IDENTIFICATION OF PACTOLUS, AN INTEGRIN-BETA SUBUNIT-LIKE CELL-SURFACE PROTEIN PREFERENTIALLY EXPRESSED BY CELLS OF THE BONE-MARROW

We have sought to develop methodologies to identify genes that are pre ferentially expressed during the differentiation of mast cells from th eir hematopoietic stem cell precursors, By using a modified differenti al display protocol, we compared a subset of transcripts expressed in bone marrow cells differentiated into immature mast cells with the exo genous addition of stem cell factor (SCF) or interleukin 3, One gene w as identified that was preferentially expressed in the SCF-derived cel ls and encodes a novel murine integrin beta subunit-like molecule, dub bed Pactolus-1 (Pactolus). Two distinct forms of Pactolus mRNA were de tected which, via alternative splicing, are predicted to encode a memb rane-bound form and truncated version of the protein, The full-length Pactolus gene product is very similar to a number of beta subunit inte grin chains, particularly beta 2, with the notable exceptions of the a pparent deletion of the metal-binding site within the putative metal i on-dependent adhesion site-like domain of the Pactolus gene product an d a cytoplasmic domain that shares no obvious homology to similar doma ins of the other beta subunit integrin proteins, Although the Pactolus sequence was first identified in immature mast cell samples, screenin g of murine tissues indicated the highest level of Pactolus expression was found in the bone marrow, suggesting that the expression of Pacto lus is confined to immature and maturing bone marrow-derived cells, an d that the SCF-derived mast cells are more representative of this stat e than are the interleukin 3-derived mast cells, Immunoprecipitation o f Pactolus revealed a cell-surface protein with an apparent molecular mass of about 95 kDa, Surprisingly, no associating a integrin subunit could be identified suggesting that either Pactolus does not associate with another integrin subunit or the association is too weak to be id entified. These data suggest that Pactolus represents a gene and gene product related to those of the integrin beta subunits but whose funct ion(s) may be quite distinct from those of the integrin beta subunits.