Mouse Clara cell 10-kDa (CC10) protein: cDNA nucleotide sequence and molecular basis for the variation in progesterone binding of CC10 from different species.

A protein similar to the rat Clara cell 10-kDa protein (CC10) was isolated from mouse lung homogenate by conventional chromatography. cDNA for the mouse CC10 protein was identified in the mouse lung cDNA library by using radiolabeled rat CC10 cDNA as the probe. The isolated cDNA was sequenced and the deduced primary amino acid sequence was compared to the known sequences of rabbit and hare uteroglobins and human and rat CC10 proteins. The cDNA sequence was confirmed by N-terminal amino acid sequencing of the purified protein. The purified mouse CC10 was tested for its ability to bind progesterone, and the binding was found to be 27% lower than rat CC10 and 48% lower than rabbit uteroglobin. The relative binding of mouse, rat, and human CC10 may reflect subtle structural perturbations. The only notable difference between mouse and rat CC10 is in the beta bend between helices 1 and 2, at residue 16. This difference also exists between rat and human CC10. The mouse CC10 sequence compares favorably with human CC10, which does not bind progesterone; however, the mouse CC10 does not contain M60, which has been proposed to block the binding of progesterone with human CC10. The wide variation in progesterone binding among this family of proteins casts doubt on the importance of such binding as a physiologic function.