This thesis demonstrates, that polyionic fusion peptides own multiple functions in protein purification, renaturation and directed association. A Fab-fragment and an enzyme (α-glucosidase from Saccharomyces cerevisiae) were genetically modified by appending a polyionic fusion peptide. The fusion peptides consist of ten either positively or negatively charged aminoacids and an addional cysteine residue. It is demonstrated, that the modified proteins can be produced in recombinant Escherichia coli and purified on complementarily charged ion exchange materials. Further, the polyanionic fusion peptide is used for matrix-assisted refolding of α-glucosidase. A special kind of a covalent affinity chromatography was developed in order to characterize proteins possessing C-terminal polyanionic fusion peptides. Further, complementarily charged polyionic fusion peptides function as dimerization motif. Using complementarily charged polyionic fusion peptides for directed association and covalent linkage by the engineered cysteine-residues, a Fab-Fragment-enzyme-conjugate was formed. Antigen binding activity and enzymatic activity were not impaired compared to the isolated state.