the proteins of interest are simply connected end-to-end via fusion of n or c termini between the proteins. this provides a flexible bridge structure allowing enough space between fusion partners to ensure proper folding. however, the n or c termini of the peptide are often crucial components in obtaining the desired folding pattern for the recombinant protein, making simple end-to-end conjoining of domains ineffective in this case. for this reason, a protein linker is often needed to maintain the functionality of the protein domains of interest.