Virus maturation is virtually universal and allows the transition from an initially assembled non-infectious and fragile particle to a robust infectious virion. Siphophage HK97 is an accessible model system, to study assembly and maturation, for which intermediates can be isolated and myriad structural, biophysical and biochemical data are available. The capsid protein fold similarity between HK97, all other dsDNA bacteriophages and some animal viruses suggests that they share common maturation principles. The proposed project seeks to unravel the mechano-chemical reorganization program underlying the structural transitions observed during HK97 procapsid maturation using hybrid methods. It relies on a multi-disciplinary approach involving X-ray crystallography, electron microscopy and tomography to obtain multi-scale data ranging from atom to whole-cell. We aim to obtain structures of three different HK97 procapsid maturation intermediates to shed light on mechanisms involved in initial assembly of an energy-loaded particle as well as on forces driving forward maturation. We will also use electron tomography to study in vivo aspects of HK97 assembly and maturation in Escherichia coli infected cells. The outcome will allow apprehending mechanisms, dynamics and energetics of viral maturation as well as provide a basis for the use of HK97 as a delivery system for nano-medical applications.