Putting a lid on protein folding: structure and function of the co-chaperonin, GroES

The co-chaperonin GroES is an essential partner in protein folding mediated by the chaperonin, GroEL. Two recent crystal structures of GroES provide a structural basis to understand how GroES forms the lid on the folding-active cis ternary complex, and how the GroEL-GroES complex enhances folding.

Characterization of the active intermediate of a GroEL-GroES-mediated protein folding reaction

Recent studies of GroE-mediated protein folding indicate that substrate proteins are productively released from a cis ternary complex in which the nonnative substrate is sequestered within the GroEL channel underneath GroES. Here, we examine whether protein folding can occur in this space. Stopped-flow fluorescence anisotropy of a pyrene-rhodanese-GroEl complex indicates that addition of GroES and ATP (but not ADP) leads to a rapid change in substrate flexibility at GroEL. Strikingly, when GroES release is blocked by the use of either a nonhydrolyzable ATP analog or a single-ring GroEL mutant, substrates complete folding while remaining associated with chaperonin. We conclude that the cis ternary complex, in the presence of ATP, is the active state intermediate in the GroE-mediated folding reaction: folding is initiated in this state and for some substrates may be completed prior to the timed release of GroES triggered by ATP hydrolysis.

A kinetic explanation for the rearrangement pathway of BPTI folding

Bovine pancreatic trypsin inhibitor (BPTI) does not fold by simple sequential formation of its native disulphide bonds. Instead, an initially formed intermediate, termed N', first rearranges to a more stable species in a slow process that requires substantial unfolding. We find that direct oxidation of N' is also inhibited by native structure which slows both the intermolecular step in oxidation--formation of a mixed disulphide bond with the oxidizing agent GSSG--as well as the subsequent intramolecular step. Folding does not occur appreciably by direct oxidation because the high GSSG concentrations required for efficient mixed disulphide formation cause N' to accumulate as a nonproductive, double-mixed disulphide species. The need to unfold previously acquired native structure, observed in the folding of BPTI, may be a common feature of disulphide-linked folding reactions.

Mechanism of GroEL action: productive release of polypeptide from a sequestered position under GroES

The chaperonin GroEL is a large, double-ring structure that, together with ATP and the cochaperonin GroES, assists protein folding in vivo. GroES forms an asymmetric complex with GroEL in which a single GroES ring binds one end of the GroEL cylinder. Cross-linking studies reveal that polypeptide binding occurs exclusively to the GroEL ring not occupied by GroES (trans). During the folding reaction, however, released GroES can rebind to the GroEL ring containing polypeptide (cis). The polypeptide is held tightly in a proteolytically protected environment in cis complexes, in the presence of ADP. Single turnover experiments with ornithine transcarbamylase reveal that polypeptide is productively released from the cis but not the trans complex. These observations suggest a two-step mechanism for GroEL-mediated folding. First, GroES displaces the polypeptide from its initial binding sites, sequestering it in the GroEL central cavity. Second, ATP hydrolysis induces release of GroES and productive release of polypeptide.

Contribution of individual side-chains to the stability of BPTI examined by alanine-scanning mutagenesis

Bovine pancreatic trypsin inhibitor (BPTI) serves as an important model system for the examination of almost all aspects of protein structure. Systematic studies of the effects of mutation on the thermodynamic stability of BPTI, however, have been limited by the extreme stability of the protein. A derivative of BPTI containing only the 5-55 disulfide bond, termed [5-55]Ala, has been shown previously to fold into a structure very similar to that of native BPTI and to be a functional trypsin inhibitor. [5-55]Ala undergoes a reversible thermal unfolding transition with a melting temperature of 39 degrees C, and is therefore well suited for stability studies. Using an alanine-scanning mutagenesis approach, we have examined the contribution to stability of each side-chain in the [5-55]Ala derivative of BPTI. These studies demonstrate the importance of the two hydrophobic cores composed largely of clusters of aromatic residues, as well as the internal hydrogen-bonding network, in stabilizing BPTI. Overall, there is a strong relationship between change in buried surface area and stability for both polar and hydrophobic residues, with proportionality constants of 50 and 20 cal/A2, respectively. None of the alanine substitutions substantially stabilized [5-55]Ala. Nonetheless, approximately 60% (28/46) of the alanine mutants were destabilized by less than 10 degrees C, suggesting that a form of BPTI with up to half of its residues being alanine could fold into a stable structure resembling the native one.

