title: Functional role of the extended loop 2 in the myosin 9b head for binding F-actin.
abstract: The mammalian class IX myosin Myo9b can move considerable distances along actin filaments before it dissociates. This is remarkable, because it is single headed and because the rate-limiting step in its ATPase cycle is ATP hydrolysis. Thus, it spends most of its cycling time in the ATP-bound state that has a weak affinity for F-actin in other myosins. It has been speculated that the very extended loop 2 in the Myo9b head domain comprises an additional actin-binding site that prevents it from dissociation in the weak binding states. Here we show that two regions in the loop 2 determine the F-actin concentrations needed to maximally activate the steady-state ATPase activity. Together these two regions regulate the amount capable of binding F-actin and the affinity of the nucleotide-free state. The isolated loop 2 behaved like an entropic spring and bound stoichiometrically and with high affinity to F-actin. Subfragment 1 from skeletal muscle myosin II bound to F-actin simultaneously with the isolated loop 2 of Myo9b and could not displace it. Furthermore, the present results imply also a regulatory role for the tail region. Taken together, the results demonstrate that the extended loop 2 in Myo9b binds F-actin and influences the binding of the conventional stereo-specific actin-binding site.
authors.affiliation: Institute of General Zoology and Genetics, Westfalian Wilhelms-University Münster, Münster 48149, Germany.
authors.forename: Sandra
authors.initials: S
authors.lastname: Struchholz
authors.forename: Kerstin
authors.initials: K
authors.lastname: Elfrink
authors.forename: Uwe
authors.initials: U
authors.lastname: Pieper
authors.forename: Georg
authors.initials: G
authors.lastname: Kalhammer
authors.forename: Ulrike
authors.initials: U
authors.lastname: Honnert
authors.forename: Anika
authors.initials: A
authors.lastname: Grützner
authors.forename: Wolfgang A
authors.initials: WA
authors.lastname: Linke
authors.forename: Wanqin
authors.initials: W
authors.lastname: Liao
authors.forename: Martin
authors.initials: M
authors.lastname: Bähler
mesh_terms: D000199:Actins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D000818:Animals; D001665:Binding Sites / Q000502:physiology; D002460:Cell Line; D006868:Hydrolysis; D009218:Myosins / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism; D011485:Protein Binding / Q000502:physiology; D017433:Protein Structure, Secondary / Q000502:physiology; D017434:Protein Structure, Tertiary / Q000502:physiology; D051381:Rats
chemical_list: D000199:Actins; C113469:myosin IXB; D000255:Adenosine Triphosphate; D009218:Myosins