The SAH domain of myosin 10.

When does a myosin dimerise?  Are the coiled coil predictions always right for every myosin?  We think not.  In some cases, the coiled coil contains many highly charged residues and is likely to be a single alpha helix.

This is the output from the coiled-coil prediction programs Coils (red) and Paircoil (blue) for Myosin 10.  It shows high probability of coiled coil between residues 800 and 950, and therefore it would be expected that this myosin is a dimer.

However, a closer examination of the sequence, particularly that in red, shows a high abundance of charged residues (R,E and K).  Coiled coils are made up of a repeating set of residues (a,b,c,d,e,f,g, as shown below the amino acid sequence here, in which the a and d residues are normally hydrophobic.  However, this is not the case in myosin 10.
What sort of structure does this sequence have?  We think it is a single alpha helix, due to the many potential charged interactions shown when the helix is drawn unwound as above. This is a model of the single alpha helix showing one of the R-E seams running down the helix.  Results from circular dichroism and analytical ultracentrifuge experiments on this peptide support this structure.

Other myosins also have this type of charged pattern of residues in their 'predicted' coiled coils such as myosin 6, myosin 7a and the Dictyostelium myosin MyoM.