Crowded conditions such as those in cells can affect proteins’ structure, function, and activity
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'The most dramatic example of shape change in a nonspherical protein that Wittung-Stafshede has studied, in collaboration with Cheung, is the borrelia protein VlsE, which is involved in Lyme disease. Under crowded conditions, this protein changes its shape from elongated to spherical, exposing an antigen in the process (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0803672105).
The sometimes-dramatic changes that proteins undergo in crowded conditions raise the question of just how relevant are all those years of work in dilute solutions.'
and this without even considering interactions between other micro organisms!