The role of zinc in the binding of killer cell Ig-like receptors to class I MHC proteins.

The binding of killer cell Ig-like Receptors (KIR) to their Class I MHC ligands was shown previously to be characterized by extremely rapid association and dissociation rate constants. During experiments to investigate the biochemistry of receptor-ligand binding in more detail, the kinetic parameters of the interaction were observed to alter dramatically in the presence of Zn(2+) but not other divalent cations. The basis of this phenomenon is Zn(2+)-induced multimerization of the KIR molecules as demonstrated by BIAcore, analytical ultracentrifugation, and chemical cross-linking experiments. Zn(2+)-dependent multimerization of KIR may be critical for formation of the clusters of KIR and HLA-C molecules, the "natural killer (NK) cell immune synapse," observed at the site of contact between the NK cell and target cell.

Atomic force microscopy of gastric mucin and chitosan mucoadhesive systems.

Atomic force microscopy has been utilized to probe, at a molecular level, the interaction between purified pig gastric mucin (PGM) and a mucoadhesive cationic polymer, chitosan (sea cure 210+), with a low degree (approx. 11%) of acetylation. Images were produced detailing the structures of both PGM and chitosan in 0.1 M acetate buffer (pH 4.5), followed by the complex of the two structures in the same buffer. PGM in 0.1 M acetate buffer revealed long linear filamentous structures, consistent with earlier electron microscopy and scanning tunnelling micoscopy studies. The chitosan molecules also adopted a linear conformation in the same buffer, although with a smaller average length and diameter. They appeared to adopt a stiff-coil conformation consistent with earlier hydrodynamic measurements. The complexes formed after mixing PGM and chitosan together revealed large aggregates. In 0.1 M ionic strength buffer they were of the order of 0.7 microm in diameter, consistent with previous electron microscopy studies. The effect of ionic strength of the buffer on the structure of the complex was also studied and, together with molecular hydrodynamic data, demonstrates that the interaction is principally electrostatic in nature.

Structure and mucoadhesion of mussel glue protein in dilute solution.

Purified mussel adhesive protein mefp-1 (Mytilus edulis foot protein 1) has been studied regarding its state of oligomerization and gross conformation in dilute solution. Sedimentation equilibrium in the analytical ultracentrifuge of a dilute solution of protein (0.4 mg/mL) in acetate buffer at pH 4.5 and I = 0.10 M yielded an apparent molecular weight (whole distribution weight average, Mw, app) of 114 000 +/- 5000 via the "M" procedure, a value in almost exact agreement with the monomeric molecular weight obtained by MALDI mass spectrometry. At this low concentration, it is reasonable to assume thermodynamic ideality, i.e., Mw,app approximately Mw. This result, together with plots of point weight average apparent molecular weight versus concentration for three different loading concentrations (0.4, 0.8, 1.0 mg/mL), clearly demonstrates that this protein is essentially monomeric in dilute solution. Sedimentation velocity experiments yielded an estimate of the sedimentation coefficient s020,w = 2.34 +/- 0.17 S, which for M = 110 000 gives a frictional ratio f/f0 = 3.2 +/- 0.3. The interpretation of this, in terms of an extended rather than globular conformation for the structure of mefp-1 in dilute solution, is considered, within plausible limits of molecular hydration, and models for the structure in solution are considered, in light of the thermodynamic nonideality behavior of these molecules and previously published circular dichroism data. The significance of these observations in terms of the bioadhesive properties of mefp-1 is described, and the very strong interaction in dilute solution with a mucin glycoprotein is demonstrated.