EM projection maps to constrain these 3D models; however, we were pleased to note that a recent EM map of the Drosophila M construct bound to protofilament bracelets supports our model for kinesin 13 M domain inter- actions with a curved substrate (Tan et al., 2008). Online supplemental material Fig. S1 shows eigenimages and additional class averages, Fig. S2 shows derivation of alternate site-binding templates, and Fig. S3 shows EM analy- sis of M construct from human MCAK. Online supplemental material is avail- able at http://www.jcb.org/cgi/content/full/jcb.200812052/DC1. We thank Craig Yoshioka, Elizabeth Wilson-Kubalek, Matthew Lee, Crystal Moran, and members of the Milligan and the Automated Molecular Imaging groups (The Scripps Research Institute, La Jolla, CA) for helpful discussions and Steve Bradlow and Henry Bradlow for mathematical assistance. This work was supported by grants from the National Institutes of Health (GM52468 to R.A. Milligan and GM69429 to L. 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