Practice question

In many archaea, particularly those lacking pseudomurein or outer proteinaceous sheath, envelope consists of cytoplasmic membrane plus outermost crystalline surface layer that serves as cell wall equivalent. This S-layer is not composed of peptidoglycan, lipids, or teichoic acids characteristic of bacteria; it is assembled from one or two species of proteins or glycoproteins that self-organize into hexagonal or tetragonal lattice with nanometer-scale pores. Subunits are often heavily glycosylated with sulfated or branched oligosaccharides that confer additional stability against proteases and extremes. Lattice formation is driven by non-covalent interactions, capable of self-assembly in vitro, and anchored via stalk-like domains interacting with membrane lipids or pseudomurein. In Sulfolobus, S-layer consists of two glycoproteins SlaA forming outer canopy and SlaB forming membrane-anchoring stalks. In halophiles, single glycoprotein forms matrix. Thus proteinaceous or glycoprotein nature distinguishes archaeal S-layers from peptidoglycan-based walls and explains resistance to lysozyme, beta-lactams, and cell wall antibiotics targeting murein biosynthesis pathways. This mechanistic insight is relevant for competitive examinations such as CSIR-NET and GATE, where understanding molecular detail rather than memorization enables accurate interpretation of experimental data and pathway interconnections.