Are there any examples of proteins with no or minimal sequence identity, but highly similar structure?

An example possibly worth taking a look at is the variable surface glycoprotein of the bloodstream form of Trypanosoma brucei.

Trypanosomes are the causative agent of sleeping sickness, and the entire cell surface is covered by a single glycoprotein called the variable surface glycoprotein, or VSG. (see here for more information)

These parasitic protozoans have the ability to 'fool' the immune response by antigenic variation. At any one time just a single VSG is expressed on the surface, but (very rarely) a trypanosome may express a different VSG gene product. Thus if the immune response succeeds in eliminating all trypanosomes expressing VSG-A, but there exists in the population just one parasite which has switched coat, this variant will poliferate as it effectively presents a different antigen to the host's defenses (and a second immune response will result).

There is litte sequence identity between different VSGs but they are homologous (descended from a common ancestor).

The crystal structure of the N-terminal domain of two different VSGs (MITat1.2 and ILTat1.24) have been solved (Blum et al. 1993; Freymann et al., 1993). Despite the low sequence identity (~16%), the structures are almost identical.

One other very interesting property of all VSGs is that they are covalently anchored to the plasma membrane via a (C-terminus) glycosylphosphatidylinositol anchor (or GPI anchor).

References

• Blum, M. L., Down, J. A., Gurnett, A. M., Carrington, M., Turner, M. J., and Wiley, D. C. (1993) Nature 362, 603-609 [pubmed]




• Freymann, D., Down, J., Carrington, M., Roditi, I., Turner, M., and Wiley, D. (1990) 2.9 A resolution structure of the N-terminal domain of a variant surface glycoprotein from Trypanosoma brucei. J. Mol. Biol. 216, 141-160 [Pubmed]

The following reference contains much useful background information, and is free to all.

• Chattopadhyay, A, Jones, N.G, Nietlispach,D, Nielsen, P.R., Voorheis, H.P., Mott H.R., Carrington, M. (2005) Structure of the C-terminal domain from Trypanosoma brucei variant surface glycoprotein MITat1.2. J. Biol. Chem., 280, 7228-7235 [Pubmed] [pdf]