Poster: Cytoskeleton: Structure & Function

Abs # 1102: Characteristics of characean myosin, the fastest motor protein in the world

Presenter:	Hachikubo, You , 96s4028@bio.s.chiba-u.ac.jp
Authors	Hachikubo, You  (A)   Awata, Jun-ya  (A)   Kashiyama, Taku  (B)   Ito, Kohji  (A)   Shimada, Kiyo  (C)   yamamoto, Keiichi  (A)
Affiliations:	(A): Department of Biology, Chiba University (B): Pharmacology Department, Juntendo University School of Medicine (C): Kazusa DNA Laboratory

It is well known that the cytoplasmic streaming of characean cells is regulated by Ca2+. However, we found previously that both the actin activated MgATPase and the in vitro motile activity of purified characean myosin were not regulated by Ca2+. Since there is a possibility that the insensitivity of characean myosin to Ca2+ is due to the dissociation of some calmodulin molecules from the light chain binding site of characean myosin during the course of protein purification, we reexamined the motile activity of characean myosin in vitro in the presence of excess characean calmodulin. We intentionally used freshly prepared crude characean myosin to minimize the possibility of calmodulin dissociation. We could not observe, however, any drastic inhibition of the motile activity of the characean myosin by Ca2+. The result suggests that the cessation of characean cytoplasmic streaming is not caused by the direct inhibition of myosin activity by Ca2+. Animal class V myosin is known to move on an actin filament without detaching from it for a long distance (processive movement). We studied the processivity of characean myosin because its structure is similar to that of myosin V. We first studied the effect of MgADP on the motile activity of characean myosin. The extent of inhibition by MgADP on the motile activity of characean myosin was almost the same as in skeletal muscle myosin (class II, typical non-processive myosin). We then measured the density of characean myosin required to produce a continuous movement of actin filament. The value was about 200 molecules/(micrometer)2, that is also similar to that for skeletal muscle myosin. These results suggest that the characean myosin is not a processive motor protein.