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A newly identified gene, MK, is transiently expressed in the early stages of embryonal carcinoma cell differentiation and in the mid-gestation period of mouse embryogenesis (Kadomatsu, K., Tomomura, M., and Muramatsu, T. (1988) Biochem. Biophys. Res. Commun. 151, 1312-1318). Analysis of various MK cDNA clones revealed differences in the 5'-region. So far three classes of cDNA clones (MK1, MK2, and MK3) have been identified; they were different in the 5'-untranslated region but shared the rest of the sequence. Ribonuclease protection, RNA blotting, and primer extension revealed that MK2-type RNA was the major MK RNA in retinoic acid-treated embryonal carcinoma cells. In addition, the number of A residues in an oligo(A) stretch in the 5'-side of the common sequence differed from 9 to 29. The number was 9 in the most frequent cases, when the putative MK polypeptide had a molecular weight of about 15,500 and had a signal peptide-like sequence. Hybrid selected MK RNA yielded the predicted polypeptide upon in vitro translation. When pancreatic microsomal membranes were included in the translation system, the translation product of MK RNA was processed and entered into the lumen of the membranes. These results suggest that the product of the MK gene is an extracellular polypeptide.
Tomomura et al. (Fri,) studied this question.