In contrast with the failure of visceral endoderm formation resulting in embryonic day (E)6.5 lethality of A-/A- mice, replacement with NM II-B or chimeric NM IIs restores a normal visceral endoderm. This finding is consistent with NM II's role in cell adhesion and also confirms an essential, isoform-independent requirement for NM II in visceral endoderm function. The knock-in mice die between E9.5 and 12.5 because of defects in placenta formation associated with abnormal angiogenesis and cell migration, revealing a unique function for NM II-A in placenta development. In vitro results further support a requirement for NM II-A in directed cell migration and focal adhesion formation. These findings demonstrate an isoform-specific role for NM II-A during these processes, making replacement by another isoform, or chimeric NM II isoforms, less successful. The failure of these substitutions is not only related to the different kinetic properties of NM II-A and II-B, but also to their subcellular localization determined by the C-terminal domain. These results highlight the functions of the N-terminal motor and C-terminal rod domains of NM II and their different roles in cell-cell and cell-matrix adhesion. We have successfully produced the full length NMHC II proteins in the Sf9-baculovirus system. 1). These proteins include wild type NMHC II-A, II-A with the N93K point mutation, II-A with D1424N point mutation, II-A with E1841K point mutation, II-B or IIB with B2 insert (IIB-B2), II-B with R709C point mutation, two chimeric NMHC II proteins (one is the amino-terminal globular head of II-A fused to the rod of II-B: IIAB;another is II-B globular head preceding the II-A rod: IIBA), as well as IIB-B2 S1. 2). Our initial results show that NM II-A and II-B, even the chimeric proteins, display similar filament structures including length and size, however, some point mutation located at the C-terminal of NM II-A have important effects on its length of filament. The identification of the key point mutations are underway. 3). In addition, in contrast to our previous report of NM IIB-B2 HMM, the full-length NM IIB-B2 protein displays some ATPase activity but no observed motility.
