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To find the answer, the researchers used biophysical imaging techniques that produce molecular pictures of the virulence molecules: the Rockefeller scientists used X-ray crystallography to get an atomic resolution structure of SipA, and the Virginia researchers developed an electron microscopy image of SipA-actin filaments - an arrangement too large and diverse to crystallize. The two images were then combined in such a way that the scientists could superimpose the SipA structure upon the larger and less detailed electron microscopy representation, and through extensive image processing, tease apart the role that SipA plays in forming filaments.

They found that SipA is shaped like a molecular staple, with a rounded middle and two extending arms that project from either side of the molecule. The middle core of the SipA molecule binds to actin and positions the long arms so that they reach about and tether together more distant actin molecules, holding the filament together. Removing the arms produced SipA proteins that lost this ability.

"Actin is dynamic, and can be floating as free molecules in the cell or in small chains. SipA drives all the actin to link together and form a lot of scaffolding underneath the attached bacterium," Stebbins says.

By building the actin girders, SipA forces the host cell to expand into a shape that resembles curtain ruffles, which then surrounds the bacterium. Through a process called macropinocytosis, Salmonella is taken inside the host cell and replicates itself to spread the infection.

"Salmonella is very well adapted, and quite sophisticated," Stebbins says. "It induces its own uptake into a host cell, but once inside, it sends out proteins that turn off the effects induced by SipA and other virulence factors."

Findings from the study will allow researchers to probe how other bacteria use virulence factors similar to SipA to change the shape of cells they infect.

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Contact: Joseph Bonner
bonnerj@rockefeller.edu
212-327-8998
Rockefeller University
25-Sep-2003