Base pairing at the 5' splice site with U1 small nuclear RNA promotes splicing of the upstream intron but may be dispensable for slicing of the downstream intron

We previously reported that exon skipping in vivo due to point mutations in the 5' splice site (5'ss) signal of an internal mammalian exon can be prevented by coexpression of U1 small nuclear RNAs, termed shift-U1s, with complementarity to sequence upstream or downstream of the mutated site. We now show by S1 nuclease protection experiments that a typical shift-U1 restores splicing of the upstream intron, but not necessarily of the down stream intron. This indicates that the normal 5'ss sequence acts as an enhancer for splicing of the upstream intron, that it owes this activity to base pairing with U1, and that the enhancer activity is reproduced by base pairing of U1 with other sequences in the area. Shift-U1s are dispensable when the 3'ss sequence of the upstream intron is improved, which suggests that base pairing of U1 with sequences at or near the downstream end of the exon normally functions by compensating for a weakness in the upstream 3'ss. Accordingly, U1 appears to be involved in communication across the exon, but our data indicate at the same time that extensive base pairing between U1 and the 5'ss sequence is not necessary for accurate splicing of the downstream intron. These findings are discussed in relation to the coordinate selection exon termini proposed by the exon definition model.

Cloning and characterization of the human homologue of a dystrophin related phosphoprotein found at the Torpedo electric organ post-synaptic membrane

Dystrophin is the protein product which is absent in Duchenne muscular dystrophy (DMD). In mammalian skeletal muscle, dystrophin is found in association with several integral and peripheral membrane proteins, forming a complex known as the dystrophin glycoprotein complex (DGC). In an expressed sequence tag (EST) database search to identify new dystrophin related genes, we isolated EST00891 which showed 57% homology to the cysteine-rich domain of dystrophin and localized to 18q12.1-12.2. This EST is also highly homologous (90%) to the Torpedo californica post-synaptic 87 kDa phosphoprotein. Screening human adult brain and skeletal muscle cDNA libraries with this EST resulted in cloning multiple cDNAs which encode several splice forms all homologous to the C-terminal domain of dystrophin. The largest open reading frame isolated shows 94% homology (86% identity) to the Torpedo 87 kDa protein and 50% homology to the cysteine-rich and carboxy-terminal domains of dystrophin. The other cDNAs isolated encode smaller splice forms of this gene which we have named dystrobrevin. The tissue distribution of dystrobrevin mRNA shows five distinct transcripts which are preferentially expressed between different tissues. In addition, antibodies against either the Torpedo 87 kDa protein or human dystrobrevin demonstrate that at least three of the splice forms are translated as proteins in human brain tissue extracts.

U1 snRNA promotes the selection of nearby 5' splice sites by U6 snRNA in mammalian cells

Communication between exon boundaries is a central feature of the exon definition model of pre-mRNA splice-site selection and an exon-bridging interaction involving U1 small nuclear RNA (snRNA) paired with the 5' splice site (5'ss) has been identified previously. It has become increasingly clear, however, that the 5'ss is not defined relative to the base-pairing interaction with U1, suggesting that a connection in the proposed line of communication between exon boundaries is missing. To explore this issue, we have first sought to characterize the role in mammalian 5'ss selection of a previously suggested base-pairing interaction with U6 snRNA. Using transfection experiments, we show that mutations at positions 5 and 6 of a 5'ss associated with an internal exon can be suppressed by compensatory changes in the first two positions of a conserved hexanucleotide of U6 RNA. The specificity of the effect was established by covariation experiments as well as by experiments with two splice sites arranged in tandem. Suppression of 5'ss mutations by U6 was more efficient when U1 could pair nearby than when pairing was restored further away and individual U1 RNAs stimulated U6-defined proximal sites more efficiently than distal sites. These results are interpreted to suggest that U1 acts to direct 5'ss choice by U6 to matching sequences nearby. Our work supports a central role for base-pairing with U6 snRNA in mammalian 5'ss selection and suggests how the interaction may be established properly despite the limited complementarity involved.

Smokers' knowledge and understanding of advertised tar numbers: health policy implications

OBJECTIVES

This article examines health policy implications of providing smokers with numerical tar yield information in cigarette advertising.

METHODS

Results of a national probability telephone survey regarding smokers' knowledge and understanding of numerical tar yields and deliveries are reported.