Satisfaction with ambulatory care of persons with AIDS: predictors of patient ratings of quality

OBJECTIVE

To examine the relation of patient characteristics and site of care to the perception of ambulatory care quality by persons with AIDS (PWAs).

DESIGN

Patient surveys and medical record review were used to determine PWAs' perceptions of their ambulatory care, self-perceived health status, primary care relationships, sociodemographic characteristics, and severity of illness.

SETTING

A public-hospital HIV clinic, an academic group practice, and a staff-model health maintenance organization (HMO) that together care for 20% of all Massachusetts PWAs.

PATIENTS

All active patients as of February 12, 1990, and all new AIDS patients at each of the three sites during the subsequent 13 months.

MEASUREMENTS AND MAIN RESULTS

The primary outcome measure was a six-item scale of patient-rated quality of care (PRQC), a newly developed measure that combined patients' ratings of their physician care, nursing care, involvement in medical decisions, and overall quality of care. Multiple logistic regression was carried out with low PRQC (lowest quartile) as the dependent variable, to identify correlates of patient perceptions of poor quality. Patients who had a primary nurse were significantly less likely to have low PRQC scores (OR = 0.50, 95% CI = 0.26 to 0.97). Black patients and patients who used injection drugs were significantly more likely to rate their care in the lowest quartile (OR = 2.22, 95% CI = 1.04 to 4.78; and OR = 2.43, 95% CI = 1.13 to 5.23, respectively), as were those who had lower self-perceived health status, after controlling for confounders; no association was found by site or severity.

CONCLUSIONS

These results show that primary nursing may be an important determinant of how PWAs rate the quality of their ambulatory care. Furthermore, PWAs who are black or who are injection drug users are less satisfied than are others with the quality of their ambulatory AIDS care.

All roads lead to Rome? The multiple pathways of protein folding

Recent studies have found that protein folding reactions often proceed through two or more kinetically distinct pathways. In at least some cases, the observed folding intermediates act as kinetic traps, slowing the rate at which folding is completed. These findings have important implications for understanding how proteins fold in vitro and in vivo.

Correlates of employment after AIDS diagnosis in the Boston Health Study

OBJECTIVES

The purpose of this study was to determine the impact of personal and job characteristics on the time to employment loss after diagnosis of the acquired immunodeficiency syndrome (AIDS) and to examine how job loss affects patients' income.

METHODS

Data were collected from 305 patients with AIDS at three sites in Boston, Mass, between February 1990 and July 1991. Life-table methods were used to estimate the number of months employed after diagnosis. A Cox proportional hazards model was used to estimate the effect of risk factors on the probability of ceasing employment in a month.

RESULTS

Seventy-six percent of respondents were working at the time of diagnosis; 53% still had a job at the time of the baseline interview, which averaged 16 months later, but about one in three was on sick or disability leave. Mental and physical demands of jobs significantly influenced the likelihood of employment loss. The loss of earnings reduced monthly income by 75%.

CONCLUSIONS

Job characteristics affect the likelihood of employment loss, which in turn has a deleterious effect on income. Programs supporting persons with AIDS during the transition out of work or enabling them to modify their job demands may also reduce these problems.

Changes in insurance status and access to care for persons with AIDS in the Boston Health Study

The purpose of this study was to measure unmet needs and changes in insurance status for persons with acquired immunodeficiency syndrome (AIDS). Thirty-six percent of the study's Boston-area respondents (n = 305) had a change in insurance coverage between AIDS diagnosis and interview. Medicaid coverage increased from 14% to 41%. Pneumocystis carinii pneumonia prophylaxis was nearly universal. Only 5% did not receive zidovudine, and intravenous drug users were at higher risk. Approximately 14% to 15% of patients reported problems in obtaining medical and dental services; Blacks, homeless persons, and those who were not high school graduates were at higher risk. Use of selected treatments for which there were clear clinical guidelines was adequate, yet disadvantaged groups were more likely than other persons with AIDS to face obstacles to other services.

