Transmembrane orientation and receptor‐like structure of the Rhizobium meliloti common nodulation protein NodC

The 46.8‐kd NodC protein of Rhizobium meliloti is a membrane protein, essential for nodule formation. Gene fusions of nodC to a portion of the λ c I repressor gene were used to define the membrane‐anchor domain which is necessary for membrane insertion of the NodC protein into the membrane. The transmembrane orientation of NodC was confirmed by surface‐specific radiolabeling and proteolysis experiments. A highly hydrophobic transmembrane‐anchor domain was found near the carboxyl terminus, separating a large extracellular domain which contains an unusual cysteine‐rich cluster from a short putative intracellular domain. Cross‐linking studies showed that the NodC protein exists in the membrane probably as a dimer. The domain structure of the NodC protein shows striking similiarities with cell surface receptors. In nodules of various legumes a truncated form of the NodC protein was detected. The processed NodC was associated with the bacteroids and the amount of this protein increased during nodule development.