The radiation inactivation method as a tool to study structure-function relationships in proteins

Modulation of PC1/3 activity by self-interaction and substrate binding

Prohormone convertase (PC)1/3 is a eukaryotic serine protease in the subtilase family that participates in the proteolytic maturation of prohormone and neuropeptide precursors such as proinsulin and proopiomelanocortin. Despite the important role of this enzyme in peptide synthesis, how PC1/3 activity is regulated is still poorly understood. Using ion exchange chromatography and two-dimensional gel electrophoresis we found that natural PC1/3 present in AtT-20 cells and bovine chromaffin granules, as well as recombinant PC1/3 secreted from overexpressing Chinese hamster ovary cells, exists as multiple ionic forms. Gel filtration and cross-linking studies revealed that protein oligomerization and aggregation contribute greatly to variability in surface charge. The most acidic forms of PC1/3 contained both inactive aggregates as well as oligomerized 87-kDa PC1/3 that exhibited stable activity which was partially latent and could be revealed by dilution. No such latency was observed for the more basic, 66/74-kDa forms of PC1/3. Fractions containing these species were stabilized by preincubation with micromolar concentrations of either fluorogenic substrate or peptides containing pairs of basic residues. In addition, the most active form of 87-kDa PC1/3, a probable homodimer, was activated by preincubation with these same peptides. Cleavage by PC1/3 is often the initiating step in the biosynthetic pathway for peptide hormones, implying that this is a natural step for regulation. Our data suggest that enzyme oligomerization and peptide stabilization represent important contributing factors for the control of PC1/3 activity within secretory granules.

Effects of radiation on frozen lactate dehydrogenase

Results concerning the influence of 6-MeV electron beam irradiation, of 2.45-GHz, 565-W microwaves, and of the combined electron and microwave irradiation, at -21 degrees C and -196 degrees C, on lactate dehydrogenase activity are presented. The microwave-irradiated samples exhibited a non-linear behaviour (successive activation and inactivation of the enzyme molecules), suggesting the major influence of the non-thermal component of microwave radiation. The combined electron and microwave irradiation led to a decrease of activity similar to the one caused by electron beam irradiation, which seemed to prove that microwave influence was insignificant in the dose, power and time ranges used. The radiation target analysis of the enzymatic decrease due to electron irradiation indicated a very large aggregation of the enzyme molecules. Our data suggest that radiation target analysis is not suitable to measure the molecular mass of lactate dehydrogenase, when frozen enzyme suspensions are irradiated. The D2O-protected enzyme, when exposed to electron irradiation, showed an even larger aggregation according to radiation target analysis, while the microwave irradiation of the protected enzyme led to a similar, though lesser, non-linear behaviour of the frozen enzyme molecules.

Protein gamma-radiolysis in frozen solutions is a macromolecular surface phenomenon: fragmentation of lysozyme, citrate synthase and alpha-lactalbumin in native or denatured states

PURPOSE

To test whether radiolysis-induced fragmentation in frozen aqueous protein solution is dependent on solvent access to the surface of the protein or to the molecular mass of the polypeptide chain.

MATERIALS AND METHODS

60Co gamma-irradiation of three proteins at -78 degrees C: lysozyme, citrate synthase and alpha-lactalbumin in their native state, with or without bound substrate, or denatured (random coil in urea/acid-denatured state).

RESULTS

By SDS-polyacrylamide gel electrophoresis/analysis of the protein-fragmentation process, it was found that for a given protein D37 values (dose to decrease the measured amount of protein, with an unaltered polypeptidic chain, to 37% of the initial amount) varied according to the state of the protein. D37 for denatured proteins was always much smaller than for native states, indicating a greater susceptibility to fragmentation. In urea, contrary to the native state, no well-defined fragments were observed. Radiolysis decay constants (K= 1/D37) increased with solvent-accessible surface area of these proteins estimated from their radii of gyration in the various states. This is shown also in previous data on native or SDS-denatured proteins. Denatured proteins which have a large surface area exposed to the solvent compared with native ones are more fragmented at equal doses.

CONCLUSIONS

It is concluded that D37 is directly related to the exposed surface area and not to the molecular mass of the polypeptide chain.

