The present invention concerns methods and compositions for extending the technique of native chemical ligation of a wider range of peptides, polypeptides, other polymers and other molecules via an amide bond (see FIG. 1). The invention further provides methods and uses for such proteins and derivatized proteins. The invention is particularly suitable for use in the synthesis of optionally polymer-modified, synthetic bioactive proteins, and of pharmaceutical compositions that contain such proteins. ##STR1##