Protein composition of rhapidosomes isolated from Aquaspirillum itersonii

Rhapidosomes are tubular microstructures composed of proteins that are found in a variety of bacteria and algae. These structures, which are resistant to disruption by many denaturing agents, have potential application as a biomaterial and may serve as a new model for the study of self assembly. When rhapidosomes were purified and analysed by SDS-PAGE the presence of three proteins with molecular masses of 53, 29 and 18 kDa was revealed. However, lysozyme treatment of the rhapidosome preparations containing the three proteins resulted in the selective release of the 18 kDa protein from the rhapidosome complex. N-Terminal sequence analysis and amino acid composition analysis were performed on all three proteins. The amino acid composition of the 18 kDa protein closely matched the amino acid composition of protein H (a peptidoglycan-associated protein from Pseudomonas aeruginosa). These results suggest that the 18 kDa protein may be a contaminating peptidoglycan-associated protein and not part of the rhapidosome structure. Western blot analysis using antisera raised against rhapidosomes revealed that the 53 kDa component protein does not react with the antisera. We speculate that the 53 kDa protein may form the inner core of the rhapidosomes, whilst the 29 kDa protein forms the outer sheath of these structures.