Fig. 4. Domain organization of kinesin I and KIF1A. For kinesin I, its heavy and light polypeptide chains are shown in red and green, respectively. Amino-acid residues in parentheses are referenced to the mouse ubiquitous isoform. Cargo-binding domains (CBD) are indicated by black on white labels; domains involved in regulation of motor activity are indicated by black on white labels. KIF1A, like other KIF1 kinesins, have much shorter coiled-coil regions than kinesin I, and these promote dimerization of KIF1 polypeptides that are brought into close proximity on the cargo surface (Klopfenstein et al., 2002; Tomishige et al., 2002). Thus, KIF1 polypeptides are monomers in the cytosol (Nangaku et al., 1994; Okada et al., 1995), and through dimerization, presumably on the cargo surface, acquire the ability to walk along microtubules processively, like native kinesin I does (Tomishige et al., 2002). It is appealing to consider that dimerization regulates motor activity of KIF1 kinesins like folding regulates kinesin I activity (see text). The function of the FHA domain, which marks all KIF1 family members, is unknown, but may be involved in regulating dimerization. Liprin and MAGUK cargo-binding domains (LBD and MBS) overlap. The pleckstrin homology domain (PH) is probably a separate cargo-binding domain (Klopfenstein et al., 2002). The amino-acid residues in parentheses are referenced to the mouse isoform of KIF1A.