Molecular mass and volume in radiation target theory

Radiation target analysis is based on the action of ionizing radiation directly on macromolecules. Interactions of this radiation with the molecules leads to considerable structural damage and consequent loss of biological activity. The radiation sensitivity is dependent on the size of the macromolecules. There has been confusion and discrepancy as to whether the molecular mass or the molecular volume was the determinant factor in the sensitivity. Some proteins are known to change their hydrodynamic volume at low pH, and this characteristic can be utilized to compare the radiation sensitivities of these proteins in the two states. The results show that the radiation sensitivity of proteins depends on the mass of the molecule and is independent of the molecular volume/shape.

Radiation inactivation suggests that human multidrug resistance-associated protein 1 occurs as a dimer in the human erythrocyte membrane

Molecular masses of functional units of two components of 2, 4-dinitrophenyl-S-glutathione (DNP-SG) transport across the erythrocyte membrane determined by radiation inactivation were 437 +/- 69 kDa for the high-affinity component and 466 +/- 67 kDa for the low-affinity component. These results confirm that the multidrug resistance-associated protein (MRP) 1 is responsible for the high-affinity DNP-SG transport across the erythrocyte membrane and suggest that MRP1 exists in the membrane as a dimer. The molecular size of the low-affinity transporter is similar if not identical to that of MRP1. Moreover, while the molecular mass of the DNP-SG-ATPase activity of the erythrocyte membrane corresponds also to that of MRP (375 +/- 36 kDa), the molecular mass of the functional unit of dinitrophenol-stimulated ATPase is significantly lower (232 +/- 26 kDa), which suggests that thisactivity is linked to a different protein, perhapsaminophospholipid translocase.

Endogenous basic fibroblast growth factor isoforms involved in different intracellular protein complexes

Four forms of basic fibroblast growth factor (bFGF or FGF-2) result from an alternative initiation of translation involving one AUG (155-amino acid form) and three CUGs (210-, 201- and 196-amino acid forms). These different forms of bFGF show different intracellular biological activities. To identify their intracellular targets, the 210- and 155-amino acid forms of bFGF were independently transfected into CHO cells and their correct subcellular localizations were verified, the 155-amino acid bFGF form being essentially cytoplasmic whereas the 210-amino acid protein was nuclear. The radiation fragmentation method was used to determine the target size of the different bFGF isoforms in the transfected CHO cells and to show that the 210- and 155-amino acids bFGF isoforms were included in protein complexes of 320 and 130 kDa respectively. Similar results were obtained using the SK-Hep1 cell line, which naturally expressed all forms of bFGF. Co-immunoprecipitation assays using different chimaeric bFGF-chloramphenicol acetyltransferase proteins showed that different cellular proteins are associated with different parts of the bFGF molecule. We conclude that bFGF isoforms are involved in different molecular complexes in the cytosol and nucleus, which would reflect different functions for these proteins.

Radiation-inactivation analysis of the oligomeric structure of the renal sodium/D-glucose symporter

The radiation-inactivation size (RIS) of the rat renal brush-border membrane sodium/D-glucose cotransporter was estimated from the loss of transport activity in irradiated membrane vesicles. The RIS depended on the electrochemical conditions present when measuring transport activity. A RIS of 294 +/- 40 kDa was obtained when transport was measured in the presence of a sodium electrochemical gradient. Under sodium equilibrium conditions, the RIS was 84 +/- 25 kDa in the presence of a glucose gradient, and 92 +/- 20 kDa in its absence. In the absence of a sodium gradient, but in the presence of an electrical potential gradient, the RIS increased to 225 +/- 49 kDa. The 294 kDa result supports earlier suggestions that the Na+ gradient-dependent glucose transport activity is mediated by a tetramer. Individual monomers appear, however, to carry out glucose transport under equilibrium exchange conditions or when a glucose gradient serves as the only driving force. The electrical potential gradient-driven glucose transport RIS appears to involve three functional subunits.

Lysozyme fragmentation induced by gamma-radiolysis

Irradiation of lysozyme in frozen states in the absence of oxygen induces specific fragmentation at defined sites along the backbone chain. This paper localizes radio-fragmentation sites by two methods. First, N-terminal sequencing of radiolysis fragments after separation by SDS-polyacrylamide gel electrophoresis and estimation of their molecular masses. Secondly, after purification of radiolysis fragments by reverse phase-HPLC and determination of their molecular mass by electro-spray-ionization mass-spectrometric analysis, combined to N-terminal sequencing and total amino acid analysis. Evidence for the breakage of the peptide bond itself (CO-NH) is given, with radio-fragmentation sites mostly found at the surface of irradiated lysozyme in solvent exposed loops and turns.

