Brush border myosin filament assembly and interaction with actin investigated with monoclonal antibodies

R40.76 binds to the α domain of ZO-1: role of ZO-1 (α+) in epithelial differentiation and mechano-sensing

The barrier function of epithelia and endothelia depends on tight junctions, which are formed by the polymerization of claudins on a scaffold of ZO proteins. Two differentially spliced isoforms of ZO-1 have been described, depending on the presence of the α domain, but the function of this domain is unclear. ZO-1 also contains a C-terminal ZU5 domain, which is involved in a mechano-sensitive intramolecular interaction with the central (ZPSG) region of ZO-1. Here we use immunoblotting and immunofluorescence to map the binding sites for commercially available monoclonal and polyclonal antibodies against ZO-1, and for a new polyclonal antibody (R3) that we developed against the ZO-1 C-terminus. We demonstrate that antibody R40.76 binds to the α domain, and the R3 antibody binds to the ZU5 domain. The (α+) isoform of ZO-1 shows higher expression in epithelial versus endothelial cells, and in differentiated versus undifferentiated primary keratinocytes, suggesting a link to epithelial differentiation and a potential molecular adaptation to junctions subjected to stronger mechanical forces. These results provide new tools and hypotheses to investigate the role of the α and ZU5 domains in ZO-1 mechano-sensing and dynamic interactions with the cytoskeleton and junctional ligands.

The S100A4 metastasis factor regulates cellular motility via a direct interaction with myosin-IIA

S100A4, a member of the Ca(2+)-dependent S100 family of proteins, is a metastasis factor that is thought to regulate the motility and invasiveness of cancer cells. Previously, we showed that S100A4 specifically binds to nonmuscle myosin-IIA and promotes the unassembled state. S100A4, thus, provides a connection between the actomyosin cytoskeleton and the regulation of cellular motility; however, the step or steps in the motility cycle that are affected by S100A4 expression have not been investigated. To examine how the biochemical properties of S100A4 affect cell motility, we determined the effect of S100A4 expression on protrusive behavior during chemoattractant-stimulated motility. Our studies show that S100A4 modulates cellular motility by affecting cell polarization, with S100A4 expressing cells displaying few side protrusions and extensive forward protrusions during chemotaxis compared with control cells. To establish a direct link between S100A4 and the regulation of myosin-IIA function, we prepared an antibody to the S100A4 binding site on the myosin-IIA heavy chain that has comparable effects on myosin-IIA assembly as S100A4. Microinjection experiments show that the antibody elicits the same effects on cell polarization as S100A4. Our studies show for the first time that S100A4 promotes directional motility via a direct interaction with myosin-IIA. These findings establish S100A4 as a critical regulator of myosin-II function and metastasis-associated motility.

Metastasis-associated protein Mts1 (S100A4) inhibits CK2-mediated phosphorylation and self-assembly of the heavy chain of nonmuscle myosin

A role for EF-hand calcium-binding protein Mts1 (S100A4) in the phosphorylation and the assembly of myosin filaments was studied. The nonmuscle myosin molecules form bipolar filaments, which interact with actin filaments to produce a contractile force. Phosphorylation of the myosin plays a regulatory role in the myosin assembly. In the presence of calcium, Mts1 binds at the C-terminal end of the myosin heavy chain close to the site of phosphorylation by protein kinase CK2 (Ser1944). In the present study, we have shown that interaction of Mts1 with the human platelet myosin or C-terminal fragment of the myosin heavy chain inhibits phosphorylation of the myosin heavy chain by protein kinase CK2 in vitro. Mts1 might also bind directly the beta subunit of protein kinase CK2, thereby modifying the enzyme activity. Our results indicate that myosin oligomers were disassembled in the presence of Mts1. The short C-terminal fragment of the myosin heavy chain was totally soluble in the presence of an equimolar amount of Mts1 at low ionic conditions (50 mM NaCl). Depolymerization was found to be calcium-dependent and could be blocked by EGTA. Our data suggest that Mts1 can increase myosin solubility and therefore suppress its assembly.

Mutations in MYH9 result in the May-Hegglin anomaly, and Fechtner and Sebastian syndromes. The May-Heggllin/Fechtner Syndrome Consortium

The autosomal dominant, giant-platelet disorders, May-Hegglin anomaly (MHA; MIM 155100), Fechtner syndrome (FTNS; MIM 153640) and Sebastian syndrome (SBS), share the triad of thrombocytopenia, large platelets and characteristic leukocyte inclusions ('Döhle-like' bodies). MHA and SBS can be differentiated by subtle ultrastructural leukocyte inclusion features, whereas FTNS is distinguished by the additional Alport-like clinical features of sensorineural deafness, cataracts and nephritis. The similarities between these platelet disorders and our recent refinement of the MHA (ref. 6) and FTNS (ref. 7) disease loci to an overlapping region of 480 kb on chromosome 22 suggested that all three disorders are allelic. Among the identified candidate genes is the gene encoding nonmuscle myosin heavy chain 9 (MYH9; refs 8-10), which is expressed in platelets and upregulated during granulocyte differentiation. We identified six MYH9 mutations (one nonsense and five missense) in seven unrelated probands from MHA, SBS and FTNS families. On the basis of molecular modelling, the two mutations affecting the myosin head were predicted to impose electrostatic and conformational changes, whereas the truncating mutation deleted the unique carboxy-terminal tailpiece. The remaining missense mutations, all affecting highly conserved coiled-coil domain positions, imparted destabilizing electrostatic and polar changes. Thus, our results suggest that mutations in MYH9 result in three megakaryocyte/platelet/leukocyte syndromes and are important in the pathogenesis of sensorineural deafness, cataracts and nephritis.

