Summary: The and genes encoding the transferrin receptor proteins Tbp 1 and Tbp2 from a serotype 7 strain of were cloned, sequenced, and expressed in . The gene was preceded by putative promoter and regulatory sequences and was separated from the downstream gene by a 13 bp intercistronic sequence suggesting that the two genes may be coordinately transcribed. Determination of the nucleotide sequence of this region facilitated PCR amplification of the region from a serotype 1 strain for comparative purposes. The deduced amino acid sequences of the Tbp1 proteins had regions of homology with Lbp and Tbp1s and with TonB-dependent outer membrane (OM) receptors of . The deduced amino acid sequences of the Tbp2 proteins were nearly identical to those presented in previous studies. Upon high-level expression of the gene, a large proportion of the recombinant Tbp1 was found in inclusion bodies and could not be affinity-isolated with immobilized porcine transferrin. Most of the remaining expressed Tbp1 was present in the OM fraction, was expressed at the surface of cells, and retained binding activity that was specific for the C-lobe of porcine transferrin. Although recombinant Tbp2 was found in inclusion bodies during high-level expression, a significant proportion was associated with a novel OM fraction that appeared in sucrose density gradients which was distinct from the OM fraction containing recombinant Tbp1. The recombinant Tbp2 was accessible at the surface yet was unable to bind porcine transferrin. In contrast to previous observations, the binding by recombinant Tbp2 was specific for the C-lobe of porcine transferrin. These results indicate that the transferrin receptor proteins have similar properties to the receptor proteins in spp. and , and that functional studies performed with recombinant receptor proteins need to consider differences in processing and export of these proteins when expressed in heterologous hosts.


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