Streptococcus agalactiae CAMP factor: Functional Roles of N- and C-terminal Domains & Target Membrane Effect on Toxin Activity

Abstract

CAMP factor is a well-known pore-forming toxin secreted by Streptococcus agalactiae that forms discrete pores on susceptible membranes whether natural or artificial. This work reports haemolysis and liposome permeabilization experiments with intact CAMP factor and with fragments representing its N- and C-terminal domains, as well as fluorescence experiments using an NBD-labelled cysteine mutant. The collective findings indicate that the N-terminal domain of CAMP factor is responsible for membrane permeabilization while the C-terminal accounts for membrane binding and likely participates in oligomerization. Based on previous work, which suggested that varying target membrane lipid composition affects CAMP factor activity and evidence pointing to a cholesterol requirement, further studies were conducted on the requirement of specific lipids for CAMP factor activity, as well as varying lipid chain length and degree of saturation on liposomes with and without cholesterol. The results show that cholesterol is not required for membrane permeabilization. Additionally, no clear difference is observed when chain length or degree of saturation is varied as it concerns dimyristoyl and palmitoyl/oleoyl lipids; however, dioleoyl lipids seems to limit CAMP factor pore formation in the presence of cholesterol. An explanation for this result is proposed.