Involvement of disulphide bonds in the renal sodium/phosphate co-transporter NaPi-2

The rat renal brush border membrane sodium/phosphate co-transporter NaPi-2 was analysed in Western blots with polyclonal antibodies raised against its N-terminal and C-terminal segments. Under reducing conditions, proteins of 45-49 and 70-90 kDa (p45 and p70) were detected with N-terminal antibodies, and proteins of 40 and 70-90 kDa (p40 and p70) were detected with C-terminal antibodies. p40 and p45 apparently result from a post-translational cleavage of NaPi-2 but remain linked through one or more disulphide bonds. Glycosidase digestion showed that both polypeptides are glycosylated; the cleavage site could thus be located between Asn-298 and Asn-328, which have been shown to constitute the only two N-glycosylated residues in NaPi-2. In the absence of reducing agents, both N-terminal and C-terminal antibodies detected p70 and a protein of 180 kDa (p180), suggesting the presence of p70 dimers. Much higher concentrations of beta-mercaptoethanol were required to produce a given effect in intact membrane vesicles than in solubilized proteins, indicating that the affected disulphide bonds are not exposed at the surface of the co-transporter. Phosphate transport activity decreased with increasing concentrations of reducing agents [beta-mercaptoethanol, dithiothreitol and tris-(2-carboxyethyl)phosphine] and was linearly correlated with the amount of p180 detected. The target sizes estimated from the radiation-induced loss of intensity of p40, p70 and p180 were all approx. 190 kDa, suggesting that NaPi-2 exists as an oligomeric protein in which the subunits are sufficiently close to one another to allow substantial energy transfer between the monomers. When protein samples were pretreated with beta-mercaptoethanol [2.5% and 5% (v/v) to optimize the detection of p40 and p70] before irradiation, target sizes estimated from the radiation-induced loss of intensity of p40 and p70 were 74 and 92 kDa respectively, showing the presence of disulphide bridges in the molecular structure of NaPi-2.

The renal brush border membrane sodium/sulfate cotransporter functions in situ as a homotetramer

The functional molecular size of the renal Na+/SO4(2-) cotransporter was analysed with the radiation inactivation and fragmentation method. Purified brush border membrane vesicles preserved in a cryoprotective medium were exposed to gamma-radiations. Initial rates of SO4(2-) influx into these vesicles were estimated with membranes irradiated with 0, 4 and 8 Mrad. In each case, SO4(2-) uptake by irradiated membranes was significantly reduced but remained linear during the first 5 sec of incubation. To avoid artifacts arising from a decrease in the driving force caused by modifications in membrane permeability, this incubation period was chosen to measure the effect of irradiation on the SO4(2-) transport activity. Increasing irradiation doses resulted in a monoexponential decrease in transport activity allowing the molecular size to be estimated at 238 +/- 6 kDa (SD, n = 3). Recently, a cDNA for the Na+/SO4(2-) cotransporter was cloned and expressed in Xenopus laevis oocytes (Markovich D. et al. (1993) Proc. Natl Acad. Sci. U.S.A. 90, 8073-8077). The deduced amino acid sequence of this cotransporter predicts a molecular weight of 66 kDa. We suggest that the in situ activity of the renal brush border membrane Na+/SO4(2-) cotransporter requires the presence of four intact and identical subunits arranged as a homotetramer.

The mathematics of radiation target analyses

Radiation target theory has been extended to complex biochemical systems. Mathematical analyses are presented for multiple forms of biological active proteins, for the presence of large inhibitors or activators, for compounds which regulate rate or affinity and for multiple-step reactions. Several predictions of these models have been verified experimentally.

Target size analysis of the peptide/H(+)-symporter in kidney brush-border membranes

