A NOVEL MICROTUBULE-BASED MOTOR PROTEIN (KIF4) FOR ORGANELLE TRANSPORTS, WHOSE EXPRESSION IS REGULATED DEVELOPMENTALLY

To understand the mechanisms of transport for organelles in the axon, we isolated and sequenced the cDNA encoding KIF4 from murine brain, an d characterized the molecule biochemically and immunocytochemically. C omplete amino acid sequence analysis of KIF4 and ultrastructural studi es of KIF4 molecules expressed in Sf9 cells revealed that the protein contains 1,231 amino acid residues (M(r) 139,550) and that the molecul e (116-nm rod with globular heads and tail) consists of three domains: an NH2-terminal globular motor domain, a central alpha-helical stalk domain and a COOH-terminal tail domain. KIF4 protein has the property of nucleotide-dependent binding to microtubules, microtubule-activated ATPase activity, and microtubule plus-end-directed motility. Northern blot analysis and in situ hybridization demonstrated that KIF4 is str ongly expressed in juvenile tissues including differentiated young neu rons, while its expression is decreased considerably in adult mice exc ept in spleen. Immunocytochemical studies revealed that KIF4 colocaliz ed with membranous organelles both in growth cones of differentiated n eurons and in the cytoplasm of cultured fibroblasts. During mitotic ph ase of cell cycle, KIF4 appears to colocalize with membranous organell es in the mitotic spindle. Hence we conclude that KIF4 is a novel micr otubule-associated anterograde motor protein for membranous organelles , the expression of which is regulated developmentally.