SEQUENCE AND SUBMOLECULAR LOCALIZATION OF THE 115-KD ACCESSORY SUBUNIT OF THE HETEROTRIMERIC KINESIN-II (KRP(85 95)) COMPLEX/

The heterotrimeric kinesin-II holoenzyme purified from sea urchin (Str ongylocentrotus purpuratus) eggs is assembled from two heterodimerized kinesin-related motor subunits of known sequence, together with a thi rd, previously uncharacterized 115-kD subunit, SpKAP115. Using monospe cific anti-SpKAP115 antibodies we have accomplished the molecular clon ing and sequencing of the SpKAP115 subunit. The deduced sequence predi cts a globular 95-kD non-motor ''accessory'' polypeptide rich in alpha -helical segments that are generally not predicted to form coiled coil s. Electron microscopy of individual rotary shadowed kinesin-II holoen zymes also suggests that SpKAP115 is globular, with a somewhat asymmet ric morphology. Moreover, the SpKAP115 subunit lies at one end of the 51-nm-long kinesin-II complex, being separated from the two presumptiv e motor domains by a similar to 26-nm-long rod, in a manner similar to the light chains (KLCs) of kinesin itself. This indicates that SpKAP1 15 and the KLCs may have analogous functions, yet SpKAP115 does not di splay significant sequence similarity with the KLCs. The results show that kinesin and kinesin-II are assembled from highly divergent access ory polypeptides together with kinesin related motor subunits (KRPs) c ontaining conserved motor domains linked to divergent tails. Despite t he lack of sequence conservation outside the motor domains, there is s triking conservation of the ultrastructure of the kinesin and kinesin- II holoenzymes.