IpaC from Shigella and SipC from Salmonella possess similar biochemical properties but are functionally distinct

Jc. Osiecki et al., IpaC from Shigella and SipC from Salmonella possess similar biochemical properties but are functionally distinct, MOL MICROB, 42(2), 2001, pp. 469-481
Citations number
Categorie Soggetti
Journal title
ISSN journal
0950-382X → ACNP
Year of publication
469 - 481
SICI code
Invasion plasmid antigen C (IpaC) is secreted via the type III secretion sy stem (TTSS) of Shigella flexneri and serves as an essential effector molecu le for epithelial cell invasion. The only homologue of IpaC identified thus far is Salmonella invasion protein C (SipC/SspC), which is essential for e nterocyte invasion by Salmonella typhimurium. To explore the biochemical an d functional relatedness of IpaC and SipC, recombinant derivatives of both proteins were purified so that their in vitro biochemical properties could be compared. Both proteins were found to: (i) enhance the entry of wild-typ e S. flexneri and S. typhimurium into cultured cells; (ii) interact with ph ospholipid membranes; and (iii) oligomerize in solution; however, IpaC appe ared to be more efficient in carrying out several of the biochemical proper ties examined. Overall, the data indicate that purified IpaC and SipC are b iochemically similar, although not identical with respect to their in vitro activities. To extend these observations, complementation analyses were co nducted using S. flexneri SF621 and S. typhimurium SB220, neither of which is capable of invading epithelial cells because of non-polar null mutations in ipaC and sipC respectively. Interestingly, both ipaC and sipC restored invasiveness to SB220 whereas only ipaC restored invasiveness to SF621, sug gesting that SipC lacks an activity possessed by IpaC. This functional diff erence is not at the level of secretion because IpaC and SipC are both secr eted by SF621 and it does not appear to be because of SipC dependency on th is native chaperone as coexpression of sipC and sicA in SF621 still failed to restore detectable invasiveness. Taken together, the data suggest that I paC and SipC differ in either their ability to be translocated into host ce lls or in their function as effectors of host cell invasion. Because IpaB s hares significant sequence homology with the YopB translocator of Yersinia species, the ability for IpaC and SipC to associate with this protein was e xplored as a potential indicator of translocation function. Both proteins w ere found to bind to purified IpaB with an apparent dissociation constant i n the nanomolar range, suggesting that they may differ with respect to effe ctor function. Interestingly, whereas SB220 expressing sipC behaved like wi ld-type Salmonella, in that it remained within its membrane-bound vacuole f ollowing entry into host cells, SB220 expressing ipaC was found in the cyto plasm of host cells. This observation indicates that IpaC and SipC are resp onsible for a major difference in the invasion strategies of Shigella and S almonella, that is, they escape into the host cell cytoplasm. The implicati ons of the role of each protein's biochemistry relative to its in vivo func tion is discussed.