Morphological transformation of liposomes caused by assembly of encapsulated tubulin and determination of shape by microtubule-associated proteins (MAPs)

To examine the role of cytoskeletons in cellular morphogenesis, we generated liposomes encapsulating tubulin, with or without microtubule-associated proteins (MAPs), and observed their transformation using dark-field microscopy. When tubulin was polymerized with MAPs in liposomes, liposomes were transformed into a "bipolar" shape with a central sphere and two tubular membrane protrusions that aligned in a straight line. On the other hand, when pure tubulin was polymerized in liposomes without MAPs, they initially transformed into a bipolar shape but subsequently re-transformed into a "monopolar" shape, i.e. a sphere with only one straight tubular portion. This re-transformation occurred in two ways: first, by shortening of one of the tubular portions due to microtubule disassembly; or second, by fluctuation of the central sphere toward one of the ends without shortening of the tube portion. MAPs prevented this re-transformation, and their role in stabilizing the shape of transformed liposomes was studied by the co-sedimentation method. The results show that MAPs, particularly MAP1 and MAP2, mediate binding between microtubules and the liposomal membrane. However, MAP2 by itself did not bind to liposomes, but was able to stabilize bipolar liposomes. This stabilization is caused not only by direct links between microtubules and liposomes, but also by prevention of Brownian motion of microtubules through an increase in friction.