A unique loop contributing to the structure of the ATP-binding cleft of skeletal muscle myosin communicates with the actin-binding site

Multiple myosin motors interact with sodium/potassium-ATPase alpha 1 subunits

The alpha1 (α1) subunit of the sodium/potassium ATPase (i.e., Na⁺/K⁺-ATPase α1), the prototypical sodium pump, is expressed in each eukaryotic cell. They pump out three sodium ions in exchange for two extracellular potassium ions to establish a cellular electrochemical gradient important for firing of neuronal and cardiac action potentials. We hypothesized that myosin (myo or myh) motor proteins might interact with Na⁺/K⁺-ATPase α1 subunits in order for them to play an important role in the transport and trafficking of sodium pump. To this end immunoassays were performed to determine whether class II non-muscle myosins (i.e., NMHC-IIA/myh9, NMHC-IIB/myh10 or NMHC-IIC/myh14), myosin Va (myoVa) and myosin VI (myoVI) would interact with Na⁺/K⁺-ATPase α1 subunits. Immunoprecipitation of myh9, myh10, myh14, myoVa and myoVI from rat brain tissues led to the co-immunoprecipitation of Na⁺/K⁺-ATPase α1 subunits expressed there. Heterologous expression studies using HEK293 cells indicated that recombinant myh9, myh10, myh14 and myoVI interact with Na⁺/K⁺-ATPase α1 subunits expressed in HEK293 cells. Additional results indicated that loss of tail regions in recombinant myh9, myh10, myh14 and myoVI did not affect their interaction with Na⁺/K⁺-ATPase α1 subunits. However, recombinant myh9, myh10 and myh14 mutants having reduced or no actin binding ability, as a result of loss of their actin binding sites, displayed greatly reduced or null interaction with Na⁺/K⁺-ATPase α1 subunits. These results suggested the involvement of the actin binding site, but not tail regions, of NMHC-IIs in their interaction with Na⁺/K⁺-ATPase α1 subunits. Overall these results suggest a role for these diverse myosins in the trafficking and transport of sodium pump in neuronal and non-neuronal tissues.

Intermolecular cross-linking of a novel rice Kinesin k16 motor domain with a photoreactive ATP derivative

A fluorescent photoreactive ATP derivative, 2'(3')-O-(4-benzoylbenzoyl)-1,N(6)-etheno-ATP (Bz(2)-epsilonATP), was synthesized and reacted with the rice kinesin K16 motor domain (K16MD). In the presence of ADP or ATP, UV irradiation of the K16MD solution containing Bz(2)-epsilonATP resulted in a new 100 kDa band, which was an intermolecular cross-linked product of motor domains. In contrast, no cross-linking was observed in the absence of nucleotides. For the motor domain of mouse brain kinesin and skeletal muscle myosin subfragment-1, no such intermolecular photo cross-linking by Bz(2)-epsilonATP was observed. Our results indicate that Bz(2)-epsilonATP acts unusually as a photoreactive crosslinker to detect conformational changes in K16MD induced by nucleotide binding resulting in the formation of dimers.

Conformational change of the loop L5 in rice kinesin motor domain induced by nucleotide binding

Loop L5 of kinesin is located near the ATPase site, in common with kinesins of various animal species. The rice plant-specific kinesin K16 also has a corresponding loop that is slightly shorter than that of mouse brain kinesin. The present study was designed to monitor conformational changes in loop L5 during ATP hydrolysis. For this purpose, we introduced one reactive cysteine into the L5 of rice kinesin and modified it with fluorescent probes. The cysteine in L5 was labeled with a fluorescent probe 2-(4'(iodoacetamide) anilino-naphthalene-6-sulfonic acid sodium salt) [IAANS]. IAANS was incorporated into L5 at an almost equimolar ratio in the absence of nucleotides. In contrast, the incorporated amount was reduced to 0.62 and 0.32 mol IAANS/mol motor domain in the presence of ATP and ADP, respectively. Upon nucleotide addition, the fluorescent intensity of IAANS incorporated into L5 was significantly reduced to 63% and 51% for ATP and ADP, respectively. These results suggest that L5 of rice kinesin significantly changes its conformation during ATP hydrolysis.