GroEL-mediated protein folding proceeds by multiple rounds of binding and release of nonnative forms

The chaperonin GroEL is a ribosome-sized double-ring structure that assists in folding a diverse set of polypeptides. We have examined the fate of a polypeptide during a chaperonin-mediated folding reaction. Strikingly, we find that, upon addition of ATP and the cochaperonin GroES, polypeptide is released rapidly from GroEL in a predominantly nonnative conformation that can be trapped by mutant forms of GroEL that are capable of binding but not releasing substrate. Released polypeptide undergoes kinetic partitioning: a fraction completes folding while the remainder is rebound rapidly by other GroEL molecules. Folding appears to occur in an all-or-none manner, as proteolysis and tryptophan fluorescence indicate that after rebinding, polypeptide has the same structure as in the original complex. These observations suggest that GroEL functions by carrying out multiple rounds of binding aggregation-prone or kinetically trapped intermediates, maintaining them in an unfolded state, and releasing them to attempt to fold in solution.

Physician discretion and racial variation in the use of surgical procedures

BACKGROUND

Racial variation in the use of surgical procedures raises concern about equitable access. The goal of our study was to examine racial differences in utilization across a broad range of procedures in Massachusetts and to assess whether racial variation is related to physician discretion.

METHODS

We obtained fiscal year 1988 hospital discharge data for all Massachusetts residents, identified 10 clinically important surgical procedures, and calculated age- and sex-adjusted rate ratios for white and black patients. Level of discretion was determined by using a modified Delphi technique.

RESULTS

Whites had higher rates for eight procedures (abdominal aortic aneurysm repair, appendectomy, cardiac valve replacement, carotid endarterectomy, cholecystectomy, lumbar disk procedures, open reduction/internal fixation of the femur, and tonsillectomy) and lower rates for two procedures, hysterectomy and prostatectomy. Of the eight procedures for which utilization was higher among whites, four were ranked as moderate- or high-discretion procedures and four were ranked as low-discretion procedures. Hysterectomy, the only procedure for which utilization was substantially higher among blacks (white:black rate ratio < 0.90), was ranked as a high-discretion procedure.

CONCLUSIONS

With the exception of hysterectomy and prostatectomy, procedure rates for whites were greater than those for blacks for a wide range of surgical procedures. Racial variation exists for low-discretion procedures as well as for those associated with moderate and high discretion. Variation among low-discretion procedures that is not explained by medical need suggests the possibility of race-related differences in access to care or in the way patients and physicians make clinical decisions.

Efficient catalysis of disulphide bond rearrangements by protein disulphide isomerase

Protein disulphide isomerase (PDI) is a highly abundant and ubiquitous eukaryotic protein that is essential for viability in yeast. Although PDI is thought to catalyse disulphide bond formation and isomerization during protein biosynthesis, PDI has been found previously to have only moderate effects (approximately 25-fold) on the rate of oxidative folding of proteins in vitro. In addition, PDI has been implicated in several apparently unrelated cellular functions. For example, PDI is the beta-subunit of prolyl 4-hydroxylase and is part of the triglyceride transfer complex. The oxidative folding of bovine pancreatic trypsin inhibitor (BPTI) is slow and inefficient in vitro. Here we report that PDI increases by a factor of 3,000-6,000 the rates of folding of kinetically trapped BPTI folding intermediates, in which native structure impedes disulphide bond formation. By contrast, PDI has only small effects on the rate of disulphide bond formation in intermediates that are oxidized readily in the absence of PDI. These results suggest that an important function of PDI is to catalyse disulphide bond formation and rearrangements within kinetically trapped, structured folding intermediates.

Discussion of preferences for life-sustaining care by persons with AIDS. Predictors of failure in patient-physician communication

OBJECTIVES

To assess the determinants of communication about resuscitation between persons with acquired immunodeficiency syndrome (AIDS) and their physician.

