Functional insight from the tetratricopeptide repeat-like motifs of the type III secretion chaperone SicA in Salmonella enterica serovar Typhimurium.

SicA functions both as a class II chaperone for SipB and SipC of the type III secretion system (T3SS)-1 and as a transcriptional cofactor for the AraC-type transcription factor InvF in Salmonella enterica subsp. enterica serovar Typhimurium. Bioinformatic analysis has predicted that SicA possesses three tetratricopeptide repeat (TPR)-like motifs, which are important for protein-protein interactions and serve as multiprotein complex mediators. To investigate whether the TPR-like motifs in SicA are critical for its transcriptional cofactor function, the canonical residues in these motifs were mutated to glutamate (SicA(A44E), SicA(A78E), and SicA(G112E)). None of these mutants except SicA(A44E) were able to activate the expression of the sipB and sigD genes. SicA(A44E) still has a capacity to interact with InvF in vitro, and despite its instability in cell, it could activate the sigDE operon. This suggests that TPR motifs are important for the transcriptional cofactor function of the SicA chaperone.