Scientists are studying the structure and function of receptor x, a single-polypeptide protein that is found in the membrane around certain types of cells. receptor x contains no alpha- helices or beta sheets. a specific molecule outside the cells is recognized and bound by receptor x. the binding of the molecule to receptor x causes the cells to have a particular response. to study the structure and function of receptor x the scientists are altering the sequence of small sections of the receptor. both the normal sequence of a five-amino acid section of receptor x and an altered sequence that contains two amino acid substitutions are shown in figure 1.