DESIGN AND SETTING

Structured patient interview at a staff-model health maintenance organization (HMO), an internal medicine group practice at a private teaching hospital, and an AIDS clinic at a public hospital.

PATIENTS

289 persons with AIDS.

MAIN RESULTS

Only 38% of patients had discussed their preferences for resuscitation with their physician. Using logistic regression, we found that patients were less likely to have discussed resuscitation with their physician if they were nonwhite (odds ratio [OR], 0.49; 95% confidence interval [CI], 0.24 to 0.99), had never been hospitalized (OR, 0.52; 95% CI, 0.27 to 0.99), or were cared for in the HMO (OR, 0.44 relative to the private teaching hospital; 95% CI, 0.23 to 0.82). Patients were more likely to have discussed their preferences if they were not currently taking zidovudine (OR, 1.76; 95% CI, 1.02 to 3.03) and if they had decided to defer life-sustaining therapy (OR, 2.30; 95% CI, 1.35 to 3.91). Among nonwhites, those with a nonwhite physician were more likely to have discussed resuscitation (OR, 4.38; 95% CI, 1.13 to 16.93). Of patients who had not discussed their preferences for life-sustaining care, 72% wanted to do so. Patient desire for discussion of this issue did not vary by race, severity of illness, hospitalization status, use of zidovudine, or site of care.

CONCLUSIONS

A majority of persons with AIDS in this study had not discussed their preferences for life-sustaining care with their physician, despite the desire to do so. Interventions to improve patient-physician communication about resuscitation for nonwhites and other groups at risk of inadequate discussion might lead to clinical decisions that are more consistent with patient preferences.

The pro region of BPTI facilitates folding

The in vitro folding pathway of bovine pancreatic trypsin inhibitor (BPTI) has been described previously in terms of the disulfide-bonded intermediates that accumulate during folding of the protein. Folding is slow, occurring in hours at pH 7.3, 25 degrees C. In addition, approximately half of the BPTI molecules become trapped as a dead-end, native-like intermediate. In vivo, BPTI is synthesized as a precursor protein that includes a 13 residue amino-terminal pro region. This pro region contains a cysteine residue. We find that, in vitro, both the rate of formation and the yield of properly folded BPTI are increased substantially in a recombinant model of pro-BPTI. The cysteine residue is necessary for this effect. Moreover, a single cysteine residue, tethered to the carboxy-terminal end of BPTI with a flexible linker of repeating Ser-Gly-Gly residues, is sufficient to assist in disulfide formation. Thus, the pro region appears to facilitate folding by providing a tethered, solvent-accessible, intramolecular thiol-disulfide reagent.

Rates of avoidable hospitalization by insurance status in Massachusetts and Maryland

OBJECTIVE

To determine whether uninsured and Medicaid patients have higher rates of avoidable hospitalizations than do insured patients.

DESIGN

We used 1987 computerized hospital discharge data to select a cross-sectional sample of hospitalized patients. Population estimates from the Current Population Survey were used to estimate rates of admission, standardized for age and sex.

SETTING

Nonfederal acute care hospitals in Massachusetts and Maryland.

PATIENTS

All patients under 65 years of age who were uninsured, privately insured, or insured by Medicaid. Hospitalizations for obstetric and psychiatric conditions were excluded.

MAIN OUTCOME MEASURES

Relative risk of admission for 12 avoidable hospital conditions (AHCs) identified by a physician panel.

RESULTS

Uninsured and Medicaid patients were more likely than insured patients to be hospitalized for AHCs. Rates for uninsured patients were significantly greater than for privately insured patients in Massachusetts for 10 of 12 individual AHCs, and in Maryland for five of 12 AHCs. After adjustment for baseline utilization, the results were statistically significant for 10 of 12 AHCs in Massachusetts and seven of 12 AHCs in Maryland. For Medicaid patients, rates were significantly greater than for privately insured patients for all AHCs in each state before adjustment, and for nine of 12 and seven of 12 AHCs in each state, respectively, after adjustment for baseline utilization.