RESULTS

Few smokers knew the tar level of their own cigarettes (the exception being smokers of 1- to 5-mg tar cigarettes), and a majority could not correctly judge the relative tar levels of cigarettes. Smokers were unsure whether switching to lower-tar cigarettes would reduce their personal health risks. Many smokers relied on absolute numbers in making trade-offs between number of cigarettes smoked and their tar levels, thus confusion machine-rated tar-yields with actual amounts ingested.

CONCLUSIONS

The wisdom of the present method of providing tar and nicotine numbers in ads and recommendations for modifying the test protocol are now under discussion. This research indicates that these tar numbers and their implications are poorly understood. The paper recommends revisions in tar ratings to make them more useful and a required statement on cigarette packages to more explicitly relate tar levels to major health risks.

Agonist-induced photoincorporation of a p-benzoylphenylalanine derivative of substance P into membrane-spanning region 2 of the Torpedo nicotinic acetylcholine receptor delta subunit

The neuropeptide substance P acts, at micromolar concentrations, as a noncompetitive antagonist of nicotinic acetylcholine receptors (AChRs) of both neuronal and muscle subtypes. The mechanism of this inhibition has been shown to be most consistent with stabilization of a nonconducting desensitized state of the AChR, via binding to a site distinct from both the agonist site and the high affinity noncompetitive antagonist site. We have used a radioiodinated photoreactive analogue of substance P, containing the amino acid p-benzoyl-L-phenylalanine in place of the Phe8 residue of substance P, to identify the sites of interaction of substance P within the Torpedo california AChR. AChR-rich membrane suspensions were photolabeled in the absence or presence of the agonist carbamylcholine and/or nonradioactive substance P, and incorporation into AChR subunits was assessed by autoradiography after sodium dodecyl sulfate-polyacrylamide gel electrophoresis. In the absence of agonist 125I incorporation was detected in each subunit and was insensitive to substance P, whereas in the presence of carbamylcholine there was a 2-fold increase in photoincorporation into the AChR delta subunit that was inhibited by the addition of an excess of substance P. The sites of specific photoincorporation in the delta subunit were initially mapped by use of Staphylococcus aureus V8 protease to a 14-kDa fragment extending from delta Ile-192 to Glu-280. Further fragmentation of this 14-kDa fragment with trypsin and S. aureus V8 protease established that the sites of specific incorporation were restricted to the region delta Ser-253 to Glu-280, which contains the membrane-spanning region 2 that is known to form the lining of the ion channel. These results establish that in the presence of agonist at least a part of the undecapeptide substance P binds within the ion channel in the desensitized state of the AChR, and it is likely that the binding of substance P to this site is responsible for the action of substance P as a noncompetitive AChR antagonist.

Suppression of mammalian 5' splice-site defects by U1 small nuclear RNAs from a distance

One of the earliest events in the process of intron removal from mRNA precursors is the establishment of a base-pairing interaction between U1 small nuclear (sn) RNA and the 5' splice site. Mutations at the 5' splice site that prevent splicing can often be suppressed by coexpression of U1 snRNAs with compensatory changes, but in yeast, accurate splicing is not restored when the universally conserved first intron base is changed. In our mammalian system as well, such a mutation could not be suppressed, but the complementary U1 caused aberrant splicing 12 bases downstream. This result is reminiscent of observations in yeast that aberrant 5' splice sites can be activated by U1 snRNA from a distance. Using a rapid, qualitative protein expression assay, we provide evidence that 5' splice-site mutations can be suppressed in mammalian cells by U1 snRNAs with complementarity to a range of sequences upstream or downstream of the site. Our approach uncouples in vivo the commitment-activation step of mammalian splicing from the process of 5' splice-site definition and as such will facilitate the genetic characterization of both.

