The 36-kD protein-tyrosine kinase substrate p36 has been variously postulated to be involved in membrane-cytoskeletal interactions, membrane traffic, and the regulation of phospholipase A2, and its phosphorylation may play some role in malignant transformation by avian sarcoma viruses. Because embryonic tissues are resistant to transformation by avian sarcoma viruses, we have examined the expression of p36 in the developing avian embryonic limb. The level of p36 increased progressively from day 5 to day 14 of development. It was largely absent from day-5 mesenchyme, and was induced during the differentiation of mesenchymal cells into connective tissue and cartilage, but was not induced in differentiating muscle. In contrast, p36 was detected in ectodermal cells at all developmental stages examined. When day-5 limbs were dissociated and cultured, p36 was induced in all adherent cells, beginning at 2-4 h after plating, and reaching levels comparable to those observed with intact day-14 limb tissue within 48 h. The accumulation of p36 in culture was dependent on substratum adherence, suggesting that its stability is regulated by cell attachment or spreading. These findings are consistent with a structural or mechanical role for p36.

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