Bacterial magnetic particles (BacMPs) produced by the magnetotactic bacterium Magnetospirillum magneticum AMB-1, are used as magnetic supports or carriers for a variety of biomedical and environmental applications. Although protein expression systems on BacMPs have been established in previous studies, the expression efficiency was dependent on the introduced protein sequences. Recombinant human proteins are often poorly expressed on BacMPs because of proteolytic degradation by endogenous proteases. We constructed a Ion protease gene deletion mutant strain (Delta lon) of M. magneticum AMB-1 by homologous recombination to increase the efficiency of functional protein display on BacMPs using Delta lon host cells. Wild-type and Delta lon-M. magneticum AMB-1 cells were transformed using expression plasmids for human proteins, thyroid-stimulating hormone receptor (TSHR) and the class II major histocompatibility complex (MHC II) molecules onto BacMPs. Although mRNA expression of both TSHR and MHC II was the same level in the wild-type and Delta lon transformants, the protein expression levels in Mon transformants were significantly increased versus wild-type cells. Furthermore, the amounts of two different human proteins on BacMPs were successfully improved. This phenomenon could be due to the reduction of the degradation of target proteins in the Delta lon strain. This is the first report to construct a protease deletion mutant in magnetotactic bacteria. The Mon strain is a useful host to provide BacMPs displaying target proteins for various experimental, and ultimately, clinical applications. (C) 2013, The Society for Biotechnology, Japan. All rights reserved.