CHARACTERIZATION OF THE STRUCTURE, FUNCTION, AND CONFORMATIONAL STABILITY OF PORB CLASS 3 PROTEIN FRONT NEISSERIA-MENINGITIDIS - A PORIN WITH UNUSUAL PHYSICOCHEMICAL PROPERTIES

PorB proteins constitute the vast majority of channels in neisserial o uter membranes and can be subdivided within meningococcal strains into two distinct and mutually exclusive families that are designated as c lass 2 and class 3 proteins. We recently characterized the functional activity and conformational stability of a PorB class 2 protein from N eisseria meningitidis (Minetti, C. A. S. A., Tai, J. Y., Blake, M. S., Pullen, J. K., Liang, S. M., and Remeta, D. P. (1997) J. Biol. Chem. 272, 10710-10720). To evaluate the structure-function relatedness amon g the PorB proteins, we have employed a combination of electrophoretic and spectroscopic techniques to assess the conformational stability o f zwittergent-solubilized class 3 trimers, The functional, physicochem ical, and structural properties of the meningococcal class 2 and class 3 proteins are comparable with the notable exception that the latter exhibits a significantly higher susceptibility to SDS. The SDS-induced dissociation and partial unfolding of PorB class 3 is characterized b y a single two-state transition with a midpoint at 0.35% SDS. The nati ve trimeric assembly dissociates reversibly, forming partially folded monomers that retain the characteristic P-sheet content of the transme mbrane domain with a concomitant increase in random coil structure ari sing from unfolding the rigid surface loops. These results provide new insight into the elucidation of porin folding pathways and the factor s that govern the overall structural stability of meningococcal protei ns.