Role of the COOH-terminal nonhelical tailpiece in the assembly of a vertebrate nonmuscle myosin rod

A short nonhelical sequence at the COOH-terminus of vertebrate nonmuscle myosin has been shown to enhance myosin filament assembly. We have analyzed the role of this sequence in chicken intestinal epithelial brush border myosin, using protein engineering/site-directed mutagenesis. Clones encoding the rod region of this myosin were isolated and sequenced. They were truncated at various restriction sites and expressed in Escherichia coli, yielding a series of mutant myosin rods with or without the COOH-terminal tailpiece and with serial deletions from their NH2-termini. Deletion of the 35 residue COOH-terminal nonhelical tailpiece was sufficient to increase the critical concentration for myosin rod assembly by 50-fold (at 150 mM NaCl, pH 7.5), whereas NH2-terminal deletions had only minor effects. The only exception was the longest NH2-terminal deletion, which reduced the rod to 119 amino acids and rendered it assembly incompetent. The COOH-terminal tailpiece could be reduced by 15 amino acids and it still efficiently promoted assembly. We also found that the tailpiece promoted assembly of both filaments and segments; assemblies which have different molecular overlaps. Rod fragments carrying the COOH-terminal tailpiece did not promote the assembly of COOH-terminally deleted material when the two were mixed together. The tailpiece sequence thus has profound effects on assembly, yet it is apparently unstructured and can be bisected without affecting its function. Taken together these observations suggest that the nonhelical tailpiece may act sterically to block an otherwise dominant but unproductive molecular interaction in the self assembly process and does not, as has been previously thought, bind to a specific target site(s) on a neighboring molecule.

Parallel modulation of brush border myosin conformation and enzyme activity induced by monoclonal antibodies

Monoclonal antibodies binding to distinct epitopes on the tail of brush border myosin were used to modulate the conformation and state of assembly of this myosin. BM1 binds 1:3 of the distance from the tip of the tail to the head and prevents the extended-tail (6S) monomer from folding into the assembly-incompetent folded-tail (10S) state, whereas BM4 binds to the tip of the myosin tail, and induces the myosin to fold into the 10S state. Thus, at physiological ionic strength BM1 promotes and BM4 blocks the assembly of the myosin into filaments. Using BM1 and BM4 together, we were able to prevent both folding and filament assembly, thus locking myosin molecules in the extended-tail 6S monomer conformation at low ionic strength where they normally assemble into filaments. Using these myosin-antibody complexes, we were able to investigate independently the effects of folding of the myosin tail and assembly into filaments on the myosin MgATPase. The enzymatic activities were measured from the fluorescent profiles during the turnover of the ATP analogue formycin triphosphate (FTP). Extended-tail (6S) myosin molecules had an FTPase activity of 1-5 X 10(-3) s-1, either at high ionic strength as a monomer alone or when complexed with antibody, or at low ionic strength as filaments or when maintained as extended-tail monomers by the binding of BM1 and BM4. Folding of the molecules into the 10S state reduced this rate by an order of magnitude, effectively trapping the products of FTP hydrolysis in the active sites.

Modulation of cellular morphology and locomotory activity by antibodies against myosin

Three monoclonal antibodies directed against chicken brush border myosin were used to study the possible function of myosin in microfilament organization and locomotion of chicken fibroblasts. These antibodies bind to distinct and separate epitopes on the heavy chain of chicken nonmuscle myosin and display differential effects of myosin filament formation and actin-myosin interaction (Citi, S., and J. Kendrick-Jones. 1988. J. Musc. Res. Cell Motil. 9: 306-319). When injected into chicken fibroblasts, all antibodies induced breakdown of stress fibers. Concomitantly, a large proportion of the cells developed extensive lamellae which altered their morphology drastically. These cells showed also increased locomotory activity. All effects were concentration dependent and reversible. The most drastic alterations were observed with cells injected with antibody quantities exceeding the quantity of cellular myosin (molar ratios of antibody to myosin greater than 3:1). The finding that antibodies with different effects on myosin filament formation in vitro all induce similar intracellular processes suggests that it is the antibody-induced decrease in functional myosin that triggers an increase in plasma membrane dynamics and locomotory activity, rather than differences in myosin filament length or conformation.