intermediate form of the human M and N glycooctapeptides. Its carbohydrate content comprised two threonine-linked saccharide units that, although similar in composition to the human threonine-linked units, were fewer in number than the three units found in the corresponding human glycooctapeptides. Structural similarities to the human antigens strongly suggest that the amino terminus bears the major antigenic determinants of the molecule, and the occurrence in this region of numerous, albeit rare, variants among humans and in chimpanzees indicates that the corresponding coding sequence of the structural gene is particularly susceptible to mutational events. We conclude that the chimpanzee M gene product is a variant of the human type and that the chimpanzee gene is an allele of the human polymorphic M-N locus.

A family study of multiple mutations of alpha and delta glycophorins (glycophorins A and B).

Glycophorins alpha and delta are the carriers of the antigens of the MNSs blood system; this report documents the presence of three glycophorin mutations in two individuals of a 16 member family. Erythrocytes were examined by serology, sodium dodecyl sulfate electrophoresis, and immunoblotting. The inheritance pattern and immunoblot profile revealed: (1) A variant Dantu glycophorin showed properties consistent with a delta-alpha glycophorin hybrid structure, previously noted in other individuals. The gene responsible for the Dantu glycophorin in this family is linked to a gene coding for an M-specific alpha glycophorin. (2) Another variant glycophorin, Mi-III glycophorin, was first revealed by immunoblotting and subsequently confirmed by erythrocyte antigen typing. This autosomal dominant trait is associated with N blood group activity and the inheritance pattern indicates that it could be a variant of delta glycophorin. (3) In the individuals with both Dantu and Mi-III glycophorins a delta glycophorin deficiency was observed suggesting that a deletion or alteration of delta gene may exist cis to the Dantu gene. Our findings that document clustering of multiple mutations in MNSs gene loci in the propositus family are very unusual as such variants are relatively rare.