RnoEX0056976 @ rn6

In contrast to any of the previously described isoforms of nonmuscle myosin 2 (NM II); II-A, II-B0, II-B1, II-C0 and II-C1, or to smooth muscle myosin, the actin-activated MgATPase activity of nonmuscle heavy meromyosin (HMM) II-B2 (formed by inclusion of HsaEX0040948) 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.

The B2-inserted isoform, which contains an insert of 21 amino acids (MmuEX0030347) near the actin-binding region (loop 2), is important for postnatal development of cerebellar Purkinje cells. Deletion of the B2 alternative exon results in abnormal development of cerebellar Purkinje cells. Cells lacking the B2-inserted isoform show reduced numbers of dendritic spines and branches. Some of the B2-ablated Purkinje cells are misplaced in the cerebellar molecular layer. All of the B2-ablated mice demonstrated impaired motor coordination.