Active-site peptides of acetylcholinesterase of Electrophorus electricus: labelling of His-440 by 1-bromo-[2-14C]pinacolone and Ser-200 by tritiated diisopropyl fluorophosphate

To characterize the structure of the active site of acetylcholinesterase (AChE) from the electric organ of E. electricus, we identified sites of incorporation of two active-site affinity labels, [3H]diisopropyl fluorophosphate ([3H]DFP), and 1-bromo-2-[14C]pinacolone ([14C]BrPin). AChE was isolated, purified, inactivated and digested with trypsin, and peptides containing 3H or 14C were purified by reverse-phase HPLC and characterized by N-terminal sequence analysis. [3H]DFP, labelling Ser-200, was found in a single peptide, QVTIFGESAGAASVGMHLLSPDSR, 83% identical with the sequence from Thr-193 to Arg-216 deduced for AChE of T. californica, with Gln, Ala, Leu, and Asp in place of Thr-193, Gly-203, Ile-210 and Gly-214, respectively, and 87% identical with that from bovine and human brain AChEs. Inactivation by [14C]BrPin led to two radioactive peptides. One, ASNLVWPEWMGVIHGYEIEFVFGLPLEK, was 96% identical with that extending from Ala-427 to Lys-454 of T. californica. Release of 14C in cycle 14 established reaction of [14C]BrPin with active-site His-440, protected by 5-trimethylammonio-2-pentanone (TAP). The other peptide, LLXVTENIDDAER, 77% homologous with that of T. californica extending from Leu-531 to Arg-543, had label associated with the third cycle, not protected by TAP, corresponding to Asn-533. The slow inactivation of eel AChE by reaction of [14C]BrPin at His-440 contrasts with that of AChE from T. nobiliana, where it reacts rapidly with a free cysteine, Cys-231, not present in eel AChE. For both AChEs, inactivation by BrPin prevents subsequent reaction with [3H]DFP, and prior inactivation by DFP does not prevent reactions with [14C]BrPin.

HIV-related oral manifestations in two cohorts of women in San Francisco

The goals of this study were to compare the prevalence of oral lesions in women infected with human immunodeficiency virus (HIV) and HIV-negative women, and to determine the association of oral lesions with route of HIV transmission and with level of immunosuppression in infected women. As part of a prospective 4-year study, oral examinations and blood tests were performed, at 6-month intervals, on 176 HIV-infected women and on 117 HIV-negative women at risk for HIV infection. We evaluated participants for the following oral conditions: hairy leukoplakia, candidiasis, ulcers, warts, non-Hodgkin's lymphoma, Kaposi's sarcoma, and parotid enlargement. As previously reported in men, the prevalence of oral lesions was significantly higher among HIV-infected (22%) than HIV-negative women (3%) [odds ratio (OR) = 8.2; 95% confidence interval (CI) 2.8, 23.5], particularly candidiasis (14%) and hairy leukoplakia (10%). Among HIV-infected women with CD4 cell count nadir > or = 200 cells/microliters, the prevalence of hairy leukoplakia was higher among those infected heterosexually than among injection drug users (OR = 5.5; 95% CI: 1.5; 19). The OR for the association between oral lesions and CD4 cell count nadir (< 200 vs. > 500 cells/microliters) was 8.9 (95% CI: 2.6, 30), indicating a strong positive association with level of immunosuppression.

Identifying the lipid-protein interface of the Torpedo nicotinic acetylcholine receptor: secondary structure implications

To identify amino acid residues of the Torpedo nicotinic acetylcholine receptor (AchR) interacting with membrane lipid, we have used the photoactivatable, hydrophobic probe 3-trifluoromethyl-3-(m-[125I]-iodophenyl)diazirine([125I]TID). The pattern of [125I]TID incorporation into the M3 and M4 hydrophobic segments of each subunit was the same both in the presence and absence of the agonist carbamoylcholine and in the presence of an excess of nonradioactive TID, consistent with nonspecific photoincorporation from the lipid-protein interface. [125I]TID reacted with five residues in alpha-M4 [Blanton, M.P., & Cohen, J. B. (1992) Biochemistry 31, 3738-3750] but with only two or three residues in M4 segments of beta-, gamma-, and delta-subunits. In delta-M3, [125I]TID reacted with Met-293, Ser-297, Gly-301, Val-304, and Asn-305 as well as with Ile-288 preceding M3. Residues at corresponding positions were labeled in beta-M3 (Met-285, Ile-289, Phe-293) and in gamma-M3 (Phe-292, Leu-296, Met-299, and Asn-300) as well as gamma-Ile-283. Within alpha-M3, Phe-284 and Ser-287 were labeled. The periodicity of labeled residues provides the first direct evidence that M3 as well as M4 segments of each subunit are organized as transmembrane alpha-helices each with substantial contact with lipid. In addition, in alpha-M1 [125I]TID reacted nonspecifically with Cys-222, Leu-223, Phe-227, and Leu-228, a pattern of incorporation inconsistent with the labeling pattern expected either for a "face" of an alpha-helix or a beta-sheet.

