Myosin is an actin based motor protein that generates motion using chemical energy released through ATP hydrolysis. Myosins play many important roles within plant cells such as organelle trafficking [1, 2], remodelling [3, 4], and inheritance . They are also known to be involved in development of various plant parts like root hairs, pollen etc[6, 7]. Though there are around 24 classes of myosins reported in eukaryota , only three classes - class VIII, XI and XIII are seen in plants. The similarity of plant myosin sequences with animal and fungal class V myosins , derived from phylogenetic analysis, suggests a common ancestor from which plants and Opisthokonts  might have evolved.
Myosins, in general, function through an ATP hydrolysis cycle by converting the hydrolysis energy to allosteric conformational changes within the motor head as well as the neck region leading to motion. Swinging cross-bridge hypothesis, proposed by H. E. Huxley, has been the most popular model to explain the molecular mechanism of energy transduction in myosins. Numerous biochemical and biophysical experiments thereafter helped to improve it to the present day swinging lever arm model , according to which, immediately after the release of ADP from the previous cycle, ATP binds to myosin head that is in an actin-bound post-stroke conformation. Upon hydrolysis of ATP, myosin head gets transformed to pre-stroke conformation which is actually actin unbound form. Upon rebinding to the next actin molecule, Pi is released first and the ADP bound myosin head changes conformation from weak to strong actin binding state. This will be followed by ADP release and conformational changes at the head domain back to post-stroke state where myosin strongly interacts with actin.
In plants, from algae to angiosperms, the high velocity cytoplasmic streaming (of the range 40-60 μm/s) is driven by myosins . Myosin XI, found in plants, is the fastest known motor and it moves processively along actin filaments towards the plus-end, performing cellular functions like cytoplasmic streaming and vesicle transport [1, 2]. Myosin XI is architecturally similar to class V myosin in animals, with a motor head followed by six IQ motifs, a coiled-coil and a globular cargo-binding domain called DIL . Myosin XI, just like myosin V, functions as a dimer formed through coiled-coil interaction between the α-helical tail regions of the monomers. Both the classes have comparable step size (an average of 35nm) and same directionality towards the plus end of actin filament.
Due to the difficulties in crystallization of actin-myosin complex, the actual actin binding residues are not known even in well-characterized myosins like myosin II and V. However, docking studies have revealed the actomyosin interface residues (see ref.  for a review). Actomyosin interface is extensive in the rigour state because of the interaction of a single head with regions on two adjacent actin molecules. Rigour state contact between actin and myosin head can be divided into four regions: a large primary binding site on the face of actin, which on three sides, is flanked by three additional sites from surface loops . In this study, using the Evolutionary Trace method, we have identified crucial residues at the actin binding site, at the ATP binding domain and at the beginning of neck region that could contribute to the fast release of ADP and the high velocity. During the process of accumulation of myosin XI sequences from plants, we have recognised nine Myosin XI sequences from sorghum and seven from grape through genome-wide survey and gene prediction.
Algal myosin XI, isolated from Chara corallina, slides F-actin in vitro at a speed equivalent to cytoplasmic streaming speed of 40- 60 μm/s, which is 10 times the speed generated by myosin V [16, 17]. Studies with tobacco myosin XI heavy isoform, by Tominaga and coworkers, revealed that single myosin XI molecules move at velocity 7 μm/s along the actin and generate relatively smaller force, in the order of 0.5 pN  much smaller than the force generated by muscle myosin II  and by myosin V [19–21]. In an attempt to elucidate the mechanism of this fast movement of Chara myosin, Ito and co-workers measured its kinetic properties . The rate constant of ADP dissociation from actin-motor domain complex of Chara myosin XI was estimated to be more than 2800 s-1 and the rate constant of ATP-induced dissociation of motor domain from the actin was 2200 s-1 at a physiological concentration of ATP. The estimated time spent on actin, in strongly bound state, was estimated to be <0.82ms. This value is the shortest among known values for various myosins and it has a duty ratio of <0.3 and a Vmax of actin-activated ATPase activity of 390 s-1. ADP release, which is the rate limiting step in all other myosin types, is dramatically accelerated in this plant myosin. Most of the myosins possess positively charged residues on loop 2 where as Chara myosin has no net positive charge on loop2. Instead, positively charged residues are harboured on loop 3. Ito and coworkers investigated the effect of positively charged residues on the loops at the actin binding region and provided evidence for its partial role on the high velocity movement through mutation studies . Still, the sequence signatures that lead to the specialization of these myosins as the fastest motors and the actual molecular mechanism of such a rapid process are not known completely. In this study, based on sequence analysis and molecular modelling, we propose that the sequence signatures at the switch I region, the ATP-binding site and the neck region as partly responsible for the observed high rate of ADP release, which in turn lead to the specialization of these myosins as the fastest ones.