The apparent functional molecular mass of the kidney peptide/H(+)-symporter was determined by radiation inactivation in brush-border membrane vesicles (BBMV) of rat kidney cortex. Purified BBMV were irradiated at low temperatures with high energy electrons generated by a 10-MeV linear accelerator at doses from 0 to 30 megarads. Uptake studies were performed with [3H]cefadroxil, a beta-lactam antibiotic which serves as a substrate for the kidney peptide/H(+)-symporter. Inhibition of influx of [3H]cefadroxil into BBMV was used to determine the functional molecular mass of the transporter. Additionally, direct photoaffinity labeling of the transport- and/or binding proteins for [3H]cefadroxil in control and irradiated BBMV was performed to determine the molecular mass of the putative transporter by SDS-polyacrylamide gel electrophoresis. Initial rates of pH-gradient dependent uptake of [3H]cefadroxil decreased progressively as a function of radiation dose. The apparent radiation inactivation size (RIS) of the transport function was found to be 414 +/- 16 kDa. Direct photoaffinity labeling yielded labeled membrane proteins with apparent molecular masses of 130 kDa and 105 kDa, respectively. The proteins displayed different labeling characteristics with respect to incubation time, specificity and the response to irradiation. It appears that only a 105 kDa protein is directly involved in transport function since (a) only it showed a specific pH gradient dependent labeling pattern and (b) the covalent incorporation of [3H]cefadroxil into this protein decreased parallel to the loss of transport function in irradiated BBMV. The peptide/H(+)-symporter in kidney brush-border membranes therefore appears to have a monomer mass of 105 kDa and may function in an oligomeric arrangement.

Molecular size of the functional complex and protein subunits of the renal phosphate symporter

The oligomeric structure of the rabbit renal brush-border membrane sodium/phosphate cotransporter was examined with the radiation inactivation and fragmentation technique. The size of its functional complex (its "radiation inactivation size") was estimated from the rate of decay of its sodium-dependent transport activity as a function of the radiation dose. A radiation inactivation size of 223 +/- 42 kDa was obtained. The polypeptide constituting the monomeric unit of the Na1+/Pi symporter was detected by immunoblotting with polyclonal anti-peptide antibodies directed against the 14 amino acid C-terminal portion of the symporter molecule. Its apparent molecular size estimated by comparison with standards following SDS-polyacrylamide gel electrophoresis was 64,000. This value is in good agreement with its known molecular mass of 51,797 Da calculated from the amino acid sequence deducted from the nucleotide sequence of its gene since this protein is probably glycosylated. The loss of labeling intensity of the polypeptide of M(r) = 64,000 was also measured as a function of radiation dose. The molecular size calculated from these data (its "target size") was 165 +/- 20 kDa. The target size estimated for the rat phosphate cotransporter was 184 +/- 46 kDa, and its previously reported radiation inactivation size was 234 +/- 14 kDa. These results strongly suggest that the renal Na1+/Pi cotransporter exists as an oligomeric protein, probably a homotetramer. The fact that the values obtained for the target size are about 3/4 those obtained for the radiation inactivation size of these cotransport proteins indicates that their subunits are closely associated since most of their subunits appear to be fragmented by a single ionizing radiation hit.

Functional size of C-terminal protein carboxyl methyltransferase from kidney basolateral plasma membranes

The functional sizes of the C-terminal isoprenylcysteine protein carboxyl methyltransferase (PCMT) from kidney cortex basolateral plasma membranes and yeast membranes have been estimated by the radiation inactivation and fragmentation method. Attempts to solubilize the methyltransferase with detergents were unsuccessful as they resulted in the irreversible denaturation of its enzymatic activity. The radiation inactivation sizes of the methyltransferases were 98 and 24 kDa for kidney and yeast, respectively. Kinetic experiments showed that irradiation affects the Vmax of the reaction but not the apparent Km for either S-adenosyl-L-methionine and N-acetyl farnesylcysteine. The functional size reported here for the kidney membrane is about 4-times larger than the size predicted for the Saccharomyces cerevisiae C-terminal PCMT deduced from the nucleotide sequence of its gene (28 kDa). These results suggest that mammalian methyltransferase has a functional size different from that of the yeast; tetramerization of monomers is one possible hypothesis for this difference.

Radiation-inactivation analysis of vacuolar H(+)-ATPase and H(+)-pyrophosphatase from Beta vulgaris L. Functional sizes for substrate hydrolysis and for H+ transport

The functional sizes of the vacuolar H(+)-ATPase (V-ATPase; EC 3.6.1.34) and H(+)-pyrophosphatase (PPase; EC 3.6.1.1) from vacuolar membranes of red beet (Beta vulgaris L.) were estimated by radiation inactivation, both for substrate hydrolysis and for H+ transport. For the V-ATPase, the radiation-inactivation size for H+ transport was 446 (403-497) kDa and that for ATP hydrolysis was 394 (359-435) kDa. The low values of both of these estimates suggest that not all subunits which may co-purify with V-ATPases are required for either hydrolysis or transport. For the PPase, the radiation-inactivation size for hydrolysis was 91 (82-103) kDa, suggesting that the minimum functional unit for hydrolysis is the 81 kDa monomer. In contrast to the V-ATPase, the PPase gave a radiation-inactivation size for transport which was 3-4-fold larger than that for hydrolysis (two estimates for transport gave 307 and 350 kDa), indicating that a single catalytic subunit is insufficient for transport activity.