U1 small nuclear RNAs with altered specificity can be stably expressed in mammalian cells and promote permanent changes in pre-mRNA splicing

Pre-mRNA 5' splice site activity depends, at least in part, on base complementarity to U1 small nuclear RNA. In transient coexpression assays, defective 5' splice sites can regain activity in the presence of U1 carrying compensatory changes, but it is unclear whether such mutant U1 RNAs can be permanently expressed in mammalian cells. We have explored this issue to determine whether U1 small nuclear RNAs with altered specificity may be of value to rescue targeted mutant genes or alter pre-mRNA processing profiles. This effort was initiated following our observation that U1 with specificity for a splice site associated with an alternative H-ras exon substantially reduced the synthesis of the potentially oncogenic p21ras protein in transient assays. We describe the development of a mammalian complementation system that selects for removal of a splicing-defective intron placed within a drug resistance gene. Complementation was observed in proportion to the degree of complementarity between transfected mutant U1 genes and different defective splice sites, and all cells selected in this manner were found to express mutant U1 RNA. In addition, these cells showed specific activation of defective splice sites presented by an unlinked reporter gene. We discuss the prospects of this approach to permanently alter the expression of targeted genes in mammalian cells.

The 87K postsynaptic membrane protein from Torpedo is a protein-tyrosine kinase substrate homologous to dystrophin

Postsynaptic peripheral membrane proteins at the neuromuscular junction have been proposed to participate in the immobilization of the nicotinic acetylcholine receptor at the synapse. An 87 kd cytoplasmic peripheral membrane protein has been demonstrated to colocalize with the nicotinic acetylcholine receptor in the Torpedo electric organ and at the mammalian neuromuscular junction. We have cloned the cDNA encoding the 87K protein from Torpedo electric organ, and the predicted protein sequence is homologous to the C-terminal domains of dystrophin, the protein product of the Duchenne muscular dystrophy gene. The 87K protein displays a restricted pattern of expression detected only in electric organ, brain, and skeletal muscle. Analysis of the in vitro and in vivo phosphorylation of the 87K protein indicates that it is multiply phosphorylated on serine, threonine, and tyrosine residues. The 87K protein is in a complex with other proteins associated with the postsynaptic membrane, including dystrophin and a 58 kd protein. These results suggest that the 87K protein is involved in the formation and stability of synapses and is regulated by protein phosphorylation.

Labeling of cysteine 231 in acetylcholinesterase from Torpedo nobiliana by the active-site directed reagent, 1-bromo-2-[14C] pinacolone. Effects of 2,2'-dipyridyl disulfide and other sulfhydryl reagents

Acetylcholinesterase (AcChE, EC 3.1.1.7) was isolated from the electric organ of T. nobiliana and treated with the active-site-directed alkylating agent 1-bromo-2-[14C]pinacolone ([14C]BrPin), or with BrPin, which acts initially as a competitive inhibitor, Ki = 0.18 mM, and then inactivates the enzyme, k2 = 1.8 x 10(-4) s-1. AcChE aliquots were digested with trypsin and fractionated by reversed phase high performance liquid chromatography. Inactivation caused a decrease in one absorption peak and an increase in another, identified as the peptide beginning at Ala-222 and extending to Arg-242. 5-Trimethylammonio-2-pentanone, a competitive inhibitor, isosteric with acetylcholine, retarded the inactivation and decreased the quantity of labeled peptide. On sequencing, the 14C label was found associated with Cys-231. This was confirmed by comparison with synthesized S-pinacolonylcysteine, by study of effects of blocking the sequencing by o-phthalaldehyde, and by inactivation by 2,2'-dipyridyl disulfide (2-PDS), a thiol-specific reagent that acts initially as a competitive inhibitor, Ki = 0.042 mM, and then inactivates the enzyme, k2 = 5.0 x 10(-4) s-1. This is retarded by 5-trimethylammonio-2-pentanone, and prior inactivation by 2-PDS prevents subsequent reaction of [14C]BrPin in the active site. BrPin inactivates AcChEs from Electrophorus electricus and from human erythrocyte, but 2-PDS does not. Neither reagent inactivates butyrylcholinesterases from human and horse serum.