CONCLUSION

Our findings suggest that patients who are uninsured or who have Medicaid coverage have higher rates of hospitalization for conditions that can often be treated out of hospital or avoided altogether. Our approach is potentially useful for routine monitoring of access and quality of care for selected groups of patients.

Kinetic role of nonnative species in the folding of bovine pancreatic trypsin inhibitor

We have shown previously that during the oxidative folding of bovine pancreatic trypsin inhibitor only intermediates with native disulfide bonds are well populated. Nevertheless, these studies also confirmed the earlier conclusion [Creighton, T. E. (1977) J. Mol. Biol. 113, 275-293] that the rate-limiting transition in the kinetically preferred route for folding involves intramolecular disulfide bond rearrangements. Consequently, intermediates with nonnative disulfide bonds must form transiently during folding. Two specific nonnative species, denoted [30-51; 5-14] and [30-51; 5-38], in which numbers indicate residues participating in a disulfide bond, can be detected at low levels in kinetic folding experiments with bovine pancreatic trypsin inhibitor. By working with purified reversibly trapped intermediates, the role of these two nonnative species has been examined directly. These species are found to be in relatively rapid exchange with each other and with an initially formed native two-disulfide intermediate [30-51; 14-38]. Thus, the low abundance of the two nonnative species detected in kinetic folding experiments reflects primarily their low thermodynamic stability as compared to this native intermediate. To a small extent, these nonnative species form the productive native intermediate [30-51; 5-55], which is the immediate precursor to the native protein. However, an equal amount of [5-55; 14-38], a nonproductive dead-end intermediate, is also produced. Thus, the nonnative species detected during the folding of bovine pancreatic trypsin inhibitor are not committed to forming the productive native intermediate, nor do they serve to direct folding specifically toward a productive route.

The emergency department as a pathway to admission for poor and high-cost patients

BACKGROUND

--To determine the importance of the emergency department as the means of access to the hospital for the poor and the fiscal implications of providing these services, we examined the relationship between patients' socioeconomic status and admission via the emergency department. We also determined the association between entering the hospital via the emergency department and hospital resource use.

METHODS

--We conducted a study of 20,089 patients admitted to five Massachusetts hospitals (three community, two tertiary care) during a 6-month period. We determined the proportions of patients within various socioeconomic and disease groupings who entered through the emergency department. We compared length of stay and charges for patients admitted through the emergency department with those for patients admitted through other routes.

RESULTS

--Overall, 51% of patients entered via the emergency department. Elderly patients (age greater than 65 years; odds ratio, 1.87) and patients with lower socioeconomic status as measured by income, occupation, and education (odds ratios, 2.38, 1.47, and 1.69, respectively) were more likely to enter the hospital via the emergency department than other patients. After adjustment for diagnosis related group, severity as measured by DRGSCALE, and socioeconomic status as measured by income, and excluding outliers, patients admitted via the emergency department stayed 27% longer and incurred 13% higher charges than other patients (P less than .001).

CONCLUSIONS

--Our data indicate that patients with lower socioeconomic status are more likely than other patients to use the emergency department as their means of access to the hospital and that patients admitted via the emergency department use far more resources than patients in the same diagnosis related group admitted by other means. Hospitals that make emergency department services more available may be more likely to hospitalize socioeconomically disadvantaged patients and may be at a substantial financial disadvantage under per-case reimbursement systems such as Medicare.

Reexamination of the folding of BPTI: predominance of native intermediates

Bovine pancreatic trypsin inhibitor (BPTI) continues to be the only protein for which a detailed pathway of folding has been described. Previous studies led to the conclusion that nonnative states are well populated in the oxidative folding of BPTI. This conclusion has broadly influenced efforts to understand protein folding. The population of intermediates present during the folding of BPTI has been reexamined by modern separation techniques. It was found that all well-populated folding intermediates contain only native disulfide bonds. These data emphasize the importance of native protein structure for understanding protein folding.

Delayed access to health care: risk factors, reasons, and consequences

OBJECTIVE

To determine characteristics of patients reporting delays in care before hospitalization and the reasons for those delays.