Cytosolic phospholipase A2 from U937 cells: size of the functional enzyme by radiation inactivation

We have studied the cytosolic phospholipase A2 (cPLA2) of human U937 cells by radiation inactivation in order to characterize the functional form of the native enzyme by a method that was independent of the discrepancies observed by SDS-PAGE and cDNA cloning. The Radiation Inactivation Size of cPLA2 was reproducible and gave a value of 76,800-80,100 daltons. We eluted the active enzyme from polyacrylamide-gradient gel electrophoresis at a molecular weight of 77,000, confirming the irradiation result. We conclude that cPLA2 is active as the monomeric enzyme and is composed of a single major functional domain that is sensitive to irradiation.

Molecular size of the renal sodium/phosphate symporter in native and reconstituted systems

The size of the renal sodium/phosphate symporter was estimated with the radiation inactivation technique in isolated bovine brush border membrane vesicles and after reconstitution in proteoliposomes. The functional unit of the native phosphate carrier had a radiation inactivation size of 172 +/- 17 kDa. Identical values were obtained for the reconstituted carrier whether it was irradiated before or after the formation of the proteoliposomes (161 +/- 9 and 159 +/- 11 kDa, respectively). The sodium-independent uptake of phosphate was not affected significantly by radiation doses up to 10 Mrad. This activity is therefore not due to the reconstitution of a large phosphate-binding protein such as alkaline phosphatase. Furthermore, bromotetramisole, a specific inhibitor of phosphate binding to this enzyme, had no significant effect on the uptake of phosphate by the proteoliposomes.

Inactivation mechanism of tetrameric beta-galactosidase by gamma-rays involves both fragmentation and temperature-dependent denaturation of protomers

The radiation inactivation method is widely used to estimate the molecular size of membrane-bound enzymes, receptors, and transport systems in situ. The method is based on the principle that exposure of frozen solutions or lyophilized protein preparations to increasing doses of ionizing radiations results in a first-order decay of biological activity proportional to radiation inactivation size of the protein. This parameter is believed to reflect the "functional unit" of the protein defined as the minimal assembly of structure (protomers) required for expression of a given biological activity. We tested the functional unit as a concept to interpret radiation inactivation data of proteins with Escherichia coli beta-galactosidase, where the protomers are active only when associated in a tetramer. Gamma-Irradiation of beta-galactosidase at both -78 and 38 degrees C followed by quantitation of the residual unfragmented promoter band by SDS-polyacrylamide gel electrophoresis yielded the protomer size, indicating that only one protomer is fragmented by each radiation hit. By following the enzyme activity as a function of dose it was found that only the protomer that has been directly hit and fragmented at -78 degrees C was effectively inactivated. In contrast, at 38 degrees C, it was the whole tetramer that was inactivated. beta-Galactosidase cannot have two different functional units depending on temperature. The inactivation of the whole beta-galactosidase tetramer at 38 degrees C is in fact related to protomer fragmentation but also to the production of stable denatured protomers (detected by gel-filtration HPLC and differential UV spectroscopy) due to energy transfer from fragmented protomers toward unhit protomers.(ABSTRACT TRUNCATED AT 250 WORDS)

Crosslinking and radiation inactivation analysis of the subunit structure of the pyridine nucleotide transhydrogenase of Escherichia coli

The pyridine nucleotide transhydrogenase of Escherichia coli consists of two types of subunit (alpha: Mr 53,906; beta: Mr 48,667). The purified and membrane-bound enzymes were crosslinked with a series of bifunctional crosslinking agents and by catalyzing the formation of inter-chain disulfides in the presence of cupric 1,10-phenanthrolinate. Crosslinked dimers alpha 2, alpha beta and beta 2, and the trimer alpha 2 beta were obtained. A small amount of tetramer, probably alpha 2 beta 2, was also formed. Radiation inactivation was used to determine the molecular size of the transhydrogenase. The radiation inactivation size (217,000) and chemical crosslinking are consistent with the structure (Mr 205,146) being the oligomer that is responsible for biological activity.

