The present invention relates to antigen-binding fragment(Fab) and a Fab-effector fusion protein or (poly)peptide comprising thereof. The Fab of the present invention specifically binds to serum albumin and thereby has extendedin vivohalf-life. The Fab of the present invention is characterized by not having cysteine residues that are responsible for the interchain disulfide bond in C<；sub>；H1<；/sub>；domain and C<；sub>；kL<；/sub>；domain as well. The Fab-effector fusion protein or (poly)peptide of the present invention can be produced in periplasm ofE. coliwith high yield, and has increasedin vivohalf-life. Further, the present invention providesE. colistrain which produces various kinds of Fab-effector fusion proteins or (poly)peptides, and a pharmaceutical composition comprising the fab-effector fusion proteins or (poly)peptides.
