Adhesion forces in the self-recognition of oligosaccharide epitopes of the proteoglycan aggregation factor of the marine sponge Microciona prolifera

Cell aggregation in the marine sponge Microciona prolifera is mediated by a multimillion molecular-mass aggregation factor, termed MAF. Earlier investigations revealed that the cell aggregation activity of MAF depends on two functional domains: (i) a Ca²⁺-independent cell-binding domain and (ii) a Ca²⁺-dependent proteoglycan self-interaction domain. Structural analysis of involved carbohydrate fragments of the proteoglycan in the self-association established a sulfated disaccharide β-d-GlcpNAc3S-(1→3)-α-l-Fucp and a pyruvated trisaccharide β-d-Galp4,6(R)Pyr-(1→4)-β-d-GlcpNAc-(1→3)-α-l-Fucp. Recent UV, SPR, and TEM studies, using BSA conjugates and gold nanoparticles of the synthetic sulfated disaccharide, clearly demonstrated self-recognition on the disaccharide level in the presence of Ca²⁺-ions. To determine binding forces of the carbohydrate–carbohydrate interactions for both synthetic MAF oligosaccharides, atomic force microscopy (AFM) studies were carried out. It turned out that, in the presence of Ca²⁺-ions, the force required to separate the tip and sample coated with a self-assembling monolayer of thiol-spacer-containing β-d-GlcpNAc-(1→3)-α-l-Fucp-(1→O)(CH₂)₃S(CH₂)₆S- was found to be quantized in integer multiples of 30 ± 6 pN. No binding was observed between the two monolayers in the absence of Ca²⁺-ions. Cd²⁺-ions could partially induce the self-interaction. In contrast, similar AFM experiments with thiol-spacer-containing β-d-Galp4,6(R)Pyr-(1→4)-β-d-GlcpNAc-(1→3)-α-l-Fucp-(1→O)(CH₂)₃S(CH₂)₆S- did not show a binding in the presence of Ca²⁺-ions. Also TEM experiments of gold nanoparticles coated with the pyruvated trisaccharide could not make visible aggregation in the presence of Ca²⁺-ions. It is suggested that the self-interaction between the sulfated disaccharide fragments is stronger than that between the pyruvated trisaccharide.