MAP7 Targets Intracellular Transport towards the Microtubule Plus End by Recruiting Kinesin-1 to Microtubules

Abstract

Microtubule-associated proteins (MAPs) play well-characterized roles in regulating microtubule polymerization dynamics and organization. In addition, MAPs modulate the interactions between motor proteins and microtubules. By differentially regulating kinesin and dynein motors, MAPs direct trafficking towards the microtubule plus or minus end. MAP7 (ensconsin, E-MAP-115) is a ubiquitous microtubule-associated protein that organizes the microtubule cytoskeleton in mitosis and neuronal branching. MAP7 also mediates the interaction of kinesin-1 with microtubules and in some systems is required for kinesin-1 function. We expressed and purified full-length kinesin-1 and MAP7 in Sf9 cells. MAP7 recruits kinesin-1 to microtubules, increasing the frequency of both diffusive and processive runs. We performed optical trapping assays on beads transported by single and teams of kinesin-1 motors. We find that MAP7 increases the relative binding rate of single kinesin-1 motors by ∼25% and teams of kinesin-1 motors by ∼50%. To examine the role of MAP7 in regulating bidirectional transport, we isolated late phagosomes along with their native set of kinesin-1, kinesin-2, dynein motors. The fraction of motility towards the plus end shifts from 55% to 78% in the presence of 10 nM MAP7. The relative binding rate of kinesin motors increases by ∼30% while the dynein binding increases by ∼10%. Collectively, our results indicate that MAP7 enhances kinesin binding such that kinesin teams resist opposing loads and stay engaged on the microtubules for a longer period of time before detaching. MAP7 targets cargoes to the microtubule plus end in contrast to tau which reduces kinesin processivity to shift transport towards the microtubule minus end. Thus, each MAP has specific effects on motor function. Through mediating the interaction between motors and the microtubule lattice, MAPs tune the balance of plus- and minus-end directed transport.