A comparative study of rat and human Tmp21 (p23) reveals the pseudogene-like features of human Tmp21-II.

Tmp21 (p23) is involved in biosynthetic transport from the endoplasmic reticulum to the Golgi complex. We have recently characterized two cDNA-variants of human Tmp21, the Tmp21-isoforms-I and -II. Because of the lack of cDNA sequence data and protein expression data, it was not clear if Tmp21-II encodes a functional Tmp21-protein. Here we describe the cloning of the full length human Tmp21-II transcript. The putative open reading frame of Tmp21-II contains a reading frame jump and a nonsense mutation in comparison to all other Tmp21-I (p23) members. Our data indicate that hum-Tmp21-II is transcribed, but not translated. We conclude that Tmp21-II cDNA derives from a neutral pseudogene, which originates from a duplication event of the human Tmp21-isoform-I.