Structure and expression of Ick transcripts in human lymphoid cells

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Abstract
The murine Ick gene encodes a membrane‐associated protein tyrosine kinase that has been implicated in lymphocyte oncogenesis. Here we report the structure of normal human Ick transcripts and the pattern of expression of these transcripts in developing thymus and in peripheral T cell subsets. The human Ick gene encodes a 509 amino acid polypeptide that is closely related to the murine Ickencoded protein throughout its length. Analysis of the deduced amino acid sequence of human p56Ick demonstrates that an amino‐terminal domain, widely divergent among the seven known src family members, has been conserved between murine and human p56Ick, and thus probably includes sequences crucial to the lymphocyte‐specific function of this molecule. Human Ick transcripts were detected in CD4+ and CD8+ T cells, in partially purified B cells, and in Epstein‐Barr virus‐immortalized B cell lines, but not in monocytes, granulocytes, or in nonhematopoietic cell types. Human Ick transcripts are readily detectable in fetal thymocytes at 70 days of gestation, but not at 57 days of gestation, indicating that Ick expression appears coordinately with the appearance of lymphoid cells in the developing thymus. Thus Ick gene expression is a marker for cells of the lymphocyte lineage in man. We conclude that the Ick gene probably participates in a signal transduction pathway uniquely present in lymphoid cells.