= Fibronectin
<Fibronectin> is a cell-adhesive <protein> of the <extracellular matrix>. Its modular regions connect molecular recognition with attachment: a local <amino acid> sequence can contribute a binding activity without reproducing every property of the complete <protein>.
A 1984 experiment made that idea concrete. Small synthetic fragments reproduced measured cell-attachment activity, with an active four-residue sequence consisting of <arginine>, <glycine>, <aspartic acid> and <serine>. The familiar <RGD motif> names the first three residues, but the reported tetrapeptide was RGDS, not a three-residue experiment silently substituted afterward.<Cell attachment activity of fibronectin can be duplicated by small synthetic fragments of the molecule by M. D. Pierschbacher and E. Ruoslahti>[Primary abstract: tetrapeptide sequence and cell-attachment domain]{ref}
An original experimental-design comparison asks two different questions. Does a short peptide permit attachment in the assay? Does the same peptide reconstruct a whole extracellular network with all of fibronectin's interactions? A positive answer to the first does not establish the second. Sequence recognition, three-dimensional presentation and assembly can make distinct contributions.
This distinction also helps interpret receptor studies. An <integrin> can recognize an adhesive region, while the behavior of the complete cell depends on the surrounding matrix, other receptors and intracellular responses. A minimal active fragment is a way to isolate one molecular contribution, rather than proof that the rest of the parent <protein> has no function.
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