Agonist-induced changes in the structure of the acetylcholine receptor M2 regions revealed by photoincorporation of an uncharged nicotinic noncompetitive antagonist

To characterize structural changes induced in the nicotinic acetylcholine receptor (AChR) by agonists, we have mapped the sites of photoincorporation of the cholinergic noncompetitive antagonist 3-(trifluoromethyl)-3-(m-[125I]iodophenyl)diazirine (]125I]TID) in the presence and absence of 50 microM carbamylcholine. [125I]TID binds to the AChR with similar affinity under both these conditions, but agonist inhibits photoincorporation into all subunits by greater than 75% (White, B. H., Howard, S., Cohen, S. G., and Cohen, J. B. (1991) J. Biol. Chem. 266, 21595-21607). [125I]TID-labeled sites on the beta- and delta-subunits were identified by amino-terminal sequencing of both cyanogen bromide (CNBr) and tryptic fragments purified by Tricine sodium dodecyl sulfate-polyacrylamide gel electrophoresis followed by reversed-phase high-performance liquid chromatography. In the absence of agonist, [125I]TID specifically labels homologous aliphatic residues (beta L-257, delta L-265, beta V-261, and delta V-269) in the M2 region of both subunits. In the presence of agonist, labeling of these residues is reduced approximately 90%, and the distribution of labeled residues is broadened to include a homologous set of serine residues at the amino terminus of M2. In the beta-subunit residues beta S-250, beta S-254, beta L-257, and beta V-261 are all labeled in the presence of carbamylcholine. This pattern of labeling supports an alpha-helical model for M2 with the labeled face forming the ion channel lumen. The observed redistribution of label in the resting and desensitized states provides the first direct evidence for an agonist-dependent rearrangement of the M2 helices. The efficient labeling of the resting state channel in a region capable of structural change also suggests a plausible model for AChR gating in which the aliphatic residues labeled by [125I]TID form a permeability barrier to the passage of ions. We also report increased labeling of the M1 region of the delta-subunit in the presence of agonist.

Survival for women and men with AIDS

To compare trends in the length of survival for women and men after diagnosis of AIDS, data were analyzed for 139 women and 7045 men who were reported with AIDS in San Francisco between July 1981 and 31 December 1990. Patients were followed prospectively through 15 May 1991. The median survival for women (11.1 months) was significantly shorter than that for men (14.6 months). When data were stratified by year of diagnosis, significantly improved survival was observed in recent years for both women and men, although survival for women remained significantly shorter than that for men. Among those who received either zidovudine or 2',3'-dideoxyinosine, survival did not differ by gender. However, among those not receiving therapy, survival was significantly shorter for women. These results suggest that the shorter survival of women may be a result of factors other than gender, possibly including less use of antiretroviral therapy.

Prevalence and significance of acanthosis nigricans in an adult obese population

BACKGROUND AND DESIGN

Acanthosis nigricans develops commonly in obese individuals, yet its prevalence and significance in an unselected adult obese population has not been determined. To address these issues, 34 patients enrolled in the Adult Obesity Clinic at Parkland Memorial Hospital (Dallas, Tex) were chosen at random and examined.

RESULTS

Acanthosis nigricans was observed in fully 74% of patients; its prevalence correlated positively with severity of obesity. Black obese patients demonstrated a greater propensity for manifesting the cutaneous disorder than did white obese individuals. Patients with acanthosis nigricans exhibited fasting plasma insulin levels that were markedly higher than those of nonacanthotic cohorts.

CONCLUSIONS

Obesity is a significant risk factor for the development of acanthosis nigricans. Conversely, acanthosis nigricans is a reliable cutaneous marker of hyperinsulinemia in obese individuals.

