Model of Kip2 polymerizing microtubules in living cells. Frame 1: The two motor domains of kinesin Kip2 coordinate their ATPase cycles to move toward the microtubule plus-end. Each motor domain (blue or orange to differentiate the two) is bound to a tubulin heterodimer (gray box, β subunit; striped box, α subunit) in its straight conformation along a microtubule protofilament (the cylindrical microtubule is composed of 13 protofilaments). Bik1 is accumulated at the plus-end and is transported there by Kip2. Frame 2: Upon arriving at the plus-end, Kip2 is retained there by binding to Bik1. Frame 3: The unoccupied motor domain (in this case, orange) bound to ATP sequesters a free tubulin dimer in its curved conformation. Frame 4: The motor domain facilitates the incorporation of the newly sequestered dimer. The motor domain ahead (orange) hydrolyzes ATP quickly, allowing the other motor domain (blue) to be released from the microtubule protofilament and sequester the next free tubulin dimer. The Bik1-dependent retention enables Kip2 to perform multiple rounds of tubulin incorporation. Frame 5: While this Kip2 molecule is eventually released from the plus-end, such as by phosphorylation on the N-terminus, new Kip2 motors arrive with Bik1 along the lattice. In the absence of Bik1, Kip2 leaves the plus-end as soon as it arrives. Thus, Kip2 fails to promote microtubule growth. The vertical dashed line (black) indicates the length of the microtubule protofilament before Kip2 incorporated tubulin dimers.