Identification of functional regions of streptococcus agalactiae CAMP factor

Abstract

Streptococcus agalactiae CAMP factor is a protein exotoxin that contains 226 amino acid residues and forms oligomeric pores on susceptible cell membranes and liposomes. In this study, fragments of CAMP factor were created and recombinantly expressed to identify functional domains that are involved in membrane binding, oligomerization, and membrane insertion. Altogether, six truncated forms of CAMP factor were created and assayed. CAMP1-113, CAMP1-170, CAMP57-226, and CAMP171-226 showed different levels of hemolytic activity. CAMP1-56 and CAMP114-226 did not show hemolytic activity or oligomerization ability, but showed binding ability. CAMP114-226 inhibited the hemolytic activity of wild-type CAMP factor, most likely through ‘one-sided’ oligomerization. From the comparison of these fragments, it emerges that the region between residues 57 and 113 plays a crucial role in oligomerization and membrane insertion. The high binding efficiency of CAMP114-226 suggests this region has great responsibility on membrane binding. The hemolytically inactive fragments showed higher binding efficiency than some of the active fragments. For the hemolytic fragments, higher binding efficiency gave stronger hemolysis. These findings support that CAMP factor has different functional regions for pore-formation.