Structure of the noncompetitive antagonist-binding site of the Torpedo nicotinic acetylcholine receptor. [3H]meproadifen mustard reacts selectively with alpha-subunit Glu-262

[3H]Meproadifen mustard, an affinity label for the noncompetitive antagonist site of the nicotinic acetylcholine receptor (AChR), specifically alkylates the AChR alpha-subunit when the acetylcholine-binding sites are occupied by agonist (Dreyer, E. B., Hasan, F., Cohen, S. G., and Cohen, J. B. (1986) J. Biol. Chem. 261, 13727-13734). In this report, we identify the site of alkylation within the alpha-subunit as Glu-262. AChR-rich membranes from Torpedo californica electric organ were reacted with [3H]meproadifen mustard in the presence of carbamylcholine and in the absence or presence of nonradioactive meproadifen to define specific alkylation of the noncompetitive antagonist site. Alkylated alpha-subunits were isolated and subjected to chemical or enzymatic cleavage. When digests with CNBr in 70% trifluoroacetic acid or 70% formic acid were fractionated by gel filtration high performance liquid chromatography (HPLC), specifically labeled material was recovered in the void volume fractions. Based upon NH2-terminal sequence analysis, for both digests, the void volume fractions contained a fragment beginning at Gln-208 before the M1 hydrophobic sequence, whereas the sample from the digest in trifluoroacetic acid also contained as a primary sequence a fragment beginning at Thr-244 and extending through the M2 hydrophobic sequence. Sequence analysis revealed no release of 3H for the sample from digestion in formic acid, whereas for the trifluoroacetic acid digest, there was specific release of 3H in cycle 19, which would correspond to Glu-262. This site of alkylation was confirmed by isolation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and reversed-phase HPLC of a specifically labeled fragment from an endoproteinase Lys-C digest of the alkylated alpha-subunit. NH2-terminal amino acid sequencing revealed release of 3H at cycle 20 from a fragment beginning at Met-243 and extending into the M3 hydrophobic sequence. Because [3H]meproadifen mustard contains, as its reactive group, a positively charged quaternary aziridinium ion, Glu-262 of the alpha-subunit is identified as a contributor to the cation-binding domain of the noncompetitive antagonist-binding site and thus of the ion channel.

Mapping the lipid-exposed regions in the Torpedo californica nicotinic acetylcholine receptor

To identify regions of the Torpedo nicotinic acetylcholine receptor (AchR) interacting with membrane lipid, we have used 1-azidopyrene (1-AP) as a fluorescent, photoactivatable hydrophobic probe. For AchR-rich membranes equilibrated with 1-AP, irradiation at 365 nm resulted in covalent incorporation in all four AchR subunits with each of the subunits incorporating approximately equal amounts of label. To identify the regions of the AchR subunits that incorporated 1-AP, subunits were digested with Staphylococcus aureus V8 protease and trypsin, and the resulting fragments were separated by SDS-PAGE followed by reverse-phase high-performance liquid chromatography. N-terminal sequence analysis identified the hydrophobic segments M1, M3, and M4 within each subunit as containing the sites of labeling. The labeling pattern of 1-AP in the alpha-subunit was compared with that of another hydrophobic photoactivatable probe, 3-trifluoromethyl-3-(m-[125I]iodophenyl)diazirine ([125I]TID). The nonspecific component of [125I]TID labeling [White, B., Howard, S., Cohen, S. G., & Cohen, J.B. (1991) J. Biol. Chem. 266, 21595-21607] was restricted to the same regions as those labeled by 1-AP. The [125I]TID residues labeled in the hydrophobic segment M4 were identified as Cys-412, Met-415, Cys-418, Thr-422, and Val-425. The periodicity and distribution of labeled residues establish that the M4 region is alpha-helical in nature and indicate that M4 presents a broad face to membrane lipid.

Hey girlfriend: an evaluation of AIDS prevention among women in the sex industry

Increasingly, acquired immunodeficiency syndrome (AIDS) prevention programs have been developed to reach and influence street-based populations. Standard methods of evaluation do not fit the conditions of such programs. This article describes a process and outcome evaluation of an AIDS prevention program for sex workers in which qualitative and quantitative methods were combined in order to mediate research problems endemic to street-based populations. Methods included epidemiological questionnaires, open-ended interviews with participants, and ethnographic field notes. Process evaluation findings show that field staff who were indigenous to the neighborhood and population readily gained access to the community of sex workers and simultaneously became role models for positive behavior change. Outcome findings show that sex workers do feel at risk for AIDS, but usually from clients rather than from husbands or boyfriends. Accordingly, they use condoms more frequently with clients than with steady partners. Increasing condom use among sex workers with their steady partners remains an important challenge for AIDS prevention. Combining qualitative and quantitative research data provided a more comprehensive assessment of how to reach sex workers with effective AIDS risk reduction messages than either method could have provided alone.

