Entamoeba histolytica causes invasive amebiasis, a major parasitic disease of the developing world, whose primary symptoms are liver abscess and colitis. All strains of E. histolytica express a 260-kDa surface Gal/GalNAc lectin that is antigenically conserved and immunogenic. The lectin is required for adherence to human intestinal epithelial cells and contact-dependent killing of immune effector cells. By expression cloning, the carbohydrate recognition domain (CRD) was identified within the lectin heavy-subunit cysteine-rich region. Of interest for a hepatic parasite, the CRD had sequence identity to the receptor-binding domain of hepatocyte growth factor (HGF) and competed with HGF for binding to the c-Met HGF receptor. In an animal model of invasive disease, immunization with the CRD inhibited liver-abscess formation, yet in humans, a naturally acquired immune response against the CRD did not persist.

Cardiac sodium channels (Na(v)1.5) comprise a pore-forming alpha-subunit and auxiliary beta-subunits that modulate channel function. In the heart, beta1 is expressed throughout the atria and ventricles, whilst beta3 is present only in the ventricles and Purkinje fibers. In view of this expression pattern, we determined the effects of beta3 and beta1 co-expression alone, and in combination, on Na(v)1.5 stably expressed in Chinese hamster ovary cells. The current/voltage relationship was shifted -5 mV with either beta1 or beta3 co-expression alone and -10 mV with co-expression of both beta1 and beta3. In addition, beta3 and beta1/beta3 co-expression accelerated macroscopic current decay. There were significant hyperpolarizing shifts in equilibrium gating relationships with co-expression of beta1 and beta3 alone and in combination. Co-expression of beta1/beta3 together resulted in a greater hyperpolarizing shift in channel availability, and an increase in the slopes of equilibrium gating relationships. Co-expression of beta3 and beta1/beta3, but not beta1, slowed recovery from inactivation at -90 mV. Development of inactivation at -70 and -50 mV was accelerated by beta-subunit co-expression alone and in combination. beta-Subunit co-expression also reduced the late Na current measured at 200 ms. In conclusion, beta-subunits modulate Na(v)1.5 gating with important differences between co-expression of beta1 and beta3 alone and beta1/beta3 together.

Electrical excitability in neurons depends on the expression and activity of voltage-gated sodium channels in the neuronal plasma membrane. The ion-conducting alpha-subunit of the channel is associated with auxiliary beta-subunits of which there are four known types. In the present study, we describe the first detailed structure/function analysis of the beta3-subunit. We correlate the effect of point mutations and deletions in beta3 with the functional properties of the sodium channel and its membrane-targeting behaviour. We show that the extracellular domain influences sodium channel gating properties, but is not required for the delivery of beta3 to the plasma membrane when expressed with the alpha-subunit. In contrast, the intracellular domain is essential for correct subunit targeting. Our results reveal the crucial importance of the Cys21-Cys96 disulphide bond in maintaining the functionally correct beta3 structure and establish a role for a second putative disulphide bond (Cys2-Cys24) in modulating channel inactivation kinetics. Surprisingly, our results imply that the wild-type beta3 molecule can traverse the secretory pathway independently of the alpha-subunit.

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