Characterization of a novel calcium‐binding 90‐kDa glycoprotein (BM‐90) shared by basement membranes and serum

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Abstract
The protein BM‐90 was solubilized from the mouse Engelbreth‐Holm‐Swarm tumor with neutral buffers in molar yields lower (15–30%) than found for other basement membrane proteins (e.g. laminin, BM‐40). The purified protein was shown to be rich in cysteine (5 mol%) and to change in SDS electrophoresis from an 84‐kDa position to a 95‐kDa one upon reduction. BM‐90 was also shown to be a calcium‐binding protein. The N‐terminal sequence of BM‐90, as well as those of several internal peptides, showed no identity with any known protein sequences, indicating that it is a new protein. Specific radioimmunoassays showed no or only minor cross‐reactions with other known basement membrane proteins.Immunological assays demonstrated BM‐90 to be present in neutral salt extracts from mouse heart and kidney, in serum (20–40 μ/ml) and in the medium of various cultured cells (0.1–1 μ/ml). The protein in these samples was identical in size to BM‐90 purified from the tumor, indicating that negligible degradation occurs during purification. An extracellular matrix localization of BM‐90 was shown by immunofluorescence for Reichert's membrane, lens capsules and other basement membranes. Thus, BM‐90 appears to be a novel basement membrane protein whose functions remain to be studied.