S-layers as natural building blocks for nanobiotechnology and synthetic biology.

Crystalline bacterial cell surface layers (S-layers) are self-assembling protein lattices that constitute the outermost envelope structure of many Bacteria and most Archaea. Beyond their classical role as cell surface components, S-layers are increasingly recognized as programmable, two-dimensional biological materials that combine nanometer-scale precision, defined porosity, and exceptional physicochemical properties. In this review, we synthesize current understanding of S-layer architecture, assembly, and functionalization to position them as a unifying platform for nanobiotechnology and synthetic biology. We highlight how their intrinsic self-assembly and genetic engineerability enable the design of ordered biomolecular interfaces with applications ranging from molecular sieving, biosensors, biomineralization, and nanoscale patterning. Engineered S-layer fusion proteins allow the modular and spatially controlled display of functional domains, bridging bottom-up materials design with biological complexity. Beyond their technological relevance, S-layers play underappreciated roles in host-microbe interactions, where their structural regularity and surface accessibility shape immunogenicity and cellular recognition, with implications for vaccine development, targeted delivery, and microbiome engineering. We argue that overcoming current limitations in scalable production, stability, and system integration will be key to unlocking the full potential of S-layers as genetically programmable, bio-inspired interfaces, enabling a new class of adaptive nanomaterials and advancing the design principles of synthetic biological systems.