Association of the Mr 58,000 postsynaptic protein of electric tissue with Torpedo dystrophin and the Mr 87,000 postsynaptic protein

Dystrophin was purified by immunoaffinity chromatography from detergent-solubilized Torpedo electric organ postsynaptic membranes using monoclonal antibodies. A major doublet of proteins at Mr 58,000 and minor proteins at Mr 87,000, Mr 45,000, and Mr 30,000 reproducibly copurified with dystrophin. The Mr 58,000 and Mr 87,000 proteins were identical to previously described peripheral membrane proteins (Mr 58,000 protein and 87,000 protein) whose muscle homologs are associated with the sarcolemma (Froehner, S. C., Murnane, A. A., Tobler, M., Peng, H. B., and Sealock, R. (1987) J. Cell Biol. 104, 1633-1646; Carr, C., Fischbach, G. D., and Cohen, J. B. (1989) J. Cell Biol. 109, 1753-1764). The copurification of dystrophin and Mr 58,000 protein was shown to be specific, since dystrophin was also captured with a monoclonal antibody against the Mr 58,000 protein but not by several control antibodies. The Mr 87,000 protein was a major component (along with the Mr 58,000 protein) in material purified on anti-58,000 columns, suggesting that the Mr 58,000 protein forms a distinct complex with the Mr 87,000 protein, as well as with dystrophin. Immunofluorescence staining of skeletal and cardiac muscle from the dystrophin-minus mdx mouse with the anti-58,000 antibody was confined to the sarcolemma as in normal muscle but was much reduced in intensity, even though immunoblotting demonstrated that the contents of Mr 58,000 protein in normal and mdx muscle were comparable. Thus, the Mr 58,000 protein appears to associate inefficiently with the sarcolemmal membrane in the absence of dystrophin. This deficiency may contribute to the membrane abnormalities that lead to muscle necrosis in dystrophic muscle.

Structure of the agonist-binding site of the nicotinic acetylcholine receptor. [3H]acetylcholine mustard identifies residues in the cation-binding subsite

To characterize the structure of the agonist-binding site of the Torpedo nicotinic acetylcholine receptor (AChR), we have used [3H]acetylcholine mustard [( 3H]AChM), a reactive analog of acetylcholine, to identify residues contributing to the cation-binding subsite. Reaction of [3H]AChM, in its aziridinium form, with AChR-rich membrane suspensions, resulted initially in reversible, high affinity binding (K approximately 0.3 microM) followed by slow alkylation of the acetylcholine-binding site. Incorporation of label into AChR alpha-subunit was inhibited by agonists and competitive antagonists, but not by noncompetitive antagonists, and reaction with 3 microM [3H]AChM for 2 h resulted in specific alkylation of 0.6% of alpha-subunits. Within the alpha-subunit, greater than 90% of specific incorporation was contained within an 18-kDa Staphylococcus aureus V8 proteolytic fragment beginning at Val-46 and containing N-linked carbohydrate. To identify sites of specific alkylation, [3H]AChM-labeled alpha-subunit was digested with trypsin, and the digests were fractionated by reverse phase high pressure liquid chromatography. Specifically labeled material was recovered within a single peak containing a peptide extending from Leu-80 to Lys-107. NH2-terminal amino acid sequencing revealed specific release of 3H in cycle 14 corresponding to alpha-subunit Tyr-93. Identification of Tyr-93 as the site of alkylation was confirmed by radiosequence analysis utilizing o-phthalaldehyde to establish that the released 3H originated from a peptide containing prolines at residues 2 and 9. Because [3H]AChM contains as its reactive group a positively charged quaternary aziridinium, alpha-subunit Tyr-93 is identified as contributing to the cation-binding domain of the AChR agonist-binding site. The selective reaction of [3H]AChM with tyrosyl rather than acidic side chains indicates the importance of aromatic interactions for the binding of the quaternary ammonium group, and the lack of reaction with the tyrosyl or acidic side chains within alpha 190-200 emphasizes the selective orientation of acetylcholine within its binding site.

