Virus-membrane interactions : spectroscopic studies

In this thesis some new aspects of the infection process of nonenveloped viruses are reported. The interaction of a rod-shaped (TMV) and three spherical (CCMV, BMV, SBMV) plant viruses, of the filamentous bacteriophage M13, and of their coat proteins with membranes have been investigated. A comparison is made between the infection mechanisms of these non-enveloped viruses.1 EFFECT OF PLANT VIRUSES ON MEMBRANESAll plant viruses studied interact with membranes. This is demonstrated by turbidity measurements of small unilamellar vesicles with different surface charges (chapter 2). The interaction is either electrostatic or hydrophobic.Neutral vesicles always interact with viral capsids by indirect hydrophobic interaction. On the other hand, charged vesicles always interact with opposite charges at the capsids by electrostatic interaction. The location of the coat protein after interaction has been determined to test Durham's model for plant virus infection, in which the coat protein becomes an integral membrane protein, similarly as for M13 infection. The results indicate that as a result of the interactions, multilamellar vesicles are formed, containing all the protein in case of electrostatic interaction. The coat protein is associated at the bilayer surface. However, after hydrophobic interaction no protein is found in the multilamellar vesicles. For the latter type of interaction a mechanism is proposed, in which the coat protein behaves as a catalyst, only enhancing the rate of fusion and multilamellar vesicle formation. Since in either case no lipid-protein complex is formed, that is stabilized by direct hydrophobic lipid-protein interactions, Durham's model for plant virus infection is very likely to be incorrect.The recently proposed co-translational disassembly model for plant virus infection, in which the viral particle dissociates during translation of its RNA by the host ribosomes, does not include a specific role for membranes during disassembly. However, before the co-translational disassembly takes place, the particles need to be destabilized. At this moment no specific mechanism for this process has been proposed. Also the fate of the coat protein after particle disassembly remains unspecified in any model, with exception of Durham's model. The observed electrostatic interaction at the bilayer surface (e.g. with the N-terminal arm of the coat proteins, released upon assembly may play a significant role in the infection mechanism.From the experiments described in this thesis no results contradictory to co-translational disassembly have been found. Therefore, to our present opinion, the best model for plant virus infection is the co-translational disassembly model. In this view it is assumed implicitely that the virus particles arrive intact in the cytoplasm, for example by passage through the cell wall and the plasma membrane by local, transient wounding.2 EFFECT OF THE MEMBRANE ON BACTERIOPHAGE M13 COAT PROTEINM13 coat protein has been incorporated as an intrinsic protein in micelles and model membranes (chapter 3-4). The secondary structure of coat protein in SDS micelles is, predominantly α-helix (60%), while in membranes (DMPC/DMPA 80/20 w/w) the structure is entirely β-structure.In micelles at high detergent protein ratio the protein is dimeric with the central core in β-structure. The termini of the coat protein (30 residues or 60%) are in α-helix structure. The dynamics of the micellar system, investigated by time-resolved fluorescence anisotropy measurements, is characterized by rotation of the complex on the nanosecond timescale (10 ns at 20°C) and additional mobility of the Trp-26 sidechain on the subnanosecond timescale (0.5 ns). The complex has a temperature dependent overall rotation, that satisfies the Stokes-Einstein relation for spherical rotation. From this dependence it has been determined that the complex consists of two coat protein molecules and approximately 57 SDS molecules.In membranes, regardless of the lipid to protein ratio, the coat protein Is aggregated. This is concluded from three independent measurements. Fluorescence anisotropy decay measurements indicate that the single tryptophan-26 in the hydrophobic core is highly ordered on the nanosecond timescale in liquid-crystalline bilayers, whereas the surrounding lipids are not. 2H NMR measurements indicate that all the exchangeable sites at the backbone are ordered on the microsecond timescale. Both results are consistent with protein aggregation. Finally, the fraction of motionally restricted lipid, determined from spin label ESR is too low for a monomeric state of the coat protein in the membrane, also in agreement with protein aggregation.The state of the M13 coat protein in model membranes used in the experiments of this thesis is therefore best described as a β- polymeric state. Within the polymer the orientation of the coat protein is unknown. For comparison, invivo , the coat protein in the E . coli cytoplasmic membrane is known to be oriented. Its secondary structure and state of aggregation, however, are up to now unknown.3 EFFECT OF BACTERIOPHAGE M13 COAT PROTEIN ON THE MEMBRANEIn model membranes also the effect of M13 coat protein incorporation on the lipids has been investigated (chapter 4). Upon introduction of coat protein in the membrane as an intrinsic protein a fraction of the lipid molecules becomes motionally restricted.The spin labelled phospholipids show a difference in their selectivity for the coat protein: cardiolipin = phosphatidic acid>>stearic acid phosphatidylserine = phosphatidylglycerol>>phosphatidylcholine phosphatidylethanolamine. The selectivities found are related to the composition of the target E . coli cytoplasmic membrane. Typically, neutral phosphatidylethanolamine accounts for 74% of the lipid in the membrane, constituting the bulk of the lipid, while phosphatidylglycerol is present for 19% and cardiolipin for 3%. The high selectivity of cardiolipin for the coat protein forms direct, biophysical evidence for a previously suggested molecular association of cardiolipin with the coat protein. This was concluded from an increase in the cardiolipin synthesis after infection of E . coli by M13. No increase in phosphatidylglycerol synthesis, the major negatively charged lipid, is observed after infection.Using selectively deuterated palmitic acid as probe lipid, spectral broadening has been observed in presence of M13 coat protein. This result, as well as the ESR results, agrees with a two-site exchange model for the probe lipid between sites in the bulk of the membrane and motionally-restricted sites at the protein. The exchange rate is fast on the nanosecond timescale of the ESR technique, but slow on the microsecond timescale of the 2H NMR technique. The exchange rate of 10 +7Hz, deduced from simulation of the spin label ESR spectra, is in excellent agreement with these upper and lower limits