A Heparin Binding Protein Whose Expression Increases during Differentiation of Embryonal Carcinoma Cells to Parietal Endoderm Cells: cDNA Cloning and Sequence Analysis1

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Abstract
A cDNA clone isolated from a λgtll expression library of teratocarcinoma OTTBO5O specifies for a glycoprotein with a molecular weight of about 44,000. The new glycoprotein was termed heparin binding protien -44 (HIBP-44), since It was absorbed to a heparin agarose column and was eluted from it by a buffer containIng 1.5 M NaC1. HLBP-44 mRNA was intensely expressed in PYS-2 parietal endoderm cells and in the kidney, and the RNA level Increased about 10-fold during differentiation of FL) embryonal carcinoma cells to parietal endoderm cells. From the eDNA sequence, IIBP-44 was concluded to be rich in charged amino acids, and large segments of the protein appeared to form α-helixes. The protein was considered to be anchored to the membrane by a cluster of hydrophobic amino acids present in the N-terminal region. Indeed, the N-terminal sequence of HBP-44 was homologous to asialoglycoprotein receptor, which Is anchored to the membrane by the N-terminal region. Furthermore, a portion of the N-terminal region of HIBP-44 was homologous to the leucine zipper domain. Except for the N-terminal region, HBP-44 had over-all homology with structural proteins such as myosin heavy chain. We propose that BIBP-44 is extruded from plasma membranes and interacts with heparin and related molecules and that It is Involved in the interactions of plasma membranes with basement membranes.