DESIGN

Survey; personal interviews.

SETTING

Five hospitals in Massachusetts.

PATIENTS

Subjects were drawn from a consecutive sample of all adult patients (excluding obstetrics or psychiatry patients) hospitalized during the first 6 months of 1987 as part of a larger study of hospital costs. For the current study, if patients were re-admitted, we included in our analysis only data on the first admission during the study period. We obtained usable survey data from 12,068 of 17,231 eligible patients.

RESULTS

Delays in care were reported by 16% of patients. The odds of reporting delays in care among patients who were black, poor, uninsured, or without a regular physician were 40% to 80% greater than those for other patients (P less than 0.01). Most patients who reported delays thought that their problem was not serious (64%). Cost was an important factor in delaying care for patients in lower socioeconomic positions; the odds of delaying care because of cost for patients who were both poor and uninsured were 12 times greater than the odds for other patients (P less than 0.001). After controlling for diagnosis-related groups (DRGs) and severity, patients who reported delays had 9% longer hospital stays compared with others (P less than 0.001).

CONCLUSIONS

Patients generally thought to be disadvantaged are at especially high risk for delaying care for conditions that eventually lead to hospitalization. Because these delays are associated with longer hospital stays and potentially poorer health outcomes, interventions that reduce delays seem especially important.

The association of payer with utilization of cardiac procedures in Massachusetts

To investigate the importance of the payer in the utilization of in-hospital cardiac procedures, we examined the care of 37,994 patients with Medicaid, private insurance, or no insurance who were admitted to Massachusetts hospitals in 1985 with circulatory disorders or chest pain. Using logistic regression to control for demographic, clinical, and hospital factors, we found that the odds that privately insured patients received angiography were 80% higher than uninsured patients; the odds were 40% higher for bypass grafting and 28% higher for angioplasty. Medicaid patients experienced odds similar to those of uninsured patients for receiving angiography and bypass, but had 48% lower odds of receiving angioplasty. In addition, the odds for Medicaid patients were lower than for privately insured patients for all three cardiac procedures. These findings suggest that insurance status is associated with the utilization of cardiac procedures. Future studies should determine the implications these findings have for appropriateness and outcome and whether interventions might improve care.

Do the poor cost more? A multihospital study of patients' socioeconomic status and use of hospital resources

There is controversy about whether hospitalized poor patients use more resources and whether hospitals that provide care for the poor therefore merit supplementary payment under per-case prospective payment systems. We previously reported that patients of low socioeconomic status with connective-tissue disease had longer hospital stays and higher costs than patients of higher socioeconomic status at a single hospital. To examine the generalizability of this phenomenon, we interviewed 16,908 (83 percent) of 20,278 consecutive adult patients (excluding obstetrical and psychiatric patients) admitted in 1987 to five Massachusetts hospitals, to obtain information on three direct measures of socioeconomic status (income, occupation, and education). We divided each measure into three strata. Thus, there were 15 comparisons--three measures of socioeconomic status applied to each of five hospitals. After excluding outliers and adjusting for diagnosis-related group (DRG), we found that the patients of the lowest socioeconomic status had hospital stays 3 to 30 percent longer than those of patients of higher status, the differences varying with the hospital and the indicator of socioeconomic status (P less than or equal to 0.05 for 11 of the 15 comparisons). Hospital charges were 1 to 18 percent higher for the patients of lowest socioeconomic status than for those of higher status (P less than or equal to 0.05 for 9 of 15 comparisons). When we adjusted for age, severity of illness, and DRG, the patients of lowest socioeconomic status had longer stays than those of higher status in 14 of 15 comparisons (P less than 0.05 for 7 of the 15) and higher charges in 13 of 15 comparisons (P less than 0.05 for 6 of the 15). The differences between patients of high and low status ranged up to 21 percent for length of stay and 13 percent for charges. Our findings suggest that hospitalized patients of lower socioeconomic status have longer stays and probably require more resources. Supplementary payments to hospitals for the treatment of poor patients merit further consideration.