The hydrophobic photoreagent 3-(trifluoromethyl)-3-m-([125I] iodophenyl) diazirine is a novel noncompetitive antagonist of the nicotinic acetylcholine receptor

We have shown previously that the lipophilic photoreagent 3-(trifluoromethyl)3-m-([125I]iodophenyl)-diazirine ([125I]TID) photolabels all four subunits of the Torpedo nicotinic acetylcholine receptor (AChR) and that greater than 70% of this photoincorporation is inhibited by cholinergic agonists and some noncompetitive antagonists, including histrionicotoxin (HTX), but not phencyclidine (PCP; White, B.H., and Cohen, J.B. (1988) Biochemistry 27, 8741-8751). We have now examined the effects of nonradioactive TID on (a) AChR photoincorporation of [125I]TID, (b) AChR-mediated ion transport, and (c) AChR binding of several cholinergic ligands. We find that TID inhibits [125I]TID photoincorporation into the AChR to the same extent as carbamylcholine. The saturable component of [125I]TID photolabeling is half-maximal at 4 microM [125I]TID with 0.5 mol specifically incorporated per mol of AChR after 30 min photolysis with 60 microM [125I]TID. Repeated labeling of membranes at a fixed [125I]TID concentration gave results consistent with a maximal incorporation of one [125I]TID molecule per AChR. Nonradioactive TID also noncompetitively inhibits agonist-stimulated 22Na+ efflux from Torpedo vesicles with an IC50 of 1 microM. Furthermore, TID inhibits allosterically the binding of [3H]HTX, decreasing its affinity for the AChR 5-fold both in the presence and absence of agonist. In contrast, TID has little effect on [3H]PCP binding in the absence of agonist but completely inhibits it in the presence of agonist. TID enhances the cooperativity of [3H]nicotine binding. [125I]TID is thus a photoaffinity label for a novel noncompetitive antagonist binding site on the AChR that is linked allosterically to the binding sites of both agonists and other noncompetitive antagonists. The [125I]TID site is presumably located within the central pore of the AChR.

Why woman partners of drug users will continue to be at high risk for HIV infection

Women infected with Human Immunodeficiency Virus (HIV) via a sexual relationship with an infected drug using partner are the second largest group of women diagnosed with AIDS in this country. Since 1983, they have been the most rapidly growing subgroup of adults with AIDS, and the increase has been even more rapid among black and hispanic women. Because they are a diverse group and are not readily identified, women partners of drug users and their needs have been unknown to or neglected by service providers and programs that could help them avoid becoming infected. More complete understanding of their characteristics and needs can help in the development of sensitive educational, preventative, and therapeutic strategies to help slow the dramatically increasing burden of AIDS-related morbidity and mortality among them and their families.

Mapping of the acetylcholine binding site of the nicotinic acetylcholine receptor: [3H]nicotine as an agonist photoaffinity label

The agonist [3H]nicotine was used as a photoaffinity label for the acetylcholine binding sites on the Torpedo nicotinic acetylcholine receptor (AChR). [3H]nicotine binds at equilibrium with Keq = 0.6 microM to the agonist binding sites. Irradiation with 254-nm light of AChR-rich membranes equilibrated with [3H]nicotine resulted in covalent incorporation into the alpha- and gamma-subunits, which was inhibited by agonists and competitive antagonists but not by noncompetitive antagonists. Inhibition of labeling by d-tubocurarine demonstrated that the alpha-subunit was labeled via both agonist sites but the gamma-subunit was labeled only via the site that binds d-tubocurarine with high affinity. Within the alpha-subunit, 93% of the labeling was contained within a 20-kDa Staphylococcus aureus V8 proteolytic fragment beginning at Ser-173. Sequence analysis of this peptide indicated that approximately 80% of the incorporation was into Tyr-198, approximately 13% was into Cys-192, and approximately 7% was into Tyr-190. Chymotryptic digestion of the alpha-subunit confirmed that Tyr-198 was the principal amino acid labeled by [3H]nicotine. This confirmation required a novel radio-sequencing strategy employing omicron-phthalaldehyde, since the efficiency of photolabeling was low (approximately 1.0%) and the labeled chymotryptic peptide was not isolated in sufficient quantity to be identified by mass. [3H]Nicotine, which is the first photoaffinity agonist used, labels primarily Tyr-198 in contrast to competitive antagonist affinity labels, which label primarily Tyr-190 and Cys-192/Cys-193.