Using surface-bound rubidium ions for protein phasing

Rubidium is a monovalent metal that can be used as a counterion in protein solutions. X-ray anomalous scattering from rubidium ions bound to the prote in surface was used for phasing of the crystal structure of the hsp60 apica l domain from Thermus thermophilus. Multiple-wavelength anomalous dispersio n (MAD) data were collected from a crystal obtained from a solution contain ing 0.2 M rubidium salt. One molecule of protein (147 amino acids) binds on e well ordered and one poorly ordered Rb atom. Phases calculated with the p rogram SHARP were sufficient for automatic tracing and side-chain assignmen t using the program ARP/wARP. The data show that bound rubidium ions can be used to determine protein structures and to study the interaction of monov alent metal ions with proteins and other macromolecules.