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Data collected for interactions among redox centres, and interactions between redox centres and acid–base residues in a family of small multihaem cytochromes are analysed. The distance dependent attenuation of the interactions between non-surface charges, for separations that range from 8 to 23 A, can be described by a simple function derived from the Debye–Huckel formalism, fit to 9.5 and 7.6 as values for the relative dielectric constant and Debye length, respectively. However, there is considerable scatter in the data despite the structural similarities among the proteins, which is discussed in the framework of using such simple models in predicting properties of novel proteins.

Chloroplasts of higher plants contain a unique signal recognition particle (cpSRP) that consists of two proteins, cpSRP54 and cpSRP43. CpSRP43 is composed of a four ankyrin repeat domain and three functionally distinct chromodomains (CDs). In this report we confirm previously published data that the second chromodomain (CD2) provides the primary binding site for cpSRP54. However, quantitative binding analysis demonstrates that cpSRP54 binds to CD2 significantly less efficiently than it binds to full-length cpSRP43. Further analysis of the binding interface of cpSRP by mutagenesis studies and a pepscan approach demonstrates that the C-terminal α-helix of CD2 facilitates binding to cpSRP54.

Coactosin is a small (MW approximately 15 kDa) evolutionarily conserved actin filament binding protein. It displays remote sequence homology to ADF/cofilin proteins and to the ADF-H domains of twinfilin and Abp1/drebrin. However, biochemical analyses have demonstrated that coactosin has a very different role in actin dynamics from the ones of ADF/cofilin, twinfilin or Abp1/drebrin. To elucidate the molecular mechanism of coactosin/actin interaction, we determined the three-dimensional structure of mouse coactosin by multidimensional NMR spectroscopy. We find that the coactosin structure is homologous to ADF/cofilin and to the ADF-H domains of twinfilin. Furthermore, the regions that have been shown to be important for actin filament interactions in ADF/cofilins are structurally conserved in coactosin suggesting that these two proteins interact with F-actin through a conserved interface. Our analysis also identifies key structural differences between these proteins that may account for the differences in biochemical activities and cellular roles of these proteins.