M20, the small subunit of PP1M,binds to microtubules

Myosinlight chain phosphatase (PP1M) is composed of three subunits, i.e.,M20, MBS, and a catalytic subunit. Whereas MBS is assigned as a myosinbinding subunit, the function of M20 is unknown. In the present study,we found that M20 binds to microtubules. The binding activity wasrevealed by cosedimentation of M20 with microtubules and binding oftubulin to M20 affinity resin. Green fluorescent protein (GFP)-taggedM20 (M20-GFP) was expressed in chicken primary smooth muscle cells andCOS-7 cells and was used as a probe for studying the associationbetween M20 and microtubules in living cells. M20-GFP was localized onfilamentous structures in both cell types. Colocalization analysisrevealed that M20-GFP colocalized with tubulin. Treatment withnocodazole, but not cytochalasin B, abolished the filamentous structureof M20-GFP. These results indicate that M20-GFP associates withmicrotubules in cells. Microinjection of rhodamine-tubulin into theM20-expressing cells revealed that incorporation of rhodamine-tubulininto microtubules was significantly facilitated bymicrotubule-associated M20. Consistent with this result, M20 enhancedthe rate of tubulin polymerization in vitro and produced elongatedmicrotubules. These results suggest that M20 has a microtubule bindingactivity and plays a role in regulating microtubule dynamics.