The B2 alternatively spliced isoform of nonmuscle myosin II-B lacks actin-activated MgATPase activity and in vitro motility.

We report the initial biochemical characterization of an alternatively spliced isoform of nonmuscle heavy meromyosin (HMM) II-B2 and compare it with HMM II-B0, the nonspliced isoform. HMM II-B2 is the HMM derivative of an alternatively spliced isoform of endogenous nonmuscle myosin (NM) II-B, which has 21-amino acids inserted into loop 2, near the actin-binding region. NM II-B2 is expressed in the Purkinje cells of the cerebellum as well as in other neuronal cells [X. Ma, S. Kawamoto, J. Uribe, R.S. Adelstein, Function of the neuron-specific alternatively spliced isoforms of nonmuscle myosin II-B during mouse brain development, Mol. Biol. Cell 15 (2006) 2138-2149]. In contrast to any of the previously described isoforms of NM II (II-A, II-B0, II-B1, II-C0 and II-C1) or to smooth muscle myosin, the actin-activated MgATPase activity of HMM II-B2 is not significantly increased from a low, basal level by phosphorylation of the 20kDa myosin light chain (MLC-20). Moreover, although HMM II-B2 can bind to actin in the absence of ATP and is released in its presence, it cannot propel actin in the sliding actin filament assay following MLC-20 phosphorylation. Unlike HMM II-B2, the actin-activated MgATPase activity of a chimeric HMM with the 21-amino acid II-B2 sequence inserted into the homologous location in the heavy chain of HMM II-C is increased following MLC-20 phosphorylation. This indicates that the effect of the II-B2 insert is myosin heavy chain specific.

Load-dependent mechanism of nonmuscle myosin 2.

Loads on molecular motors regulate and coordinate their function. In a study that directly measures properties of internally strained myosin 2 heads bound to actin, we find that human nonmuscle myosins 2A and 2B show marked load-dependent changes in kinetics of ADP release but not in nucleotide binding. We show that the ADP release rate constant is increased 4-fold by the assisting load on one head and decreased 5-fold (for 2A) or 12-fold (for 2B) by the resisting load on the other. Thus these myosins, especially 2B, have marked mechanosensitivity of product release. By regulating the actin attachment of myosin heads, this provides a basis for energy-efficient tension maintenance without obstructing cellular contractility driven by other motors such as smooth muscle myosin. Whereas forward load accelerates the cycle of interaction with actin, resistive load increases duty ratio to favor tension maintenance by two-headed attachment.

Growth cones contain myosin II bipolar filament arrays.

Nonmuscle myosin II is among the most abundant forms of myosin in nerve growth cones. At least two isoforms of myosin II (A and B) that have overlapping but distinct distributions are found in growth cones. It appears that both myosin IIA and IIB may be necessary for normal nerve outgrowth and motility, but the molecular interactions responsible for their activity remain unclear. For instance, it is unknown if these myosin II isoforms produce bipolar "minifilaments" in growth cones similar to those observed in other nonmuscle cells. To determine if minifilaments are present in growth cones, we modified the electron microscopy preparative procedures used to detect minifilaments in other cell types. We found structures that appeared very similar to bipolar minifilaments found in noneuronal cells. They also labeled with antibodies to either myosin IIA or IIB. Thus, the activity of myosin II in growth cones is likely to be similar to that in other nonmuscle cells. Bipolar filaments interacting with oppositely oriented actin filaments will produce localized contractions or exert tension on actin networks. This activity will be responsible for the myosin II dependent motility in growth cones.