The present invention relates in general to therapeutic fusion proteins useful to treat lysosomal storage diseases and methods for treating such diseases. Exemplary therapeutic fusion proteins comprise a lysosomal enzyme, a lysosomal targeting moiety, e.g., an IGF-II peptide, and a spacer peptide. Also provided are compositions and methods for treating Mucopolysaccharidosis Type IIIB (Sanfilippo B Syndrome), comprising a targeted therapeutic fusion protein comprising alpha-N-acetylglucosaminidase (Naglu), a lysosomal targeting moiety, e.g., an IGF-II peptide, and a spacer peptide.