Abstract
Changes in environmental pH can induce morphological changes in empty proteinaceous shells of bacteriophages in vitro that are very similar to changes occurring in viral capsids in vivo after encapsidation of DNA. These changes in capsid shape and size cannot be explained with a simple elastic model alone. We propose a new theoretical framework that combines the elasticity of thin icosahedral shells with the pH dependence of capsid charge distribution. Minimization of the sum of elastic and electrostatic free energies leads to equilibrium shapes of viral shells that depend on a single elastic parameter and the detailed configuration of the imbedded protein charges. Based on the in vitro shell reconstructions of bacteriophage HK97 we elucidate the details of how the reversible transition between Prohead II and Expansion Intermediate II states of the HK97 procapsid is induced by pH changes, as well as some other features of the bacteriophage maturation.
Highlights
Changes in environmental pH can induce morphological changes in empty proteinaceous shells of bacteriophages in vitro that are very similar to changes occurring in viral capsids in vivo after encapsidation of DNA
It is quite surprising that the same morphological changes that occur in procapsids of bacteriophages during their maturation in vivo can be experimentally reproduced in vitro even in the absence of DNA9, solely by changing the pH of the bathing medium which strongly implicates the role of electrostatic interactions involved[10,11,12]
Oxidation of Prohead II results in the formation of four intermediate states that are not observed in vivo, termed the Expansion Intermediates (EI): EI-I, EI-II, EI-III, and EI-IV13
Summary
Changes in environmental pH can induce morphological changes in empty proteinaceous shells of bacteriophages in vitro that are very similar to changes occurring in viral capsids in vivo after encapsidation of DNA. These changes in capsid shape and size cannot be explained with a simple elastic model alone. In vivo genome packaging into Prohead II results in the formation of the mature Head II, which has an increased average capsid diameter of more than 20% compared to Prohead II This process is in addition accompanied by a thinning of the Head II protein shell and a transformation of its shape from spherical to icosahedral[7,9,15]. The crosslink-defective K169Y mutant virus particles follow a similar maturation pathway, but the absence of cross-links allows the process to be reversible at the very last stage[6,19]
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