A 20-year-old woman presented to the local clinic with fever and abdominal tenderness. She was at day 2 of her menstrual cycle and reported having had unprotected sexual intercourse a few weeks earlier. Gram-negative diplococci are seen in a Gram stain of vaginal secretions, many of the bacteria inside polymorphonuclear leukocytes. Which of the following is the most important virulence factor necessary for this organism to initiate infection?