Effects of ionizing radiations on proteins. Evidence of non-random fragmentations and a caution in the use of the method for determination of molecular mass

We have reinvestigated the use of ionizing radiations to measure the molecular mass of water-soluble or membrane proteins. The test was performed by using the most straightforward aspect of the technique, which consists of SDS/PAGE analysis of the protein-fragmentation process. We found that exposure of purified standard proteins to increasing doses of ionizing radiation causes progressive fragmentation of the native protein into defined peptide patterns. The coloured band corresponding to the intact protein was measured on the SDS gel as a function of dose to determine the dose (D37.t) corresponding to 37% of the initial amount of unfragmented protein deposited on the gel. This led to a calibration curve between 1/D37.t and the known molecular mass of the standard proteins whose best fit gave Mr = 1.77 x 10(6)/D37.t at -78 degrees C, i.e. 35% higher than the generally accepted value at that temperature obtained from inactivation studies. However, we have to conclude that this method is useless to determine the state of aggregation of a protein, since, for all the oligomers tested, the best fit was obtained by using the protomeric molecular mass, suggesting that there is no energy transfer between promoters. Furthermore, SDS greatly increases the fragmentation rate of proteins, which suggests additional calibration problems for membrane proteins in detergent or in the lipid bilayer. But the main drawback of the technique arises from our observation that some proteins behaved anomalously, leading to very large errors in the apparent target size as compared with true molecular mass (up to 100%). It is thus unreliable to apply the radiation method for absolute molecular-mass determination. We then focused on the novel finding that discrete fragmentation of proteins occurs at preferential sites, and this was studied in more detail with aspartate transcarbamylase. N-Terminal sequencing of several radiolysis fragments of the catalytic chain of the enzyme revealed that breaks along the polypeptide chains are localized close to the C-terminal end. Examination of the three-dimensional structure of aspartate transcarbamylase suggests that radiolysis sites (fragile bonds) might be localized in connecting loops.

Radiation-inactivation studies on brush-border-membrane vesicles. General considerations, and application to the glucose and phosphate carriers

Radiation-inactivation studies were performed on brush-border-membrane vesicles purified from rat kidney cortex. No alteration of the structural integrity of the vesicles was apparent in electron micrographs of irradiated and unirradiated vesicles. The size distributions of the vesicles were also similar for both populations. The molecular sizes of two-brush-border-membrane enzymes, alkaline phosphatase and 5'-nucleotidase, estimated by the radiation-inactivation technique, were 104800 +/- 3500 and 89,400 +/- 1800 Da respectively. Polyacrylamide-gel-electrophoresis patterns of membrane proteins remained unaltered by the radiation treatment, except in the region of higher-molecular-mass proteins, where destruction of the proteins was visible. The molecular size of two of these proteins was estimated from their mobilities in polyacrylamide gels and was similar to the target size, estimated from densitometric scanning of the gel. Intravesicular volume, estimated by the uptake of D-glucose at equilibrium, was unaffected by irradiation. Uptake of Na+, D-glucose and phosphate were measured in initial-rate conditions to avoid artifacts arising from a decrease in the driving force caused by a modification of membrane permeability. Na+-independent D-glucose and phosphate uptakes were totally unaffected in the dose range used (0-9 Mrad). The Na+-dependent uptake of D-glucose was studied in irradiated vesicles, and the molecular size of the transporter was found to be 288,000 Da. The size of the Na+-dependent phosphate carrier was also estimated, and a value of 234,000 Da was obtained.

Scrapie prion liposomes and rods exhibit target sizes of 55,000 Da

Scrapie is a degenerative neurologic disease in sheep and goats which can be experimentally transmitted to laboratory rodents. Considerable evidence suggests that the scrapie agent is composed largely, if not entirely, of an abnormal isoform of the prion protein (PrPSc). Inactivation of scrapie prions by ionizing radiation exhibited single-hit kinetics and gave a target size of 55,000 +/- 9000 mol wt. The inactivation profile was independent of the form of the prion. Scrapie agent infectivity in brain homogenates, microsomal fractions, detergent-extracted microsomes, purified amyloid rods, and liposomes exhibited the same inactivation profile. Our data are consistent with the hypothesis that the infectious particle causing scrapie contains approximately 2 PrPSc molecules.