SEQUENCE-ANALYSIS OF THE STRUCTURAL TBPA GENE - PROTEIN TOPOLOGY AND VARIABLE REGIONS WITHIN NEISSERIAL RECEPTORS FOR TRANSFERRIN IRON ACQUISITION

The gene coding for the 98-kDa meningococcal outer membrane transferri n binding protein 1 (TbpA) from strain B385 was cloned and sequenced. Sequence comparison among its deduced aminoacid sequence and those fro m TbpA and the closely related LbpA (lactoferrin binding protein) gene from three different meningococcal strains, and four isolates from tw o other bacterial pathogens, showed that TbpA variability is confined to five specific segments, designated VR1 (199-287), VR2 (306-381), VR 3 (480-546), VR4 (618-651) and VR5 (681-708). The third VR was the mos t variable among strains both at the nucleotide and amino acid levels. Six additional tbpA genes from different meningococcal strains were c loned and its VR3 sequence determined. On the basis of this data we we re able to cluster tbpA genes in two groups: D (bearing a deletion in VR3) and N (nondeleted); all N and D strains belonging to the groups o f high or low molecular weight transferrin receptor isotype, respectiv ely. However, by phenogram analysis, the prototypical strain M982 (Gro up II) was clustered with M990 (B16B6 isotype, Group I). These results point to the existence of important exposed regions as well as to the possibility of horizontal gene exchange involving this locus. A topol ogy model with 14 exposed loops and 28 membrane spanning segments was postulated. According to this tentative analysis, TbpA as well as LbpA proteins should form a gated channel in the neisserial outer membrane . The variable regions were located in the fifth, sixth, eighth, 10th and 11th loops respectively. Among TbpAs VR1, VR2, and VR3 resulted th e most relevant regions. (C) 1997 Academic Press Limited.